Pacemakers and Implantable Defibrillators*



Pacemakers and Implantable Defibrillators*


Carol Jacobson

Donna Gerity



PACEMAKERS

Arrhythmia device therapy is becoming more complex with every advance in technology, requiring clinicians to have more knowledge and greater responsibilities than ever before. Early pacemakers were single-chamber devices designed to pace only in the ventricle, and the only programmable parameters were pacing rate and output. With the introduction of dual-chamber pacemakers with the capability of pacing the atria and the ventricles, the number of programmable parameters increased dramatically. Rate-responsive pacemakers came next and are capable of increasing the pacing rate in response to the body’s need for increased cardiac output. Antitachycardia devices were developed to terminate supraventricular and ventricular tachyarrhythmias using pacing techniques, cardioversion, or defibrillation. Most recently, the development of biventricular pacing capability allows for pacing to improve hemodynamics and left ventricular (LV) function in patients with heart failure (HF) and cardiomyopathy. There have been tremendous advances in technology of devices for both bradycardia and antitachycardia therapy in recent years, with even more complex devices coming in the future. Given the number of companies in the arrhythmia device market and the increasing complexity of the devices themselves, it has become very difficult for clinicians to stay abreast of device features and function. The goal of this chapter is to present generic concepts of pacemaker and implantable defibrillator functions to provide a basic knowledge background upon which cardiac nurses can build to enhance their understanding of arrhythmia management devices.


Indications for Pacing

Pacemakers were originally designed to treat disorders of impulse initiation or impulse conduction resulting in symptomatic bradycardia. Symptomatic bradycardia is a term used to define a bradycardia that is directly responsible for symptoms such as syncope, near syncope, transient dizziness, or light-headedness, and confusion resulting from cerebral hypoperfusion caused by slow heart rate.1 Other symptoms such as fatigue, exercise intolerance, HF, dyspnea, and hypotension can also result from bradycardia. Symptomatic bradycardia can be caused by sinus node dysfunction or by conduction failure in or below the AV node. Sinus node dysfunction is the most common indication for permanent pacing, followed by AV node dysfunction.2,3

In addition to treating symptomatic bradycardia, pacemaker therapy can have beneficial effects on hemodynamics and clinical status by providing rate response for patients whose sinus node is not capable of increasing its rate appropriately in response to the body’s need for increased cardiac output (chronotropic incompetence). Dual-chamber pacemaker therapy can improve stroke volume in patients with LV dysfunction, hypertrophic cardiomyopathy, or dilated cardiomyopathy by ensuring AV synchrony and providing optimal AV intervals to enhance ventricular filling.4,5 Cardiac resynchronization therapy (CRT) with biventricular pacing improves septal wall motion, mitral valve function, and the dynamics of LV contraction in patients with severe HF or dilated cardiomyopathy.4, 5, 6, 7, 8, 9, 10, 11 The use of pacemaker therapy to prevent atrial fibrillation is an area of intense interest and investigation and has proven successful in many patients.12, 13, 14, 15, 16, 17, 18 Other indications for cardiac pacing include hypersensitive carotid sinus syndrome, neurocardiogenic syncope (vasovagal syncope), long QT syndrome, and sleep apnea.1, 2, 3, 4,19

The American College of Cardiology (ACC), American Heart Association (AHA), and Heart Rhythm Society (HRS) task force on practice guidelines recently updated the guidelines for implantation of pacemakers and antiarrhythmia devices.1 Display 28-1 lists the indications for permanent pacemaker implantation in selected clinical settings.

Temporary pacing is indicated to treat symptomatic bradycardia after AMI or cardiac surgery, or when associated with hyperkalemia or drug toxicity; bradycardia-dependent ventricular tachycardia (VT); before permanent pacemaker implantation in symptomatic patients; and in reversible conditions that will not likely result in the need for permanent pacing, such as bacterial endocarditis, Lyme disease, or cardiac trauma.20,21 Temporary pacing in acute myocardial infarction (MI) is still controversial. Inferior MI results in intranodal block that is usually benign and temporary and requires pacing only if it results in symptomatic bradycardia or bradycardia-dependent VT. When atrioventricular (AV) block occurs in anterior MI, it is usually infranodal, involves a large amount of myocardium, and is often symptomatic. Second- or third-degree AV block associated with anterior MI and bundle-branch block usually requires temporary pacing, but the mortality rate is high because of LV dysfunction secondary to the large infarction rather than to the conduction disturbance. Prophylactic temporary pacing is often performed in the presence of new right bundle-branch block (RBBB) with either anterior or posterior hemiblock, in left bundle-branch block (LBBB) with first-degree AV block, and in alternating right and LBBB.

Temporary pacing is often used after cardiac surgery to prevent or treat symptomatic bradycardia and is sometimes used prophylactically in high-risk patients during cardiac catheterization, or with electrical or chemical cardioversion. Overdrive atrial pacing is sometimes used in an attempt to terminate atrial flutter or fibrillation after cardiac surgery when atrial epicardial leads are in place.




Types of Pacemakers

Refer to Displays 28-2 and 28-3 for definitions of single- and dual-chamber pacemaker terminology. The terms defined there are used throughout the pacemaker section of this chapter and are not defined in the text unless necessary.


Permanent Pacemakers

Permanent pacemakers are usually implanted under local anesthesia in the operating room (OR), electrophysiology laboratory, or cardiac catheterization laboratory. The pulse generator is placed in a subcutaneous pocket in the pectoral area and the pacing lead is inserted either through the cephalic vein or through the subclavian vein and advanced into the right ventricular (RV) apex. If a dual-chamber pacemaker is implanted, then a second lead is placed in the right atrial appendage (Fig. 28-1). Permanent pulse generators are powered by lithium batteries with a lifespan of approximately 10 years, depending on many factors, including how the pacemaker is programmed and the percentage of time that it paces.


Temporary Pacemakers

Temporary pacing can be accomplished with transvenous, epicardial, or transcutaneous methods. Temporary pacing can be performed in emergency and elective situations, and it is usually performed in a monitored unit such as critical care or telemetry unit. Transcutaneous pacing can also be performed by paramedics or other trained personnel in emergency response vehicles or in the field.





Transvenous Pacing.

Transvenous pacing is usually performed by percutaneous puncture of the internal jugular, subclavian, antecubital, or femoral vein and threading a pacing lead into the apex of the right ventricle for ventricular pacing, the right atrium for atrial pacing, or both chambers for dual-chamber pacing (Fig. 28-2). The transvenous pacing lead is attached to an external pulse generator that is kept either on the patient or at the bedside. The procedure is usually performed under fluoroscopy in a cardiac catheterization laboratory but it can be done at the bedside with or without fluoroscopy. Transvenous pacing is usually necessary only for a few days until the rhythm returns to normal or a permanent pacemaker is inserted. Instructions for initiating transvenous pacing are covered later in this chapter.






Figure 28-1 Transvenous installation of a permanent dual-chamber pacemaker.


