27 Assessment and monitoring of the perianesthesia patient
Hemodynamic Monitoring (Invasive Monitoring): The monitoring of blood flow through the use of invasive catheters to provide pressure measurements in the systemic and pulmonary circulations, central veins, pulmonary capillary bed, and the right or left atrium, as well as cardiac output.
Left Atrial Pressure: Measured with a catheter placed directly in the left atrium. Usually monitored only in open heart cases when direct access to the left atrium can be reached. In the absence of mitral valve disease or left atrial tumor, left atrial pressure reflects left ventricular end-diastolic pressure and left ventricle preload.
The primary purpose of the postanesthesia care unit (PACU) is the critical evaluation and stabilization of patients after procedures, with an emphasis on the anticipation and prevention of complications that result from anesthesia or the operative or interventional procedure. A knowledgeable, skillful perianesthesia nurse must fully assess the condition of each patient not only at admission and at discharge but also at frequent intervals throughout the postanesthesia period. Assessment must be a continuous and complete process that leads to sound nursing judgment and the implementation of therapeutic care. Assessment includes the gathering of information from direct observation of the patient (the primary source), from the physician, other health care personnel, and from the medical record and the care plan. Traditionally perianesthesia nurses have, with only limited information, performed the role of caring for the surgical and interventional patients in the vulnerable postanesthesia state. However, for assessment of the perianesthesia patient and plan and implementation of appropriate care, preoperative information must be available as a basis for comparison with postoperative data. The perianesthesia nurse has a professional obligation to consider the patient’s history, clinical status, and psychosocial state. The necessary data may be gathered with chart review, personal preoperative visit, and consultation with other health care members who provide care to the patient. The collection of such information should be a coordinated effort with all involved members of the health care team. This chapter discusses the assessment of postprocedure patients and their common needs. Specific assessments related to patient age, the type of procedure, and problems that result from complicated diagnoses are addressed in following chapters. The assessment and management of postoperative pain is presented in Chapter 31.
The preoperative evaluation of both the physical and the emotional status of the surgical patient is extremely important, and nursing brings a unique perspective to this assessment. The scope of perianesthesia nursing practice involves the age-specific assessment, diagnosis, intervention, and evaluation of patients within the perianesthesia continuum. The scope identifies risks for problems that can result from the administration of sedation/analgesia or anesthetic agents for surgical, diagnostic, or therapeutic procedures.2 Nurses in a number of subspecialties, including perianesthesia nurses, perioperative nurses, and general unit nurses, have advocated this assessment. A preoperative visit from each nurse who will care for the patient seems redundant and can be overwhelming for the patient. More appropriately, nurses should treat each other as colleagues who communicate needs for specific information, coordinate the collection of such information, and document data to be used for planning care. Multidisciplinary communications are instrumental in the education of all those who care for the patient and in the development of communication patterns.
Because many PACUs include preoperative holding areas, the perianesthesia nurse must participate in the patient’s preoperative interview and assessment. A complete preoperative nursing assessment should include relevant preoperative physical and psychosocial condition, spiritual and cultural status, medical history (including anesthesia history), length of fasting, understanding of the procedure and postprocedure course, and the need for follow-up services. The preoperative physical assessment should include documentation of temperature, pulse, blood pressure, respirations, oxygen saturation, height, and weight and a review of systems. Nursing diagnoses are established on the analysis of data collected during the assessment phase and generation of an appropriate plan of care.
Physical assessment of the perianesthesia patient must begin immediately on admission to the PACU. The patient is accompanied from the procedure room to the PACU by the anesthesia provider or monitoring nurse, who reports to the receiving nurse on the patient’s general condition, the procedure performed, and the type of anesthesia or sedation used. In addition, the nurse should be informed of any problems or complications encountered during the procedure and anesthesia or sedation (see Chapter 26). Because all anesthetics are depressants, postoperative assessment and care generally are the same, regardless of the specific agent used. For special precautions required for certain agents, review the chapters on anesthesia (see Chapters 19 through 25).
