Introduction

The cardiovascular system consists of the heart and blood vessels. It is a closed circuit and is responsible for ensuring that blood flows throughout the body. Heart and circulatory disease, cardiovascular disease (CVD), includes all the diseases of the heart and blood vessels. The two main diseases in this category are coronary heart disease (CHD) and stroke, but CVD also includes congenital heart disease and a range of other diseases of the heart and blood vessels. CVD is the greatest cause of death in the UK, accounting for 34% of the deaths in 2007, a total of over 193 000 people (British Heart Foundation [BHF] 2008). It is the main cause of disability in the UK and as such is likely to be encountered by all nurses, whether hospital- or community-based. CVD also has an immense impact on society in human terms. Bereavement, disability, changing roles within the family and society, and fear are some examples of its consequences. CVD is costly in financial terms, imposing a significant annual burden on the UK economy. CVD cost the health care system in the UK around £3.2 million in 2006 – a cost per capita of just over £50. However, the majority of the costs of CVD fall outside the health care system and are due to illness and death in those of working age and the economic effects on their families and friends who care for them. The overall cost of CVD to the UK economy is estimated to be £30.7 billion per annum (BHF 2008).

Many cardiovascular diseases take the form of progressive debilitating illness, often becoming chronic with intermittent acute episodes. In contrast, a heart attack (myocardial infarction, MI) is often sudden and unexpected, arousing acute distress in the individual and family as they confront a life-threatening crisis. Cardiovascular nursing is evolving as nurses move on from practising the skills of advanced life support, cannulation and phlebotomy, to taking greater responsibility for decisions that influence patient care management. Nursing care has historically ranged from acute care management to long-term support (Ashworth 1992) and includes:

Government frameworks and standards of care, such as the National Service Framework for Coronary Heart Disease (Department of Health [DH] 2000), have resulted in opportunities for nurses to lead and develop CVD services in and between primary, secondary and tertiary care environments. Over the past decade, experienced nurses have increasingly become part of the multidisciplinary team, admitting and discharging patients from specialist units via triage and fast-tracking, prescribing and titrating drug therapies, coordinating specialised clinics and leading rehabilitation and health promotion programmes (Quinn & Morse 2003). For an example of a critical pathway, see website Figure 2.1.

See website Figure 2.1

Figure 2.1 The internal anatomy of the heart.

Anatomy and physiology of the heart

The heart is a hollow, four-chambered muscular organ that generates pressure changes resulting in the propulsion of blood around the vascular system. The right side of the heart pumps blood around the pulmonary system where gaseous exchange takes place and then on to the left side of the heart. The left side of the heart operates under much greater pressure to enable it to pump blood around the systemic circulation. The various chambers of the heart are illustrated in Figure 2.1.

The heart is composed of different types of tissue:

Coronary blood supply

Like all major organs, the heart requires blood flow to maintain cellular activity. It receives its blood supply from the right and left coronary arteries which arise from the aorta just beyond the aortic valve and run over the outer surface of the heart (Figure 2.2).

Figure 2.2 The coronary circulation.

The left coronary artery runs towards the left side of the heart and divides into two major branches: the left anterior interventricular branch or left anterior descending artery (LAD) and the circumflex artery (CX). The LAD follows the anterior ventricular sulcus and supplies blood to the interventricular septum and the anterior walls of both ventricles. The CX follows the coronary sulcus and supplies blood to the lateral and posterior regions of the left atrium and left ventricle. The right coronary artery (RCA) runs to the right side of the heart and divides into two branches: the posterior interventricular artery and the marginal artery. The posterior interventricular artery supplies blood to the posterior ventricular walls and the marginal artery supplies the right ventricle. The RCA supplies the sinoatrial (SA) and atrioventricular (AV) nodes in about 60% of cases (Figure 2.3).

Figure 2.3 The conducting tissues of the heart. LA, left atrium; LV, left ventricle; RA, right atrium; RV, right ventricle.

After passing through the cardiac capillary bed, the blood drains into the cardiac veins which join to form the coronary sinus on the posterior surface of the heart from where venous blood drains into the right atrium.