Epicardial Pacing.

Epicardial pacing is performed through electrodes placed on the atria or ventricles during cardiac surgery. The pacing electrode end of the lead is attached to the epicardial surface of the atria or ventricles and the other end is pulled through the chest wall, sutured to the skin, and attached to an external pulse generator. A ground wire is often placed subcutaneously in the chest wall and pulled through with the other leads. The number and placement of leads varies with the surgeon; there may be one or two atrial leads, one or two ventricular leads, and one, two, or no ground leads (Fig. 28-3). Instructions for initiating epicardial pacing are covered later in this chapter.


Transcutaneous Pacing.

Transcutaneous external pacing is a noninvasive method of pacing used as a temporary measure in emergency situations for treatment of asystole, severe bradycardia, or overdrive pacing for tachyarrhythmias until a transvenous
pacing lead can be inserted. Large-surface adhesive electrodes are attached to the anterior and posterior chest wall and connected to an external pacing unit (Fig. 28-4). The pacing current passes through skin and chest wall structures to reach the heart; therefore, high energies are required to achieve capture and sedation is usually needed to minimize the discomfort felt during pacing.






Figure 28-2 Temporary transvenous pacing lead in right ventricle inserted through antecubital vein.


Single-Chamber Pacing

Single-chamber pacing means that only the atria or the ventricles, but not both, are paced. This type of pacing requires only one pacing lead inserted into the desired chamber. Single-chamber ventricular pacing is the most frequently used temporary transvenous type of pacing and can also be used for permanent pacing. Single-chamber atrial or ventricular pacing can be performed using epicardial-pacing leads.






Figure 28-3 Epicardial pacing using atrial and ventricular pacing leads attached to a dual-chamber pacemaker.






Figure 28-4 Transcutaneous pacing. Electrodes are placed on anterior and posterior chest wall and attached to the external pacing unit.


Dual-Chamber Pacing

Dual-chamber pacing means that both the atria and the ventricles can be paced. Dual-chamber pacing is a frequently used method of permanent pacing and can also be performed via epicardial-pacing leads. Temporary transvenous dual-chamber pacing can be performed, but it is difficult to place temporary atrial leads and it is not as reliable as ventricular pacing.


Biventricular Pacing

Biventricular pacing means that both ventricles are simultaneously paced via a lead in the RV apex for RV pacing and a lead threaded through the coronary sinus into a lateral or posterior cardiac vein (or less commonly via an epicardial LV lead) for LV pacing. Figure 28-5 illustrates dual-chamber biventricular pacing leads, and this topic is discussed in more detail later in this chapter.







Figure 28-5 Biventricular pacing. There is a lead in the atrium and a lead in the right ventricle for dual-chamber pacing. The left ventricle is paced via a lead threaded through the coronary sinus and down a lateral or posterior cardiac vein in the left ventricle.


Pacing Modes

The current nomenclature used to describe the expected function of a pacemaker was established by members of the North American Society of Pacing and Electrophysiology and the British Pacing and Electrophysiology Group and is designated the NBG code for pacing nomenclature.22 The code describes the expected function of the device according to the site of the pacing electrodes and the mode of pacing. The first letter describes the chamber that is paced: A, atrium; V, ventricle; D, dual (both atrium and ventricle); O, none. The second letter describes the chamber where intrinsic electrical activity is sensed: A, atrium; V, ventricle; D, dual (both atrium and ventricle); O, none. The third letter describes the pacemaker’s response to sensing of intrinsic electrical activity: I, inhibited; T, triggered; D, dual (inhibits or triggers); O, none. The fourth letter denotes the presence or absence of rate modulation (R, rate modulation and O, none). The fifth letter specifies the location or absence of multisite pacing, which includes either biatrial or biventricular pacing, or more than one stimulation site in a single chamber (e.g. two atrial pacing sites in the right atrium). Table 28-1 illustrates the pacemaker code in detail.

The most commonly used pacing modes are VVI and DDD. The VVI mode means that the electrode is in the ventricle and paces the ventricle (first V), senses ventricular activity (second V), and inhibits its output when it senses intrinsic ventricular depolarization (I in third position). VVI is the most commonly used mode of pacing with temporary transvenous leads because it is the quickest and easiest method of pacing in an emergency, and it is difficult to get a temporary atrial lead to stay in place. VVI is also often used with epicardial leads after cardiac surgery, especially if third-degree AV block is present, and is the mode that has to be used for permanent pacing in patients with chronic atrial fibrillation. The DDD mode means that both atrial and ventricular electrodes are present and both chambers are paced (first D), both chambers are sensed (second D), and the device either inhibits or triggers an output in response to sensed intrinsic activity (D in third position means dual-response to sensing). DDD is the most frequently used permanent pacing mode, unless the patient has chronic atrial fibrillation or flutter. Other pacing modes that are sometimes used are AOO, AAI, DVI, DDI, and VDD.








Table 28-1 ▪ FIVE-LETTER PACEMAKER CODE

































First Letter: Chamber Paced


Second Letter: Chamber Sensed


Third Letter: Response to Sensing


Fourth Letter: Rate Modulation


Fifth Letter: Multisite Pacing


O = None


O = None


O = None


O = None


O = None


A = Atrium


A = Atrium


I = Inhibited


R = Rate modulation


A = Atrial


V = Ventricle


V = Ventricle


T = Triggered



V = Ventricle


D = Dual (A and V)


D = Dual (A and V)


D = Dual (I and T)



D = Dual



Basics of Pacemaker Operation

Electrical current flows in a closed-loop circuit between two pieces of metal (poles). For current to flow, there must be conductive material (i.e. a lead, muscle, or conductive solution) between the two poles. In the heart, the pacing lead, cardiac muscle, and body tissues serve as conducting material for the flow of electrical current in the pacing system. The pacing circuit consists of the pacemaker (the power source), the conducting lead (pacing lead), and the myocardium. The electrical stimulus travels from the pulse
generator through the pacing lead to the myocardium, through the myocardium, and back to the pulse generator, thus completing the circuit.23,24


Components of a Pacing System

The three basic components of a cardiac pacing system are the pulse generator, the pacing lead, and the myocardium. The pulse generator contains the power source (battery) and all of the electronic circuitry that controls pacemaker function. Most pacemakers are powered by a lithium battery. The pulse generator of a permanent pacemaker is small and thin, and is implanted in the pectoral area or sometimes in the abdominal area (see Fig. 28-1). Once a permanent pulse generator is implanted, the only way to alter its pacing parameters is with a programmer that communicates with the pacemaker through a wand placed over the pulse generator. A temporary pulse generator is a box that is kept at the bedside of the patient and is usually powered by a regular 9-volt battery. It has controls on the front that allow the operator to set certain pacing parameters easily (Fig. 28-6).