Rapid assessment of the life-sustaining cardiorespiratory system is of initial concern. The nurse ensures that the airway is patent and that respirations are free and easy; check and record the patient’s blood pressure, pulse, rate of respiration, temperature, and oxygen saturation level; and quickly inspect all dressings and drains for gross bleeding. These baseline observations, which are made immediately on admission, should be reported to the anesthesia or sedation provider in attendance and recorded in the admission note.
After these initial observations are made, systematic assessment of the patient’s total condition is essential. This assessment can be made from head to toe or by a systems review, whichever the individual nurse prefers. These observations are essentially identical, and each system of the body has an integral function, making all observations interrelated.
Because the postanesthesia patient has had some interference with the respiratory system, maintenance of adequate gas exchange is a crucial aspect of care in the PACU. Any change in respiratory function must be detected early so that appropriate measures can be taken to ensure adequate oxygenation and ventilation. The most significant respiratory problems encountered in the immediate postoperative period include hypoventilation, airway obstruction, aspiration, and atelectasis.
Respiratory assessment is coupled with the related responses of the cardiovascular and neurologic systems for total evaluation of the adequacy of gas exchange and ventilatory efficiency. Respiratory function is evaluated with clinical assessment. Pulse oximetry is used for assessment of arterial oxygenation, and capnography is used in evaluation of the adequacy of alveolar ventilation. Arterial blood gas measurements may be a part of the respiratory assessment (see Chapters 12, 29, and 30).
The resting respiratory rate of a normal adult is approximately 12 to 20 beats/min. Infants and children have a higher respiratory rate and a lower tidal volume than adults (see Chapter 49). Respirations should be quiet and easy and have a regular rate and rhythm. The chest should move freely as a unit, and expansion should be equal bilaterally. Alterations in symmetry can be caused by many factors, including pain, that may cause splinting at the incision site, consolidation, and pneumothorax. The nurse should note the character of the respirations; intercostal retractions, bulging, nasal flaring, or use of the accessory respiratory muscles, which are signs of respiratory distress. The depth of respiration is as important as the rate. Shallow respirations are the cardinal sign of continuing depression from anesthesia or preoperative medications, but can be caused by many other factors, including incisional pain, obesity, tight binders, and dressings that restrict movements of the thoracic cage or abdomen. Shallow respirations and use of the neck and diaphragmatic muscles may also indicate reparalyzation from the use of skeletal muscle relaxants such as succinylcholine, atracurium, pancuronium, and vecuronium. The presence of chest movements alone does not provide evidence that adequate gas exchange is occurring.
Airway obstruction may be present when the normal duration of inspiration versus exhalation is altered. Restlessness, confusion or anxiety, and apprehension are the earliest signs of hypoxemia and CO2 retention and should receive immediate attention for determination of cause. The patient’s color is regularly evaluated. Although this assessment is difficult to make, the results provide important information about the respiratory function. Cyanosis is a late sign of severe tissue hypoxia, and if it appears, immediate and vigorous efforts must be instituted to determine and correct the cause of hypoxia. The noninvasive monitors that are increasingly used in the PACU provide an effective means of continuous and objective assessment of gas exchange; pulse oximeters are used for monitoring hemoglobin oxygen saturation, and capnographs are used in evaluation of the adequacy of alveolar ventilation. A discussion of these monitors is forthcoming.
The presence of an artificial airway is noted; airways are used primarily to maintain a patent air passage so that respiratory exchange is not hampered. Five types of airways commonly used are: (1) the balloon-cuffed endotracheal tube (extends from the mouth through the glottis to a point above the bifurcation of the trachea); (2) the balloon-cuffed nasotracheal tube (extends from the nose to the trachea); (3) the laryngeal mask airway (inflatable silicone mask and rubber connecting tubing blindly inserted into the pharynx forming a low pressure seal around the laryngeal inlet); (4) the oropharyngeal airway (extends from the mouth to the pharynx and prevents the tongue from falling back and obstructing the trachea); and (5) the nasopharyngeal airway (extends from the nose to the pharynx). The airway must be kept clear of secretions for adequate gas exchange to occur, and suctioning may be needed if gurgling develops. The airway should not be removed until the laryngeal and pharyngeal reflexes return; these reflexes enable the patient to control the tongue, to cough, and to swallow. If the patient “reacts on the airway” (attempts to eject it), and gags, this can progress to retching and vomiting. The airway should be removed as soon as clinically possible in this instance to avoid aspiration.
