Chapter 28 – Cardiac Cycle

Abstract

The cardiac cycle refers to the complete sequence of physiological events that occur in the heart, from the beginning of one heartbeat to the beginning of the next.

Chapter 28 Cardiac Cycle

What is the cardiac cycle?

The cardiac cycle refers to the complete sequence of physiological events that occur in the heart, from the beginning of one heartbeat to the beginning of the next.

The cardiac cycle consists of two phases:

The diastolic phase, during which the ventricles are relaxed and are filling with blood. Diastole consists of four stages:
1. – Isovolumetric relaxation;
2. – Rapid ventricular filling;
3. – Slow ventricular filling;
4. – Atrial contraction.
The systolic phase, during which the ventricles contract and eject blood into the aorta and pulmonary artery. Systole consists of two stages:
1. – Isovolumetric contraction;
2. – Ejection.

Describe the events making up the cardiac cycle

Traditionally, the cardiac cycle is described from late diastole, when the atria and ventricles are relaxed and the atrioventricular (AV) valves are open:

Slow ventricular filling. The pressure within the atria is slightly higher than the intraventricular pressure. The AV valves are therefore open, allowing blood to flow slowly from atrium to ventricle.
Atrial contraction.
1. – The pacemaker cells of the sinoatrial (SA) node spontaneously depolarise, generating an action potential (see Chapter 57). The resulting electrical impulse is rapidly conducted throughout the atria, triggering atrial contraction.
2. – As a result of atrial contraction, much of the remaining atrial blood is ejected through the AV valves into the ventricles. At rest, this atrial ‘kick’ accounts for only 10% of ventricular filling: 90% of the blood flows into the ventricle passively. However, during exercise, when diastole is shortened, atrial contraction contributes up to 40% of ventricular filling.
3. – The pressure generated during atrial contraction is transmitted along the venae cavae and pulmonary veins as they have no valves: atrial contraction is represented by the a-wave on the atrial pressure waveform (Figure 28.1).
4. – The volume of blood within the ventricle at the end of atrial contraction is the end-diastolic volume.
Isovolumetric contraction. The action potential continues through the AV node and is conducted throughout the ventricles by the His–Purkinje system, represented on the electrocardiogram by the QRS complex. Initially, ventricular contraction causes a rapid rise in intraventricular pressure:
1. – Once intraventricular pressure exceeds atrial pressure, the AV valves close, resulting in the first heart sound, S₁. The mitral valve normally closes slightly earlier than the tricuspid valve, resulting in a ‘split’ S₁.
2. – There is a period of time between the closure of the AV valves and the opening of the aortic and pulmonary valves (semilunar valves) during which ventricular pressure rises without a change in ventricular volume – this is the phase of isovolumetric contraction.
3. – During isovolumetric contraction, the increased right ventricular pressure causes the tricuspid valve to bulge into the right atrium. This corresponds to the c‑wave on the atrial pressure waveform. Similarly, increased left ventricular pressure causes the mitral valve to bulge into the left atrium.
Ejection. Once ventricular pressure exceeds the pressure in the aorta and pulmonary artery, the semilunar valves open and blood is ejected from the ventricles.
1. – Right ventricular contraction pulls the tricuspid valve downwards. As the length of the right atrium increases, its pressure falls to zero and it is rapidly filled with blood. This is the origin of the x‑descent on the atrial pressure waveform.
2. – Initially, the flow of blood through the semilunar valves is rapid, but as the ventricular myocytes start to repolarise, the force of contraction wanes.
3. – In the course of ventricular relaxation, the ventricular pressure falls below that of the aorta and pulmonary artery; the semilunar valves close, resulting in the second heart sound, S₂. The aortic valve usually closes slightly earlier than the pulmonary valve. Inspiration can accentuate this difference, particularly in young people, resulting in a ‘physiological split’ S₂.
4. – Aortic valve closure is represented on the aortic pressure curve (Figure 28.1) by the dicrotic notch, a positive deflection caused by the elastic recoil of the aortic valve and the aorta.
5. – The volume of blood within the ventricle following valve closure is the end-systolic volume.
Isovolumetric relaxation. Following the closure of the semilunar valves, it takes a short time for the ventricles to further relax and their pressure to fall below that of the atria. Throughout late systole and isovolumetric relaxation, atrial pressure slowly rises due to venous return from the lungs and venae cavae. This corresponds to the v‑wave of the atrial pressure waveform (Figure 28.1).
Rapid ventricular filling. Once atrial pressure exceeds ventricular pressure, the AV valves open. Blood flows down its pressure gradient from the atria to the ventricles. During the early part of diastole, the ventricles are still undergoing relaxation and intraventricular pressure continues to decrease, and blood therefore flows rapidly into the ventricles. The fall in atrial pressure is represented by the y‑descent of the atrial pressure waveform (Figure 28.1).