Figure 28-6 Examples of temporary pulse generators and pacing cable. (A) Older model Medtronic single-chamber pacemaker. (B) New model Medtronic single-chamber pacemaker. (C) New model Medtronic dual-chamber pacemaker. (D) Pacing cable used to connect pulse generator to pacing leads.

The pacing lead is an insulated wire used to transmit the electrical current from the pulse generator to the myocardium. A unipolar lead contains a single wire and a bipolar lead contains two wires that are insulated from each other. In a unipolar lead, the electrode is an exposed metal tip at the end of the lead that contacts the myocardium and serves as the negative pole of the
pacing circuit. In a bipolar lead, the end of the lead is a metal tip that contacts myocardium and serves as the negative pole, and the positive pole is an exposed metal ring located a few millimeters proximal to the distal tip. Permanent pacing leads can be unipolar or bipolar, but bipolar is more commonly used. Permanent leads have some type of fixation device on the end of the lead that helps keep the tip in contact with myocardium. Passive-fixation leads usually have tines on the end that get caught in the trabeculae of the right ventricle and keep the lead in position. Active-fixation leads have a screw on the end that is screwed into the ventricular muscle to hold the lead in place (Fig. 28-7). Occasionally, epicardial leads with a screw-type fixation are used in permanent pacing, especially in children, and with implantable defibrillators. Temporary transvenous pacing leads are insulated wires (usually bipolar) with no-fixation device, making them more prone to dislodgment. Temporary epicardial-pacing leads are unipolar or bipolar wires with one end looped through the myocardium and the leads then pulled through the chest wall for easy access.






Figure 28-7 Pacing leads. (A) Passive fixation lead with tines on the end to hold lead in position. (B) Active fixation screw lead; top shows screw retracted, bottom shows screw extended.

A pacing cable is usually used to connect a temporary pacemaker pulse generator to the pacemaker lead, similar to an extension cord. This enhances patient comfort by allowing the pulse generator to be kept at the bedside rather than being strapped to the patient.


Bipolar Pacemaker Operation

In any pacing system, there are two metal poles that make up the pacing circuit. The term bipolar means that both of these poles are in or on the heart. In a bipolar system, the pulse generator initiates the electrical impulse and delivers it out the negative terminal of the pacemaker to the pacing lead. The impulse travels down the lead to the distal electrode (negative pole or cathode) that is in contact with myocardium. As the impulse reaches the tip, it travels through the myocardium and returns to the positive pole (or anode) of the system, completing the circuit. In a bipolar system, the positive pole is the proximal ring located a few millimeters proximal to the distal tip. As illustrated in Figure 28-8, the circuit over which the electrical impulse travels in a bipolar system is small because the two poles are located close together. This system results in a small pacing spike on the electrocardiogram (ECG) as the pacing stimulus travels between the two poles. If the stimulus is strong enough to depolarize the myocardium, then the pacing spike is immediately followed by a P wave if the lead is in the atrium, or a wide QRS complex if the lead is in the ventricle.






Figure 28-8 Bipolar pacing system. The pulse generator delivers an electrical stimulus at a predetermined rate. The stimulus travels down the lead to the distal pole in contact with myocardium (arrows from pulse generator to distal tip of lead). Current spreads through cardiac muscle while traveling to the positive pole located proximal to the distal tip. Current returns to the pulse generator (arrows from proximal pole back to pulse generator), completing the circuit.


Unipolar Pacemaker Operation

A unipolar system has only one of the two poles in or on the heart. In a permanent unipolar pacing system, the back of the pulse generator serves as the second pole. In a temporary epicardial-pacing system, a ground lead placed in the subcutaneous tissue in the mediastinum serves as the second pole. Unipolar pacemakers work the same way as bipolar systems, but the circuit over which the impulse travels is much larger because of the distance between the two poles (Fig. 28-9). This type of system results in a large pacing spike on the ECG as the impulse travels between the two poles.






Figure 28-9 Unipolar pacing system. The pulse generator delivers an electrical impulse which travels from the negative terminal of the pulse generator to the electrode at the tip of the catheter (arrows from pulse generator to distal tip of lead). Current exits through the electrode tip, stimulates the myocardium, and completes the circuit by traveling through body tissues to the positive terminal on the back of the pulse generator (arrow from distal tip of pacing lead to pulse generator).



Asynchronous (Fixed-Rate) Pacing

A pacemaker programmed to an asynchronous mode paces at the programmed rate regardless of intrinsic cardiac activity. This mode can result in competition between the pacemaker and the heart’s own electrical activity. Asynchronous pacing in the ventricle is unsafe because of the potential for pacing stimuli to fall in the vulnerable period of repolarization and cause ventricular fibrillation (VF). Asynchronous pacing in the atria is less dangerous but can cause atrial fibrillation.


Demand Pacing

The term demand means that the pacemaker paces only when the heart fails to depolarize on its own, that is, the pacemaker fires only “on demand.” In the demand mode, the pacemaker’s sensing circuit is capable of sensing intrinsic cardiac activity and inhibiting pacer output when intrinsic activity is present. Sensing takes place between the two poles of the pacemaker. A bipolar system senses over a small area because the poles are close together, and this can result in “undersensing” of intrinsic signals. A unipolar system senses over a large area because the poles are far apart, and this can result in “oversensing.” A unipolar system is more likely to sense myopotentials caused by muscle movement and inappropriately inhibit pacemaker output, potentially resulting in periods of asystole if the patient has no underlying cardiac rhythm. The demand mode should always be used for ventricular pacing to avoid the possibility of VF.


Capture

Capture means that a pacing stimulus results in depolarization of the chamber being paced. Capture is determined by the strength of the stimulus, which is measured in milliamperes (mA), the amount of time the stimulus is applied to the heart (pulse width), and by contact of the pacing electrode with the myocardium. Capture cannot occur unless the distal tip of the pacing lead is in contact with healthy myocardium that is capable of responding to the stimulus. Pacing in infarcted tissue usually prevents capture. Similarly, if the catheter is floating in the cavity of the ventricle and not in direct contact with myocardium, capture will not occur.

In permanent pacing systems, stimulus strength is programmed at implant and can be changed as necessary by using a pacemaker programmer. In temporary pacing, the output dial on the face of the pulse generator controls stimulus strength and can be set and changed easily by the operator. Temporary pulse generators usually are capable of delivering a stimulus of from 0.1 to 20 mA.






Figure 28-10 The fence analogy for pacemaker sensitivity. The height of the fence is inversely related to the sensitivity of the pacemaker: the taller the fence, the less sensitive the pacemaker is; the shorter the fence the more sensitive the pacemaker is. (A) The fence is too high for the QRS complex behind it to be visible; the pacemaker sensitivity is too low to be able to sense the QRS. (B) The fence is a good height for the pacemaker to be able to see the QRS but not the P wave or T wave or other signals. (C) The fence is too low and now the T wave is visible along with the QRS complex; the pacemaker sensitivity is so high that it senses extraneous signals.