An endotracheal tube can be removed as soon as the patient’s condition is adequately reversed, the patient can maintain an airway without the tube, and the danger of aspiration is over. Determination of this point may be difficult; the decision of when a patient needs an airway is usually much easier than the decision of when such an adjunct is not needed. If PACU policy permits removal of an airway, insertion of an airway should definitely be included and both procedures should be accompanied by appropriate education and skill training for the nurses who perform them.
First, the perianesthesia nurse should listen unaided to the patient’s respirations. Normal respiration should be quiet; noisy breathing indicates a problem. Extraneous sounds always indicate some kind of obstruction; however, quiet breathing does not always indicate the absence of problems. An accumulation of mucus or other secretions, evidenced by gurgling in any of the respiratory passages, can cause airway obstruction and should be removed immediately. Purposeful coughing with good expiratory airflow is the most effective method of clearing secretions. If the patient is not yet reactive enough to do this alone, the secretions must be suctioned orally and nasally. Nasotracheal suctioning may be useful to clear secretions and to stimulate cough, but the catheter is ineffective for reaching secretions distal to the carina. Obstruction can also occur from poor oropharyngeal muscle tone caused by the muscle-relaxant effect of general anesthesia plus the rolling back of the tongue. Patients with obstructive sleep apnea are prone to airway obstruction and should not undergo extubation until they are fully awake. Tracheal extubation should be performed only when the patient is breathing spontaneously with adequate tidal volumes, oxygenation, and ventilation.1,3 To relieve airway obstruction, use the jaw thrust maneuver by providing anterior pressure support on the angle of the jaw to open the air passages.
Crowing, a sudden violent contraction of the vocal cords, may indicate laryngospasm and can result in complete or partial closure of the trachea. Other signs and symptoms of laryngospasm include wheezing, stridor, reduced compliance, cyanosis, and respiratory obstruction. If spasms continue and are not broken with jaw thrust and positive pressure, succinylcholine may be administered with subsequent endotracheal tube insertion for maintenance of a patent airway. Total blockage of the airway caused by laryngospasm produces no sound because of the absence of moving air. Equipment and medications for management of a difficult airway should be readily available in the PACU.
Wheezing may indicate bronchospasm caused by a reflex reaction to an irritating mechanism. Bronchospasm occurs most often in patients with preexisting pulmonary disease, such as severe emphysema, reactive airway disease, pulmonary fibrosis, and radiation pneumonitis. Laryngeal edema after endotracheal intubation is not uncommon and can contribute significantly to airway obstruction. Acute changes in the patient’s skin condition, cardiovascular status, and bronchospasm after regional anesthesia must alert the nurse to a possible rare allergic reaction.
The perianesthesia nurse should auscultate the patient’s chest with a stethoscope for quality and intensity of breath sounds. Any abnormality should be located and identified and then described in the patient’s medical record. Total absence of breath sounds on one side may signal the presence of pneumothorax (collapsed lung), obstruction, or fluid or blood within the pleural space. Auscultation of breath sounds in the PACU is often difficult because the patient usually cannot sit up or respond to commands to breathe deeply with the mouth open. Positioning the patient on alternating sides during the stir-up regimen provides an opportunity for examination of the posterior lung field.
Palpation and inspection of the chest can be performed simultaneously for validation of observations such as symmetry of expansion. Crepitus and fremitus may be heard and felt. The temperature, the level of moisture and general turgor of the skin, and the presence of any edema should be noted.