Structure and function of cardiac valves

The atrioventricular valves, i.e. the tricuspid and the mitral, function in a similar manner. During ventricular diastole (relaxation), they act as a funnel to promote rapid filling of the ventricles. Most of ventricular filling is passive. During ventricular systole (contraction), intraventricular pressure rises, pushing the cusps, which are restrained by the chordae tendineae, back and up towards the atria, thus preventing backflow of blood during systole.

The semilunar valves, i.e. the pulmonary and the aortic, have three cusps each. These valves are closed during ventricular diastole and intraventricular pressure therefore increases as the ventricles contract. When the pressure in the ventricles becomes greater than that in the aorta and the pulmonary artery the semilunar valves are forced open and blood is ejected. After ventricular systole, the pressure in the aorta and pulmonary artery is greater than that in the left and right ventricles, resulting in retrograde blood flow which fills the valve cusps and snaps them closed. The closure of the valves produces the sounds referred to as heart sounds. Mitral and aortic valve closure produces the first heart sound (S1) which precedes that of the tricuspid and pulmonary valves known as the second heart sound (S2). The second heart sound is normally split because the aortic valve closes before the pulmonary valve, on inspiration, when the right ventricle takes longer to expel the increased venous return.

The conducting system of the heart

The contraction and relaxation of the muscles of the atria and ventricles needs to be coordinated so that filling and emptying of the chambers is controlled and efficient. This occurs as a result of an electrically activated stimulus response system. Specialised automatic cells generate the initial impulse which spreads throughout the myocardium in a coordinated way and triggers a transient release of calcium ions that is responsible for initiating myocardial cell contraction. The automatic cells possess three specific properties:

• automaticity – the ability to generate action potentials, spontaneously and regularly

• excitability – the ability to respond to electrical stimulation by generating an action potential

• conductivity – the ability to propagate action potentials, i.e. spread to other cells.

The main ions (electrically charged particles) involved in the electrical activation of cardiac muscle are sodium (Na⁺), potassium (K⁺) and calcium (Ca²⁺). Electrical activity occurs due to the movement of these and other ions in and out of the cells thereby altering the electrical charge of the cell. Cells at rest (known as polarised) are negatively charged on the inside whilst cells that are electrically activated (known as depolarised) are positively charged on the inside. The electrical charge on the surface of an automatic cell leaks away until a certain threshold is reached and spontaneous action potential is generated over the whole cell surface. The automatic cells are found in the cardiac conducting system (see Figure 2.3), which consists of:

• the sinoatrial (SA) or sinus node

• the atrioventricular (AV) junction (the AV node and bundle)

• ventricular conducting tissue (the right and left bundle branches).

The automatic cell with the most rapid leak of charge becomes the principal pacemaking cell and is normally located within the SA node. Both automatic and myocardial cells can transmit impulses, but the specialised cells do so in a more rapid and coordinated way. Electrical activation spreads from the SA node through both atria at a rate of about 1 m/s and reaches the most distant portion of the atria in about 0.08 s. The atria and ventricles are electrically connected via the AV junction, and when the impulse reaches the AV node there is a delay of about 0.04 s to allow blood flow from the atria to the ventricles. From the AV node, the impulse enters the rapidly conducting tissue of the bundle of His and the right and left bundle branches and the entire ventricular mass is depolarised almost simultaneously thereby producing efficient contraction and pumping. Return to the electrical resting state for each cell is called repolarisation and involves active pumping of ions against concentration gradients.

Electrocardiography

Electrocardiography is the graphic recording from the body surface of potential differences resulting from electrical currents generated in the heart (vertical axis) plotted against time (horizontal axis). This recording may be displayed on special graph paper or on an oscilloscope (monitor) and is known as an electrocardiogram (ECG). The main value of the ECG is in the detection and interpretation of cardiac arrhythmias, diagnosis of CHD and assessment of ventricular enlargement (hypertrophy).

The sequence of electrical events produced at each heartbeat has arbitrarily been labelled P, Q, R, S and T (Figure 2.4).

Figure 2.4 ECG of one cardiac cycle.

The P wave

represents atrial activation and the width of the P wave illustrates the time this takes. The PR interval, measured from the beginning of the P wave to the beginning of the first deflection of the QRS complex, represents the total time taken for atrial activation and AV nodal delay.