Sensing

The sensing circuit controls how sensitive the pacemaker is to intrinsic cardiac depolarizations. Intrinsic activity is measured in millivolts (mV), and the higher the number, the larger the intrinsic signal. For example, a 10-mV QRS complex is larger than a 2-mV QRS. When pacemaker sensitivity needs to be increased to make the pacemaker “see” smaller signals, the sensitivity number must be decreased. For example, a sensitivity of 2 mV is more sensitive than one of 5 mV.

A fence analogy may help explain sensitivity (see Fig. 28-10). Think of sensitivity as a fence standing between the pacemaker and what the pacemaker wants to see—the ventricle, for example. If there is a 10-ft-high fence (or a 10-mV sensitivity) between the two, then the pacemaker may not see what the ventricle is doing. To make the pacemaker able to see, the fence needs to be lowered. Lowering the fence to 2 feet would probably enable the pacemaker to see the ventricle. Changing the sensitivity from 10 to 2 mV is like lowering the fence—the pacemaker becomes more sensitive and is able to “see” intrinsic activity more easily. Thus, to increase the sensitivity of a pacemaker, the millivolt number (fence) must be decreased.


Initiating Temporary Pacing


Transvenous Ventricular Pacing

A transvenous pacing lead is inserted through a peripheral vein, either antecubital or femoral, or through the internal jugular or subclavian vein, and advanced into the apex of the right ventricle. The lead is sutured in place at its insertion site and a dressing is applied. Temporary transvenous pacing leads are bipolar and have two tails, one marked “positive” or “proximal” and the other marked “negative” or “distal.” These tails are connected to the pacing cable, which is then connected to the pulse generator. To initiate ventricular pacing using a transvenous lead (Fig. 28-11):







Figure 28-11 Initiating temporary transvenous pacing. The distal tail of the pacing lead is connected to the negative terminal of the pacemaker, and the proximal tail of the pacing lead is connected to the positive terminal of the pacemaker.



  • Connect the pacing cable to the top of the pulse generator (if using an older pacemaker with positive and negative connections on top, make sure cable is connected positive to positive and negative to negative).


  • Connect the other end of the pacing cable to the tails of the pacing wire: proximal tail to positive terminal of pacing cable, and distal tail to the negative terminal of pacing cable.


  • Set the rate at 70 to 80 bpm or as ordered by physician.


  • Set the output at 5 mA and adjust according to stimulation threshold.


  • Set the sensitivity at 2 mV and adjust according to sensitivity threshold.






Figure 28-12 Initiating epicardial pacing. (A) Unipolar epicardial pacing. The ventricular lead (or atrial lead for atrial pacing) is connected to the negative terminal of the pacemaker and the ground lead is connected to the positive terminal. (B) Bipolar epicardial pacing. One ventricular lead is connected to the positive terminal and one to the negative terminal of the pacemaker. (For atrial pacing, either atrial lead is connected to the positive terminal and the other atrial lead to the negative terminal of the pacemaker.)

See the section “Nursing Considerations” for the procedure for performing stimulation and sensitivity threshold tests.


Epicardial Pacing

The number and location of epicardial leads placed in surgery determine connections for epicardial pacing. There may be one or two atrial or ventricular leads with a ground or no ground lead. If only one lead is on a chamber, then unipolar pacing is performed. If there are two leads on a chamber, then bipolar pacing can be performed.

To initiate unipolar atrial or ventricular pacing (Fig. 28-12A):



  • Connect the pacing cable to the top of the pulse generator (if using an older pacemaker with positive and negative connections on top, make sure cable is connected positive to positive and negative to negative).


  • Connect the other end of pacing cable to pacing wires: negative terminal of the cable to the atrial or ventricular wire to be paced, positive terminal of the cable to the ground wire.


  • Set the rate at 70 to 80 bpm or as ordered by physician.


  • Set the output at 10 mA for atrial pacing and 5 mA for ventricular pacing, then determine stimulation threshold and set two- to three-times higher.


  • Set the sensitivity at the lowest possible number for atrial pacing and at 2 mV for ventricular pacing.

To initiate bipolar atrial or ventricular pacing (see Fig. 28-12B):



  • Connect the pacing cable to the top of the pulse generator (if using an older pacemaker with positive and negative connections on top, make sure cable is connected positive to positive and negative to negative).


  • Connect the other end of the pacing cable to pacing wires: for atrial pacing, connect one atrial pacing wire to the negative terminal of the pacing cable the other atrial pacing wire to the
    positive terminal of the cable; for ventricular pacing, connect one ventricular pacing wire to the negative terminal of the pacing cable and the other ventricular wire to the positive terminal of the cable.


  • Set the rate at 70 to 80 bpm or as ordered.


  • Set the output at 10 mA for atrial pacing and 5 mA for ventricular pacing, then determine stimulation threshold and set two- to three-times higher.


  • Set the sensitivity at the lowest possible number for atrial pacing and at 2 mV for ventricular pacing.


Dual-Chamber Temporary Pacing

Dual-chamber pacing can be performed through epicardial-pacing leads or with transvenous atrial and ventricular leads. Transvenous dual-chamber pacing is not often performed because of difficulties in placing temporary atrial leads and the unreliable stability of atrial leads. Epicardial dual-chamber pacing is often performed after cardiac surgery, but should be performed only when there are two ventricular leads in place. Two ventricular leads allow for bipolar ventricular pacing and sensing, thus reducing the possibility that the ventricular lead will sense atrial output and inappropriately inhibit ventricular pacing (crosstalk).

Dual-chamber pacing modes available depend on the type of pulse generator used for pacing. Older dual-chamber temporary pulse generators (like that shown in Fig. 28-6A) allow only DVI pacing. The newer dual-chamber units allow for DDD, DVI, DDI, and VDD pacing in addition to the single-chamber options AAI, AOO, and VVI.

To initiate dual-chamber pacing with epicardial leads:



  • Connect two pacing cables to the top of the pulse generator: one to the atrial terminal and one to the ventricular terminal.


  • Connect the atrial pacing cable to the atrial pacing wires: one atrial wire to the positive terminal and one atrial wire to the negative terminal.


  • Connect the ventricular pacing cable to the ventricular wires: one ventricular wire to the positive terminal and one ventricular wire to the negative terminal.


  • Select dual-chamber pacing mode desired (if option is provided): DDD, DDI, DVI, VDD. The DDD mode is almost always used.


  • Set AV delay at 150 milliseconds or as ordered.


  • Set atrial output at 10 mA and ventricular output at 5 mA, then determine stimulation threshold for both chambers and set output two- to three-times higher than threshold.


  • Set atrial or ventricular sensitivity as necessary, depending on pacing mode selected (atrial sensing occurs only in DDD, DDI, and VDD dual-chamber modes).



    • Set atrial sensitivity at 0.5 mV.


    • Set ventricular sensitivity at 2 mV.