A pulse oximeter is used for noninvasive measurement of arterial oxygen saturation (SaO2) in the blood (SpO2 when measured with pulse oximetry) and is a valuable adjunct to the clinical assessment of oxygenation. Many clinical indicators, such as the patient’s color and the characteristics of the respirations, are subjective, and the physical signs of cyanosis are not evident until hypoxia is severe. Pulse oximetry monitoring is objective and continuous and provides an early warning of developing hypoxemia, thus allowing intervention before signs of hypoxia appear. Consequently, pulse oximetry has been widely adopted in the PACU as a tool for both safety monitoring and patient management. As a confirmation of the importance of pulse oximetry, the American Society of PeriAnesthesia Nurses (ASPAN) PeriAnesthesia Nursing Standards and Practice Recommendations 2010-2012 recommends evaluation of all PACU patients with pulse oximetry at admission and discharge, and ASPAN recommends a pulse oximeter for every patient in all phases of perianesthesia nursing levels of care.2
A pulse oximeter consists of a microprocessor-based monitor and a sensor (Fig. 27-1). In addition to a SpO2 display, most oximeters display the pulse rate and have an adjustable alarm system that sounds when values register outside a designated range. A variety of sensors is available, each intended for application to specific sites and for use on patients of various sizes (the manufacturer’s instructions describe these requirements). The sensor is applied to a site with a good arterial supply. The most common application site is a finger or toe (hand or foot in neonates); other sites include the nose, the forehead, the earlobe, or the temple. Both reusable sensors and disposable adhesive sensors are available, and disposable sensors allow for patient-dedicated monitoring when infection control concerns are present.
A pulse oximeter uses plethysmography for detection of the arterial pulse and spectrophotometry in determination of SpO2. The pulse oximetry sensor incorporates a red and an infrared light-emitting diode as light sources and a photodiode as a light detector. In the most common type of sensor, a transmission sensor, the light sources and detector are positioned on opposite sides of an arterial bed, such as around the finger. In a reflectance oximetry sensor, they are positioned on the same surface, such as on the forehead.
With both transmission and reflectance sensors, red and infrared light passes into the tissue, and the detector measures the amount of light absorbed. Because oxyhemoglobin and deoxygenated hemoglobin differ in the absorption of red and infrared light, the detector can determine the percentage of oxyhemoglobin in the arterial pulse.
Pulse oximetry is used in many clinical settings for safety monitoring and as a patient management tool. As a safety monitor, a pulse oximeter detects hypoxemia caused by unanticipated events such as severe atelectasis, bronchospasm, airway displacement, disconnections or kinks in the breathing circuit, and cardiac arrest. As a patient management tool, pulse oximetry is valuable in titrating oxygen therapy, weaning a patient from mechanical ventilation, and evaluating response to medications or other interventions that are intended for improvement of oxygenation.
In addition to these broad applications, certain uses of pulse oximetry are of particular value in the PACU. For example, patients after surgery can become significantly hypoxemic during transport to or from the PACU. Pulse oximetry during transport can be used to diagnose undetected hypoxemia and to identify a need for supplemental oxygen. As indicated by the ASPAN standards, pulse oximetry is a valuable adjunct to clinical assessments in determination of readiness for PACU discharge. Some patients in the PACU judged to be stable and ready for transfer on the basis of clinical evaluation alone have been found to be hypoxemic after evaluation with pulse oximetry.
Consideration of the mechanisms of oxygen transport is essential for adequate interpretation of SpO2. Approximately 98% of the oxygen in blood is bound to hemoglobin; SaO2 and SpO2 reflect this blood oxygen. The remaining blood oxygen is dissolved in plasma; blood gas analysis measures the partial pressure exerted by this oxygen dissolved in plasma (PaO2 = 80 – 100 mm Hg at sea level). The dissolved oxygen is used to meet immediate metabolic needs. The oxygen bound to hemoglobin serves as the reservoir that replenishes the pool of dissolved oxygen (see Chapter 12).
The rate at which oxygen binds to hemoglobin is primarily controlled by two factors: the PaO2 and the affinity of hemoglobin for oxygen. This relationship between SaO2 and PaO2 is represented by the oxyhemoglobin dissociation curve. The curve is sigmoid in shape, and its position is affected by a number of physiologic variables that change the affinity of hemoglobin for oxygen (Fig. 27-2).
FIG. 27-2 Normal oxyhemoglobin dissociation curve is indicated with the solid line. This curve may shift (indicated with broken lines) whenever pH, temperature, PCO2, or 2, 3-DPG values are increased or decreased. SO2, Oxygen saturation; PO2, partial pressure of oxygen; PCO2, partial pressure of carbon dioxide; 2, 3-DPG, 2, 3-diphosphoglycerate.
