The Cardiac Cycle
The cardiac cycle (Fig. 18a) describes the events that occur during one beat of the heart. These are shown in the figure for the left side of the heart, together with the pressures and volumes in the chambers and main vessels. At the start of the cycle, towards the end of diastole, the whole of the heart is relaxed. The atrioventricular (AV) valves are open (right, tricuspid; left, mitral), because the atrial pressure is still slightly greater than the ventricular pressure. The pulmonary and aortic valves are closed, as the pulmonary artery and aortic pressures are greater than that in the ventricles. The cycle starts when the sinoatrial node (SA node) initiates atrial systole (Chapter 19).
Atrial systole (A). At rest, atrial contraction only contributes the last ∼15–20% of the final ventricular volume, as most of the filling has already occurred due to venous pressure. The atrial contribution increases with heart rate, as diastole shortens and there is less time for ventricular filling. There are no valves between the veins and atria, and atrial systole causes a small pressure rise in the great veins (a wave). The ventricular volume after filling is complete (end-diastolic volume, EDV) is ∼120–140 mL in humans. The end-diastolic pressure (EDP) is less than 10 mmHg, and is higher in the left ventricle than in the right due to the thicker and therefore stiffer left ventricular wall. EDV strongly affects the strength of ventricular contraction (see Starling’s law; Chapter 20). Atrial depolarization causes the P wave of the electrocardiogram (ECG); it should be noted that atrial repolarization is too diffuse to be seen on the ECG.
Ventricular systole (B, C). The ventricular pressure rises sharply during contraction, and the AV valves close as soon as this is greater than the atrial pressure. This causes a vibration which is heard as the first heart sound (S1). Ventricular depolarization is associated with the QRS complex of the ECG. For a short period, while force is developing, both the AV and outflow (semilunar) valves are closed and there is no ejection, as the ventricular pressure is still less than that in the pulmonary artery and aorta. This is called isovolumetric contraction (B), as the ventricular volume does not change. The increasing pressure makes the AV valves bulge into the atria, causing a small atrial pressure wave (c wave), followed by a fall (x descent).
Ejection. Eventually, the ventricular pressure exceeds that in the aorta or pulmonary artery, the outflow valves open and blood is ejected. The flow is initially very rapid (rapid ejection phase, C) but, as contraction wanes, ejection is reduced (reduced ejection phase, D). During the second half of ejection, the ventricles stop actively contracting, and the muscle starts to repolarize; this is associated with the T wave of the ECG. The ventricular pressure during the reduced ejection phase is slightly less than that in the artery, but initially blood continues to flow out of the ventricle because of momentum. Eventually, the flow briefly reverses, causing the closure of the outflow valve, a small increase in aortic pressure (dicrotic notch) and the second heart sound (S2). The amount of blood ejected in one beat is the stroke volume, ∼70 mL. About 50 mL of blood is therefore left in the ventricle at the end of systole (end-systolic volume, ESV). The proportion of EDV that is ejected (stroke volume/EDV) is the ejection fraction; this is normally ∼0.6, but is reduced below 0.5 in heart failure.
Diastole. Immediately after the closure of the outflow valves, the ventricles rapidly relax. The AV valves remain closed, however, because the ventricular pressure is initially still greater than that in the atria (isovolumetric relaxation, E). This is called isometric relaxation because again the ventricular volume does not change. Meanwhile, the atrial pressure has been increasing due to filling from the veins (v wave). When the ventricular pressure falls sufficiently, the AV valves open and the atrial pressure falls (y descent) as the ventricles rapidly refill (rapid filling phase, F). This is assisted by elastic recoil of the ventricular walls, essentially sucking blood into the ventricle. Filling during the last two-thirds of diastole is slower and due to venous flow alone (reduced filling phase, G). Diastole is twice the length of systole at rest, but decreases as the heart rate increases.
Ventricular pressure–volume loop
The ventricular pressure plotted against volume generates a loop (Fig. 18b), the area of which represents the work performed. Its shape is affected by the force of ventricular contraction (contractility), factors that alter refilling (EDV) and the pressure against which the ventricle has to pump (e.g. aortic pressure, afterload). An estimate of stroke work is calculated from the mean arterial pressure × stroke volume.
The peripheral arterial pulse reflects the pressure waves travelling down through the blood from the heart; these move much faster than the blood itself. The shape of the pulse is affected by the compliance (stretchiness) and diameter of the artery; stiff (e.g. atherosclerosis) or small arteries have sharper pulses because they cannot absorb the energy so easily. Secondary peaks are due to reflections of the pressure wave at bifurcations of the artery. The jugular venous pulse reflects the right atrial pressure, as there is no valve between the jugular vein and right atria, and has corresponding a, c, and v waves.
Heart sounds are caused by vibrations in the blood due, for example, to closure of the cardiac valves (see above). Normally, only the first and second heart sounds are detectable (S1, S2), although a third sound (S3) can occasionally be heard in fit young people. When the atrial pressure is raised (e.g. in heart failure), both a third and fourth sound may be heard, associated with rapid filling and atrial systole, respectively; this sounds like a galloping horse (gallop rhythm). Cardiac murmurs are caused by turbulent blood, and a benign murmur is sometimes heard in young people during the ejection phase. Pathological murmurs are associated with the narrowing of valves (stenosis), or regurgitation of blood backwards through valves that do not close properly (incompetence).