Nursing Considerations

Nursing care of patients with pacemakers requires an understanding of how pacemakers work and what to expect the pacemaker to do depending on how it is programmed. Clinicians working in pacemaker follow-up clinics or physician offices have the advantage of being able to use the pacemaker programmer and view intracardiac electrograms and marker channels to help evaluate pacemaker function. Bedside nurses in most acute care facilities do not have access to programmers and must be able to evaluate appropriate pacemaker function by looking at the ECG or rhythm strips. The next section of this chapter covers ECG analysis of pacemaker rhythm strips to assist bedside practitioners in evaluating pacemaker function. It is beyond the scope of this chapter to discuss pacemaker follow-up in a clinic or office setting.

An important function for nurses or monitor technicians is to document significant bradycardias that may require pacemaker therapy and to relate these bradycardia events with clinical symptoms whenever possible. The guidelines for pacemaker insertion state, “definite correlation of symptoms with a bradyarrhythmia is required to fulfill the criteria that define symptomatic bradycardia.”1 Many insurance companies will not cover the cost of pacemaker insertion without good clinical documentation of the need. Nurses and monitor technicians are in a prime position to be able to document the bradycardia event by mounting rhythm strips in the patient’s chart or getting a 12-lead ECG, and documenting symptoms that occur in conjunction with the bradycardia. Documentation of hypotension, syncope or near syncope, dizziness or light-headedness, confusion, fatigue, exercise intolerance, and development of symptoms of HF associated with bradycardia is an important nursing function.


Permanent Pacemakers

Implantation of a permanent pacemaker is often performed on an outpatient basis, but some patients are kept overnight for observation. The procedure is performed under local anesthesia in the cardiac catheterization laboratory, electrophysiology laboratory, or the OR, and it takes from 1 to 5 hours to complete depending on the number and location of pacing leads being inserted. In addition to routine postoperative care given to any surgical patient, permanent pacemaker insertion usually requires that the patient immobilize the operative arm in a sling for the first 24 hours to prevent lead dislodgment. The nurse must be aware of the potential complications of pacemaker insertion, including the potential for cardiac perforation leading to tamponade, and monitor for those complications. Patient teaching includes information about pacemaker function, how to count the pulse, and importance of follow-up visits to the physician. Because patients are discharged so soon after the procedure, they should be told to take their temperature and monitor the insertion site for signs of infection.


Using a Magnet With a Permanent Pacemaker.

Occasionally, the nurse is asked to place a pacemaker magnet over the permanent pulse generator. Use of a magnet usually requires a physician’s order or is covered by a written protocol detailing conditions under which a magnet can be used without a direct order.

A magnet inactivates the sensing circuit of a permanent pacemaker and causes it to revert to the asynchronous mode of pacing. This may be performed to verify a pacemaker’s ability to pace when it is being inhibited by a patient’s own natural rhythm. With a magnet in place, the pacemaker paces at a fixed rate in competition with the patient’s rhythm, thus verifying the pacemaker’s ability to deliver pacing stimuli. When a paced impulse happens to fall at a time when the ventricle is able to respond, capture should occur, verifying the pacemaker’s ability to capture. A magnet may also be used to evaluate battery status if a pacemaker is nearing its end of service. In some older pacemakers the primary indicator of battery depletion is a change in the magnet-induced pacing rate. Some models of pacemakers pace at a faster rate for the first several beats after magnet application and then pace at a
slower rate. Some pacemakers remain in the asynchronous pacing mode for the entire duration that the magnet is in place, whereas others only pace asynchronously for a certain number of beats and then revert to normal operation. It is possible to program the magnet operation off in some pacemakers, although this is rarely done. Because of the wide variety of potential responses to magnet application, it is advisable to know what pacemaker is present and what the programmed parameters are whenever possible when using a magnet.

Another indication for magnet use is to terminate a pacemaker-mediated tachycardia (PMT) in a dual-chamber pacemaker (see section “Dual-Chamber Pacing”). When using the magnet, the nurse should have the patient on a cardiac monitor and must be aware of the potential danger of a pacing spike falling in the vulnerable period and causing ventricular arrhythmias. A defibrillator should be immediately available whenever a magnet is used on a permanent pacemaker.


Pacemakers in the OR.

The biggest concern regarding pacemakers in the OR is the potential effect of electromagnetic interference (EMI) on pacemaker operation. Cautery used during surgery is a type of EMI that can cause abnormal behavior of the pacemaker. Although modern pacemakers are heavily shielded and protected from many sources of EMI, it is still possible for extraneous signals to enter the pacemaker when detected by the pacing leads. Bipolar pacing systems are less likely to be affected than unipolar systems because the sensing circuit in a bipolar system is much smaller than that in a unipolar system. Possible responses to EMI include: (1) inhibition of pacemaker output; (2) triggering of pacemaker output at rapid rates; (3) asynchronous pacing; (4) mode resetting; (5) damage to the circuitry in the pacemaker; or (6) delivery of inappropriate shocks if the device is an ICD.25 The most common responses to EMI in the OR are inhibition of pacing or reversion to a “noise mode,” usually VOO or DOO pacing (asynchronous pacing). Inhibition of pacing occurs when the pacemaker senses the cautery and interprets those signals as intrinsic ventricular activity. This feature can result in asystole if the patient is pacemaker-dependent with no reliable underlying rhythm and is the most worrisome concern when dealing with pacemakers in the OR. Many pacemakers revert to asynchronous pacing when they sense electrical “noise” from cautery or other sources of EMI. This feature allows pacing to occur in an asynchronous mode, creating the potential problem of pacemaker output occurring during the vulnerable period of ventricular repolarization and resulting in VF.

There are several interventions that can reduce the potential adverse effects of EMI during surgery. If cautery is to be used, then place the grounding pad as far away from the pulse generator as possible (e.g. on a leg rather than on the chest or back), and place it on the opposite side of the body from the pacemaker. Use cautery in short bursts rather than long, continuous applications. Observe the monitor for pacemaker response to cautery, and if cautery appears to cause inhibition of pacing place a magnet over the pacemaker while cautery is being applied. A defibrillator and other emergency equipment should be immediately available during surgery. It is advisable to interrogate a pacemaker both before and after surgical procedures involving cautery or other sources of EMI (i.e. high-intensity radiation, radiofrequency ablation, lithotripsy) to verify programmed parameters before surgery and make sure they have not changed after surgery. If the pacemaker is programmed to a rate-modulated mode (VVIR, DDDR), then it is wise to disable rate modulation by programming to VVI or DDD before surgery, because mechanical ventilators, bone hammers, surgical saws, and other equipment in the surgical environment may trigger the physiological sensors and result in rapid pacing.


Nonmedical Sources of EMI.

Pacemakers can be adversely affected by EMI in the environment, and patients should be taught about potential pacemaker interactions with common sources of EMI. Security systems or antitheft devices in department stores can potentially interact with pacemakers and cause intermittent inhibition of pacing output, inappropriate atrial tracking, or asynchronous pacing.25 Patients should be cautioned not to linger close to a security system but to pass through it and then move away. Metal detectors used in security systems in airports and other places can potentially interact with pacemakers, but this interaction is rare. It is generally safe for people with pacemakers or ICDs to walk through a metal detector gate even though the alarm may be triggered by the device. People with implanted devices can request a manual search rather than a hand-held metal detector search.