Many factors that shift the oxyhemoglobin dissociation curve are commonly seen in patients in the PACU. For example, a patient with hypothermia may have a left-shifted curve. In such a patient, a given SpO2 as measured with pulse oximetry may correspond to a lower than normal PaO2. Although oxygen saturation may be adequate, hemoglobin has a greater affinity for oxygen and is less willing to release oxygen to meet tissue needs. Warming the patient to a normothermic range facilitates oxygen unloading from the hemoglobin molecule and helps to maintain adequate tissue oxygenation.
As with any technology, important clinical issues must be considered for appropriate use of pulse oximetry. Shifts in the oxyhemoglobin dissociation curve that are caused by abnormal values of pH, temperature, partial pressure of carbon dioxide (PCO2), and 2,3-diphosphoglycerate must be considered. Consideration of the patient’s hemoglobin level is also important because a pulse oximeter cannot detect depletion in the total amount of hemoglobin. When pulse oximetry is used on a postoperative patient with a low hemoglobin level, a high SpO2 value might not reflect adequate oxygenation. The amount of hemoglobin, although it is well saturated with oxygen, may be inadequate to meet tissue needs because fewer carriers are available to transport oxygen.
Adequate oxygenation is a factor of not only adequate oxygen saturation and hemoglobin values but also of adequate oxygen delivery, which necessitates appropriate cardiac output, and the ability of the tissues to effectively use oxygen. Tissue hypoxia results when oxygen demand exceeds oxygen supply. Pulse oximetry readings therefore should be assessed in conjunction with all other indices of oxygenation.
Dysfunctional hemoglobin, variants of the hemoglobin molecule that is unable to transport oxygen, present a similar problem. Despite the high SpO2 level, hemoglobin may be insufficient to carry oxygen. Carboxyhemoglobin is hemoglobin that is bound with carbon monoxide and therefore is unavailable for carrying oxygen. Its effect must be considered in patients with burns or in tobacco smokers with carbon monoxide poisoning. In methemoglobinemia, the iron molecule on the hemoglobin is oxidized from the ferrous to the ferric state. This form of iron is unable to transport oxygen. Methemoglobinemia, although rare, can occur in patients who receive nitrate-based and other drugs and in those who are exposed to a variety of toxins. When dysfunctional hemoglobins are suspected, assessment of oxygenation with pulse oximetry must be supplemented with arterial blood gas saturations measured with a laboratory cooximeter to determine whether dyshemoglobins are present and oxygenation is adequate.
Perfusion at the sensor application site must be sufficient for the pulse oximeter to detect pulsatile flow, which is an important consideration for some patients in the PACU, such as those treated with vasoconstrictors, those with marked hypothermia, and those with significantly reduced cardiac output. A well-perfused site should be selected for application of the sensor. If in doubt, the pulse and adjacent capillary refill can be checked.
If the monitor cannot track the pulse, the patient first is evaluated for adverse physiologic changes. Next, the perianesthesia nurse should ensure that blood flow is not restricted, such as by a flexed extremity, a blood pressure cuff, an arterial line, any restraints, or a sensor that is applied too tightly. Local perfusion to the sensor site can be improved by covering the site with a warm towel or with the use of a forced air warming device. Certain sensors, such as nasal sensors, are designed for application to areas where perfusion is preserved even when peripheral perfusion is relatively poor. Finally, some pulse oximeters use an electrocardiographic signal as an aid in identification of the pulse, thus enhancing the instrument’s ability to detect a weak pulse.
Patient movement can produce false signals that interfere with the ability of the pulse oximeters to identify the true pulse, thus leading to unreliable SpO2 and pulse rate readings. The sensor should be properly and securely applied; a sensor that is loosely attached or incorrectly positioned can magnify the effect of motion. If the problem persists, consideration should be given to moving the sensor to a less active site. Pulse oximeters that use the electrocardiographic signal as an aid in identification of the pulse can have an enhanced ability to distinguish between the true pulse and artifacts produced by motion. The result is more reliable SpO2 readings.