The Q, R and S waves

are associated with ventricular activation. The first downward deflection after the P wave is always labelled the Q wave and the R wave is the first upward deflection. If a negative deflection follows an R wave, it is labelled an S wave. The width of the QRS complex shows how long the action potential takes to spread through the ventricles.

The ST segment

is the line between the S wave and the T wave and represents the early phase of ventricular muscle repolarisation, or recovery.

The T wave

represents the electrical recovery or repolarisation of the ventricular muscle. Occasionally, a U wave can be observed following the T wave. The origin of this wave is not well understood, but it is considered significant in a state of hypokalaemia. For more details on the ECG see Riley (2007a) in Further reading.

Cardiac cycle

The cardiac cycle is the cyclical contraction (systole) and relaxation (diastole) of the two atria and the two ventricles. Each cycle is initiated by the spontaneous generation of an action potential in the SA node.

During diastole, which normally lasts about 0.4 s, blood enters the relaxed atria and flows passively into the ventricles. The atria contract fractionally before the ventricles and complete ventricular filling. As the ventricles begin to contract, ventricular pressure increases and for a short time (isometric phase) all four valves are closed and the volume of blood in the ventricles remains constant. Increasing pressure eventually forces the pulmonary and aortic valves to open and blood is ejected into the pulmonary artery and aorta. When the ventricles stop contracting, the pressure within them falls below that in the major blood vessels, the aortic and pulmonary valves close and the cycle begins again with diastole (Figure 2.5).

Figure 2.5 Blood flow through the heart.

The normal heart rate is approximately 70 beats/min in the resting adult, with each cardiac cycle lasting approximately 0.8 s. With each ventricular contraction, 65–75% of the blood in the ventricle at the end of diastole is ejected. This is usually a volume of 70–80 mL of blood and is known as the stroke volume.

Cardiac output is the volume of blood ejected from one ventricle in 1 min. Although cardiac output is a traditional measure of cardiac function, it differs markedly with body size. Thus, a more informative measure is the cardiac index, which is the cardiac output per minute per metre squared of body surface area. Usually it is about 3.2 L/m².

The primary factors that determine cardiac output are:

• Preload – the amount of tension on the ventricular muscle fibres before they contract, determined primarily by the end-diastolic volume (EDV).

• Afterload – the resistance against which the heart must pump. It is determined by blood pressure in the aorta, resistance in the peripheral vessels, the size of the aortic valve opening, left ventricular size, contractility of the heart and the heart rate.

Within physiological limits, the volume of blood pumped out by a ventricle is the same as that entering the atrium on the same side of the heart, i.e. cardiac output matches venous return. This principle is often referred to as the Frank–Starling law of the heart. This means that the heart is able to adapt to changing loads of inflowing blood from the systemic and pulmonary circulations. Within certain limits, cardiac muscle fibres contract more forcibly the more they are stretched at the start of contraction. Once the venous return increases beyond a certain limit, the myocardium begins to fail. This regulation of the heart in response to the amount of blood to be pumped is known as intrinsic regulation.

Regulation of cardiac function by the autonomic nervous system

The autonomic nervous system alters the rate of impulse generation by the SA node, the speed of impulse conduction and the strength of cardiac contraction. It regulates the heart through both sympathetic and parasympathetic nerve fibres. The sympathetic fibres supply all areas of the atria and ventricles, and the effects on the heart include increased heart rate, increased conduction speed through the AV node and increased force of contraction. Parasympathetic impulses are conducted to the heart via the vagus nerve and affect primarily the SA node, the AV node and the atrial muscle mass. Parasympathetic stimulation produces decreased heart rate, decreased conduction rate through the AV node and decreased force of atrial contraction.

Sympathetic and parasympathetic control of the heart occurs by reflexes coordinated in the medulla oblongata of the brain. The group of neurones in the brain that affects heart activity and the blood vessels is known as the cardiovascular centre. This centre receives information from various sensory receptors. Baroreceptors, located in the atria, the aortic arch and carotid sinuses, alter their rate of impulse generation in response to changes in blood pressure; chemoreceptors, located for example in the carotid artery, respond to changes in the chemical composition of the blood.

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