Cell phones, personal digital assistants, laptop computers, and other wireless devices are a potential source of EMI that can inhibit pacemaker output, cause asynchronous pacing, or cause inappropriate ventricular tracking in a dual-chamber device.25 Keeping a cell phone at least 6 inches away from the pacemaker pulse generator prevents interactions. Patients should avoid carrying their cell phone in a pocket near the pulse generator and should use the ear opposite the pacemaker when talking on a cell phone. It is reasonable to consider the hand used to hold a cell phone when selecting the site for pacemaker implantation in individual patients.

Household appliances are safe for use by patients with pacemakers, as are other commonly used electrical or motor-driven appliances like lawn mowers, leaf blowers, and small tools (drills, saws, etc.). Almost all interactions with household appliances (especially washing machines) occur with improper grounding of the appliance.25


Cardioversion and Defibrillation.

Patients with pacemakers can be safely cardioverted or defibrillated if precautions are taken to protect the pacemaker from high-energy electrical forces. Paddles or defibrillation pads should not be placed directly over the pulse generator. Placing the paddles or pads in the anterior-posterior position is preferred over the standard transthoracic placement (refer to Fig. 28-4, which illustrates anterior-posterior pad placement for external pacing). Use of lower energy shocks is preferable over higher energy shocks whenever possible. The pacemaker should be interrogated after cardioversion or defibrillation to make sure it is still programmed and functioning as intended.


Temporary Pacemakers

In the care of patients with temporary pacemakers, the following additional considerations become important.


Insertion Site Care.

A temporary pacing catheter is usually inserted through a venous sheath that is sutured to the skin and treated as any central venous catheter. Maintaining a clean, dry insertion site is important to prevent infection, and hospital policies governing the care of central venous catheters and dressings should be followed. If the pacing catheter is placed via a femoral vein, then the patient needs to be on bed rest with the affected leg straight and head of bed elevated no more than 20 degrees while the femoral sheath is in place.



Care of Epicardial Leads.

Epicardial leads exit through the chest and unless they are being used for pacing, they are usually coiled and placed in a gauze dressing until needed. The exit site should be kept clean and dry according to established hospital policies on exit site care. Epicardial leads are easily dislodged, so care must be taken when handling them so as not to pull them out. Use of a pacing cable is recommended to prevent the need to strap the temporary pacemaker directly to the patient’s body. The leads and pacing cable must be securely taped to the chest to prevent dislodgement of epicardial leads. Because the exposed metal end of the leads is a direct route for electrical current from the environment to conduct directly to the heart, care must be taken to insulate the leads to prevent cardiac arrhythmias, especially VF (see section “Electrical Safety,” for more information).


Electrical Safety.

A temporary pacing lead provides a direct pathway for stray electrical current to reach the heart without the protective resistance of the skin. Even a very small electrical current can initiate atrial fibrillation or VF if it is conducted directly to the heart by pacing leads.

Some considerations for electrical safety when caring for patients with temporary pacing leads include:



  • Wear gloves when handling pacing leads.


  • Make sure that all connections between the pulse generator and pacing cable and between pacing cable and pacing leads are tight and inserted completely into their receptacles so no metal is exposed.


  • If using a pacing cable with an alligator clip connector, then wrap a glove around the connections in such a way that they are separated and insulated from each other and from the environment.


  • Cover exposed metal ends of pacing leads that are not in use with some type of insulating material.



    • Wrap a glove around the ends of transvenous leads and tape loosely.


    • Place the ends of epicardial leads in a glove (or cut a finger from a glove and place them inside) or place the metal end of each individual lead in a needle cover, small syringe, or some other insulating material.


  • Keep dressings over pacing lead insertion sites dry; wet dressings conduct electricity more easily.


  • Make sure all electrical equipment in the room is grounded and in good working order.


  • Be aware of your own body’s static electricity, especially if your unit is carpeted.



    • Never let the pacing system be the first thing you touch when entering a patient’s room.


    • Be especially careful when using slider boards to transfer patients into and out of bed, because they generate static electricity.


Stimulation Threshold Testing.

The stimulation threshold is the minimum pacemaker output necessary to capture the heart consistently. The contact of the pacing lead with the myocardium causes local tissue edema and inflammation that impedes the delivery of current to the myocardium. Peak thresholds occur approximately 3 to 4 weeks after permanent lead placement, and chronic stable thresholds are usually reached at approximately 3 months. Stimulation threshold testing with a temporary pacing system should be performed every shift to ensure an adequate safety margin for capture. The procedure for performing a stimulation threshold test is as follows:



  • Verify that the patient is in a paced rhythm; pacing rate may need to be temporarily increased to override an intrinsic rhythm.


  • Watch the cardiac monitor continuously while gradually decreasing output.


  • Note when loss of capture occurs (pacing spike not followed by appropriate waveform: P wave for atrial pacing, QRS for ventricular pacing).


  • Gradually turn output up until 1:1 capture resumes—this is the stimulation threshold.


  • Set the output two- to three-times higher than threshold to ensure adequate safety margin; for example, if consistent capture is regained at 2 mA, then set the output at 4 to 6 mA.


Sensitivity Threshold Testing.

The sensitivity threshold is the minimum voltage of intrinsic cardiac activity that can be sensed by the pacemaker. The pacemaker becomes more sensitive (can sense smaller signals) as the number on the sensitivity control gets smaller (see section “Sensing,” for further explanation).

Sensitivity testing can be performed only if the patient has a hemodynamically stable underlying rhythm. If the patient is completely pacemaker-dependent or has a very slow underlying rate, then do not perform sensitivity threshold testing. The procedure for performing a sensitivity threshold test is as follows:



  • Verify that the patient has an intrinsic rhythm (is not being paced); this may require temporarily decreasing the pacing rate to allow the underlying rhythm to emerge.


  • Slowly decrease the pacemaker’s sensitivity (by increasing the number on the sensitivity control) while watching the sense indicator light on the pulse generator or watching the cardiac monitor.



    • The sense indicator light flashes with each sensed P wave (for atrial sensing) or QRS (for ventricular sensing).


    • Pacing remains inhibited and there are no pacing spikes seen on the monitor as long as sensing continues.


  • Note when the sense indicator fails to flash with each P wave or QRS and when pacing spikes begin to appear in competition with the intrinsic rhythm; this is the sensitivity threshold.


  • Set the sensitivity at one-half of the identified threshold to ensure an adequate safety margin; for example, if the threshold is 5 mV, then set the sensitivity at 2.5 mV.