Normally, venous blood is nonpulsatile and is not detected with a pulse oximeter. In the presence of venous pulsations, the SpO2 value provided by the pulse oximeter may be a composite of both arterial and venous saturations. Venous pulsations can occur in patients with severe right-sided heart failure or other pathophysiologic states that create venous congestion and in patients receiving high levels of positive end-expiratory pressure. They can also occur when the sensor is placed distal to a blood pressure cuff or occlusive dressing and when additional tape is wrapped tightly around the sensor. When venous pulsations are present, the perianesthesia nurse should take care in interpreting the SpO2 readings and, if possible, attempt to eliminate the cause.
Because pulse oximeters are optical measuring devices, the perianesthesia nurse must be aware of additional factors that can influence the reliability of SpO2 readings. To ensure good light reception, the sensor’s light sources and detector must always be positioned according to the manufacturer’s specifications. In the presence of bright lights, such as infrared warming devices, fluorescent lights, direct sunlight, and surgical lights, the sensor must be covered with an opaque material to prevent incorrect SpO2 readings. In addition, agents that significantly change the optical-absorbing properties of blood, such as recently administered intravascular dyes, can interfere with reliable SpO2 measurements. The use of pulse oximetry with certain nail polishes, especially those that are blue, green, and reddish-brown in color, can result in inaccurate readings. If nail polish in these shades cannot be removed, the sensor should be applied to an alternate unpolished site.
Monitoring of end-tidal carbon dioxide (ETCO2) in respiratory gases provides an early warning of physiologic and mechanical events that interfere with normal ventilation. Capnography, which measures ETCO2 at the patient’s airway, is increasingly used in the PACU. It allows continuous assessment of the adequacy of alveolar ventilation, cardiopulmonary function, ventilator function, and the integrity of the airway and the breathing circuit. Consequently, it enables early detection of many potentially catastrophic events, including the onset of malignant hyperthermia, esophageal intubation, hypoventilation, partial or complete airway obstruction, breathing circuit leaks or disconnections, a large pulmonary embolus, and cardiac arrest.
Two variants of the instrument are available. A capnometer provides numeric measurement of exhaled CO2 levels. A capnograph provides the same numeric information, and it also displays a CO2 waveform. Both types of instruments usually incorporate an adjustable alarm system and often have trending and printing capabilities. The following discussion focuses on the use of capnographs because they allow more complete and effective patient assessment than do capnometers. As discussed subsequently, changes in the shape of the CO2 waveform can provide crucial diagnostic information about ventilation, similar to the way in which the waveform provided with an electrocardiogram (ECG) can provide crucial diagnostic information about the heart.
For measurement of exhaled CO2, the most common type of capnograph passes infrared light at a wavelength that is absorbed by CO2 through a sample of the patient’s respiratory gas. The amount of light that is absorbed by the patient’s gas reflects the amount of CO2 in the sample.
Capnographs differ in the manner in which they obtain respiratory gas samples for analysis. Sidestream (or diverting) capnographs transport the sample through narrow-gauge tubing to a measuring chamber. Mainstream (or nondiverting) capnographs position a flow-through measurement chamber directly on the patient’s airway. Special adapters are available to allow sidestream capnographs to be used on patients who are not intubated. The sample adapter should be placed as close to the patient’s airway as possible.
Sidestream capnographs incorporate moisture-control features that are designed to minimize clogging of the sample tube, protect the measurement chamber from moisture-induced damage, and minimize the risk of cross contamination. The design of these moisture-control systems significantly affects a monitor’s ease of use. Most systems rely on water traps, which must be emptied routinely. A new technology uses a special system of filters and tubing to dehumidify the sample and thus eliminate the need for water traps.
Capnographs also differ in calibration requirements. Many require removal of the patient from the respiratory circuit and adjustment of the instrument with special mixtures of calibration gases. Advanced capnographic technology includes automatic calibration and does not require any user calibration skills or time.