Evaluating Pacemaker Function

This section is directed primarily at temporary pacemakers because nurses can interact more directly with them than with permanent pacemakers. The same concepts apply to permanent pacemakers, but corrective measures require the use of a pacemaker programmer or an actual surgical procedure to reposition pacing leads or replace the pulse generator.

Evaluation of pacemaker function requires knowledge of the mode of pacing expected (e.g. VVI, AAI, DDD); the minimum rate of the pacemaker, or pacing interval; and any other programmed parameters in the pacemaker. The basic functions of a pacemaker include stimulus release, capture, and sensing, and they should be evaluated for both temporary and permanent pacemakers. Stimulus release refers to pacemaker output, or the ability of the pacemaker to generate and release a pacing impulse. Capture is the ability of the pacing stimulus to cause depolarization of the chamber being paced. Sensing is the ability of the pacemaker to recognize and respond to intrinsic electrical activity in the
heart. Pacemaker operation is evaluated by assessing these three functions. Single-chamber pacemaker evaluation is much less complicated than dual-chamber evaluation. Because ventricular pacing is the most common type of single-chamber pacing, evaluation of VVI pacemakers is discussed here. The concepts presented for ventricular pacemaker evaluation can also be applied to atrial pacemaker evaluation.






Figure 28-13 Normal VVI pacemaker function. (A) Capture is good, but sensing cannot be evaluated because no intrinsic QRS complexes are present. (B) Capture and sensing both normal. Beats numbers 1 and 2 are intrinsic QRS complexes that are sensed, inhibit ventricular pacing output, and reset the pacing interval. Beat number 3 is a fusion beat between the intrinsic QRS and the paced beat.

A VVI pacemaker is expected to pace the ventricle at the set rate unless spontaneous ventricular activity occurs to inhibit pacing. The minimum rate of the pacemaker, or pacing interval, is measured from one pacing stimulus to the next consecutive pacing stimulus with no intervening sensed beats between the two. In a normally functioning VVI pacemaker, pacing spikes occur at the preset pacing interval and each spike results in a ventricular depolarization (capture). If spontaneous ventricular activity occurs (either a normally conducted QRS or a PVC), that activity is sensed, the next pacing stimulus is inhibited, and the pacing interval timing cycle is reset. If no intrinsic ventricular activity occurs, a pacing stimulus is released at the end of the timing cycle. Figure 28-13 shows normal VVI pacemaker function.

The pacemaker has a refractory period, which is a period of time after either pacing or sensing in the ventricle during which the pacemaker is unable to respond to intrinsic activity. During the refractory period, the pacemaker in effect has its “eyes closed” and is not able to sense spontaneous activity. If an intrinsic QRS should occur during the pacemaker’s refractory period, it is not sensed because the pacemaker is “blind” at that time.


Stimulus Release

Stimulus release is verified on the ECG by the presence of a pacing spike. A pacing spike indicates that the pacemaker battery has enough power to initiate a stimulus and that the stimulus was delivered into the body. When evaluating a temporary pacing system, the presence of a pacing spike indicates that the connections between the pulse generator and the pacing cable and between the pacing cable and the pacing leads are intact. If any part of the system becomes disconnected, the stimulus cannot reach the body and a pacing spike is not seen. The presence of a pacing spike alone does not indicate where the stimulus was delivered, only that it entered the body somewhere.






Figure 28-14 Absence of stimulus release in a patient with a permanent pacemaker. Underlying rhythm is atrial fibrillation with complete atrioventricular block and a very slow ventricular rate. The battery in the pacemaker generator was at end of service.

Absence of pacing stimuli when they should be present can indicate a faulty pulse generator or battery, or a break or disconnection in the lead system. Pacing stimuli can also be absent when pacing is inhibited by the sensing of extraneous electrical signals, such as EMI or myopotentials. Figure 28-14 illustrates total loss of stimulus release in a patient whose permanent pacemaker battery was totally depleted.


Capture

Capture is indicated by a wide QRS complex immediately after the pacemaker spike and represents the ability of the pacing stimulus to depolarize the ventricle. Loss of capture is recognized by the presence of pacing spikes that are not followed by paced ventricular complexes (Fig. 28-15). Causes of loss of capture include the following:



  • Inadequate stimulus strength, which can be corrected by increasing the electrical output of the pacemaker (turning up the milliamperage).


  • Pacing lead out of position and not in contact with myocardium, which can sometimes be corrected by repositioning the patient; repositioning the pacing lead is usually not a nursing function and must be performed by a physician or someone trained in intracardiac catheter manipulation.


  • Pacing lead positioned in infarcted tissue, which can be corrected by repositioning the lead to a place where myocardium is healthy and capable of responding to the stimulus.



  • Electrolyte imbalances or drugs that alter the ability of the heart to respond to the pacing stimulus.


  • Delivery of a pacing stimulus during the ventricle’s refractory period when the heart is physiologically unable to respond to the stimulus; this problem occurs with loss of sensing (undersensing) and can be prevented by correcting the sensing problem (Fig. 28-16A).






Figure 28-15 (A) VVI pacemaker with intermittent loss of capture. (B) VVI pacemaker with total loss of capture. The underlying rhythm is atrial flutter with a slow ventricular response.

Loss of capture in a totally pacemaker-dependent patient is an emergency because without an effective underlying rhythm, the patient may be asystolic or severely symptomatic because of slow, ineffective rate. If the underlying rhythm is ineffective or absent, cardiopulmonary resuscitation must be performed until the capture problem is corrected or until the emergency transcutaneous pacing can be instituted. If loss of capture is intermittent, it may not result in symptoms but should be corrected as soon as possible.






Figure 28-16 (A) Loss of sensing in a VVI pacemaker. Delivery of the pacing stimulus during the heart’s refractory period makes it appear that capture is lost as well. Because the heart is physiologically unable to respond to the pacing stimulus when it falls in the refractory period, this is not a problem. The beats marked with stars are beats that were not sensed by the pacemaker. Pacing spikes 1, 2, 5, and 6 should not have occurred; their presence is due to loss of sensing. Pacing spike 4 occurred coincident with the normal QRS complex, resulting in a “pseudofusion” beat, and does not represent loss of sensing. (B) Loss of capture in a VVI pacemaker. Only one pacing spike captures the ventricle. Two QRS complexes marked with stars occur during the pacemaker’s refractory period and thus are not sensed. This does not represent loss of sensing because the pacemaker has its “eye closed” during the time intrinsic ventricular activity occurred.


Sensing

Sensing of intrinsic ventricular electrical activity inhibits the next pacing stimulus and resets the pacing interval. Sensing cannot occur unless the pacemaker is given the opportunity to sense. It
must be in the demand mode and there must be intrinsic ventricular activity for the pacemaker to have an opportunity to sense. In Figure 28-13A, sensing cannot be evaluated because there is no intrinsic ventricular activity; therefore, the pacemaker is not given an opportunity to sense. In Figure 28-13B, the occurrence of two spontaneous QRS complexes provides the opportunity to sense. In this example, sensing occurred normally, as indicated by the absence of the next two expected pacing stimuli and resetting of the pacing interval from the intrinsic QRS complex.






Figure 28-17 (A) Undersensing in a VVI pacemaker. The third QRS is an intrinsic beat that is not sensed and pacing occurs at the programmed pacing interval, resulting is a spike close to the T wave of the intrinsic beat. (B) Oversensing in a VVI pacemaker. Pacing intervals 1, 2, and 3 represent the basic pacing rate. The pacing rate slows in the middle of the strip; pacing should have occurred at the end of pacing intervals 2 and 3 but was delayed because the device sensed something that reset the pacing interval.


Undersensing.

Undersensing means that the pacemaker fails to sense intrinsic activity that is present (Figs. 28-16A and 28-17A). This can be caused by:



  • Asynchronous (fixed-rate) pacing mode in which the sensing circuit is off; this problem can be corrected by turning the sensitivity control to the demand mode.


  • Pacing catheter out of position or lying in infarcted tissue, which can be corrected by repositioning the lead; lead repositioning must be performed by a physician; however, turning the patient to the side sometimes temporarily works when the pacing lead loses contact with the ventricle.


  • Intrinsic QRS voltage may be too low to be sensed by the pacemaker; increasing the pacemaker’s sensitivity (by decreasing the number on the sensitivity control) allows it to see smaller intrinsic signals and may solve the problem.


  • Break in connections, battery failure, or faulty pulse generator; check and tighten all connections along the pacing system, and replace the battery if it is low; a chest radiograph may detect lead fracture; change the pulse generator if problems cannot be corrected any other way.


  • Intrinsic ventricular activity falling in the pacemaker’s refractory period; if a spontaneous QRS complex occurs during the time the pacemaker has its “eyes closed,” then the pacemaker cannot see it; this may occur when the pacemaker fails to capture, which can allow an intrinsic QRS to occur during the pacemaker’s refractory period; this problem is caused by loss of capture and does not reflect a sensing malfunction (see Fig. 28-16B).


Oversensing.

Oversensing means that the pacemaker is so sensitive that it inappropriately senses internal or external signals as QRS complexes and inhibits its output. Common sources of external signals that can interfere with pacemaker function include electromagnetic or radiofrequency signals or electronic equipment in use near the pacemaker. Internal sources of interference can include large P waves, large T-wave voltage, local myopotentials in the heart, or skeletal muscle potentials. Figure 28-17B illustrates oversensing in a temporary pacemaker. Because a VVI pacemaker is programmed to inhibit its output when it senses, oversensing can be a dangerous situation in a pacemaker-dependent patient, resulting in a dangerously slow rate or ventricular asystole. Oversensing is usually caused by the sensitivity being set too high, which can be corrected by reducing the pacemaker’s sensitivity by increasing the number on the sensitivity control. For example, if sensitivity is set at 0.5 mV, changing it to 2 mV decreases the sensitivity of the pacemaker. For ventricular pacing, a sensitivity of 2 mV is usually safe and can always be changed if needed to correct sensing problems.


Dual-Chamber Pacemaker Operation

Dual-chamber pacemakers have become very complicated, with multiple programmable parameters and varying functions, depending on the manufacturer. Because it is impossible to present a detailed explanation of all aspects of dual-chamber pacing in a single chapter, this section concentrates on basic dual-chamber pacing concepts that apply to all manufacturers’ products. More detailed information is best obtained by attending a formal pacing program sponsored by a pacemaker manufacturer or from a pacemaker technical manual. Dual-chamber pacemakers can function in a variety of modes, depending on how they are programmed (Table 28-2). Because the DDD mode is most commonly used, basic DDD function is described here. Display 28-3 defines terms commonly used in dual-chamber pacing.


Dual-Chamber Timing Cycles

According to the pacemaker code, DDD means that both chambers (atria and ventricles) are paced, both chambers are sensed, and the mode of response to sensed events is either inhibited or
triggered, depending on which chamber is sensed. When atrial activity is sensed, atrial pacing is inhibited and ventricular pacing is triggered at the end of the programmed AV delay. When ventricular activity is sensed, all pacemaker output is inhibited. The following timing cycles determine how a dual-chamber pacemaker functions, and Figure 28-18 illustrates many of these timing cycles24,26:








Table 28-2 ▪ DUAL-CHAMBER PACING MODES











































Mode


Chamber(s) Paced


Chamber(s) Sensed


Response to Sensing


DVI


Atrium and ventricle


Ventricle


Ventricular sensing inhibits atrial and ventricular pacing


VDD


Ventricle


Atrium and ventricle


Atrial sensing—triggers ventricular pacing





Ventricular sensing—inhibits ventricular pacing


DDI


Atrium and ventricle


Atrium and ventricle


Atrial sensing inhibits atrial pacing





Ventricular sensing inhibits ventricular pacing


DDD


Atrium and ventricle


Atrium and ventricle


Atrial sensing—inhibits atrial pacing, triggers ventricular pacing





Ventricular sensing—inhibits atrial and ventricular pacing




  • Pacing interval (or lower rate limit)—the base rate of the pacemaker, measured between two consecutive atrial pacing stimuli with no intervening sensed events; the pacing interval is a programmable parameter and determines the minimum rate at which the pacemaker paces in the absence of intrinsic cardiac activity.


  • AV delay (or AV interval)—the amount of time between atrial and ventricular pacing, or the “electronic PR interval”; this is measured from the atrial pacing spike to the ventricular pacing spike and is a programmable parameter; the AV delay timer is initiated by a paced or sensed atrial event, and if no intrinsic conduction occurs to the ventricle within that time, a ventricular pacing spike occurs at the end of the programmed AV delay.


  • Atrial escape interval (or ventriculoatrial [VA] interval)—the interval from a sensed or paced ventricular event to the next atrial pacing output; the VA interval represents the amount of time the pacemaker waits after it paces in the ventricle or senses ventricular activity before pacing the atrium; the atrial escape interval is not a programmed parameter, but rather is derived by subtracting the AV delay from the pacing interval; its length can be estimated by measuring from a ventricular spike to the next atrial pacing spike.


  • Total atrial refractory period—the period of time after a sensed P wave or a paced atrial event during which the atrial channel does not respond to sensed events; the total atrial refractory period consists of the AV delay and the postventricular atrial refractory period (PVARP).


  • PVARP—the period of time after an intrinsic QRS or a paced ventricular beat during which the atrial channel is refractory and does not respond to sensed atrial activity; PVARP is a programmable parameter but is not evident on a rhythm strip.


  • Blanking period—the very short ventricular refractory period that occurs with every atrial pacemaker output; the ventricular channel “blinks its eyes” so it will not sense the atrial output and inappropriately inhibit ventricular pacing; the blanking period is a programmable parameter but is not evident on a rhythm strip.

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Jan 10, 2021 | Posted by in NURSING | Comments Off on Pacemakers and Implantable Defibrillators*
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