For effective use of capnography, it is important to understand the components of the normal CO2 waveform (capnogram)—a square wave pattern with a plateau (Fig. 27-3). Early in exhalation, air from the anatomic dead space, which is virtually CO2 free, is measured with the instrument. As exhalation continues, alveolar gas reaches the sampling site, and the CO2 level increases rapidly. The CO2 concentration continues to increase throughout exhalation and reaches the alveolar plateau because alveolar gas dominates the sample. At the end of exhalation, the ETCO2 occurs, which in the normal lung is the best approximation of alveolar CO2 levels. The CO2 concentration then drops rapidly as the next inhalation of CO2-free gas begins.
In normal conditions, when ventilation and perfusion are well matched, ETCO2 closely approximates arterial CO2 (PaCO2). The difference between the PaCO2 and the ETCO2 level is the alveolar-arterial CO2 difference (a-ADCO2). ETCO2 is usually as much as 5 mm Hg lower than PaCO2. When the two measurements differ significantly, an anomaly in the patient’s physiology, the breathing circuit, or the capnograph is usually present. Significant divergence between ETCO2 and PaCO2 is often attributable to increased alveolar dead space. CO2-free gas from nonperfused alveoli mixes with gas from perfused regions, thus decreasing the ETCO2 measurement. Clinical conditions that cause increased dead space, such as pulmonary hypoperfusion, cardiac arrest, and embolic conditions (e.g., air, fat thrombus, amniotic fluid), can increase the a-ADCO2. Changes in the a-ADCO2 can be used in assessing the efficacy of the treatment; as the patient’s dead space improves, the partial pressure of the alveolar carbon dioxide less the partial pressure of arterial carbon dioxide (PACO2 – PaCO2) narrows. Increases in dead space ventilation lower ETCO2 and therefore increase the PaCO2 – ETCO2 gradient. Widened PaCO2 – ETCO2 examples include embolic phenomena, hypoperfusion, and chronic obstructive pulmonary disease. Alternatively, a significant PACO2 – PaCO2 value can indicate incomplete alveolar emptying (e.g., with reactive airway disease), a leak in the gas-sampling system that allows loss of respiratory gas, and contamination of respiratory gas with fresh gas.4
An abnormal capnogram provides an initial warning of many events that warrant immediate intervention. Abnormalities may be seen on a breath-by-breath basis or when the CO2 trend is examined. For this reason, visualization is preferable of both the real-time waveform and the CO2 trend on the monitor display. Examples follow for a look at changes produced by significant events that commonly occur in the PACU.
A sudden decrease in ETCO2 to a near-zero level indicates that the monitor is no longer detecting CO2 in exhaled gases (Fig. 27-4). Immediate action is crucial for detection and correction of the cause of this loss of ventilation. Possible causes include a completely blocked endotracheal tube, esophageal intubation, a disconnection in the breathing circuit, and inadvertent extubation. The latter three possibilities are particularly likely if the decrease in ETCO2 coincides with movement of the patient’s head. First, after elimination of possible clinical causes for this decrease in ETCO2, a clogged sampling tube or instrument malfunction is investigated as the cause of the problem.
An exponential decrease in ETCO2 over a small number of breaths usually signals a life-threatening cardiopulmonary event that has dramatically increased dead space ventilation (Fig. 27-5). Sudden hypotension, pulmonary embolism, and circulatory arrest with continued ventilation must be considered.
A gradual increase in the ETCO2 level while the capnogram retains its normal shape usually indicates that ventilation is inadequate to eliminate the CO2 that is produced (Fig. 27-6). This situation can be the result of a small ventilator leak or a partial airway obstruction that reduces minute ventilation. It can also reflect increased CO2 production associated with increased body temperature, the onset of sepsis, or shivering. Of particular importance, a large increase in ETCO2 can be one of the earliest signs of malignant hyperthermia, which may not begin until after emergence from anesthesia.
Assessment of the capnogram can reveal information about the quality of alveolar emptying. For example, the patient with bronchospasm is unable to completely empty the alveoli, and the resulting capnogram does not have an alveolar plateau (Fig. 27-8). The ETCO2 reported by the capnograph in this instance is not a good estimate of alveolar CO2. Effective administration of bronchodilator therapy commonly improves alveolar emptying and results in a more normal capnogram.