What is the cardiac cycle?

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Cardiac sounds and murmurs that arise from turbulence or vibrations within the heart and vascular system may be innocent or as a result of disease. It is vital to understand the timing of events in the cardiac cycle as a precondition for understanding heart murmurs. The cardiac cycle lasts for approximately 0.8 seconds and is the period from the end of one contraction to the end of the next.

This cycle has two phases:

  • Systole (period of contraction)
  • Diastole (relaxation)

What is systole?

The term ‘systole’ refers to the time period when the heart is contracting, in other words, the period in particular during which the left ventricle of the heart contracts. The last letter in “systole” is pronounced as a long “e” as in “lee.” The adjective derived from ‘systole’ is systolic.

The systolic pressure is, in particular, the maximum arterial pressure during contraction of the left ventricle of the heart. In a blood pressure read-out, the systolic pressure is normally the first number recorded. For instance, with a blood pressure reading of 120/80 (“120 over 80”), the systolic pressure is 120. “120” means 120 mm Hg (millimetres of mercury).

A systolic murmur refers to a heart murmur that is heard during systole, which is the time that the heart contracts, between the normal first and second heart sounds.

What is diastole?

Diastole will begin with the closure of the aortic and pulmonary valves. Intraventricular pressure drops but there is a very slight rise in ventricular volume (isovolumetric relaxation). As soon as ventricular pressure falls below atrial pressure, the mitral and tricuspid valves open. Ventricular filling then begins.

At first, the pressure gradient between the atria as well as the ventricles is high and ventricular filling is quick (the phase of quick early filling). Under the usual circumstances, about 70% of ventricular filling takes place during this phase. As diastole advances, ventricular pressure rises and the rate of filling slows down (the phase of diastasis).

The last 25% of the filling during ventricular diastole comes from atrial contraction (the phase of atrial systole). When the pressure in the ventricles rises beyond the pressure in the atria, the mitral, as well as tricuspid valves close and diastole, is done. Isovolumetric relaxation, as well as the first part of the rapid early filling, are active, energy-requiring processes.

In a number of disease states, diastolic filling is abnormal. For instance, with mitral stenosis, an elevated proportion of ventricular filling occurs late in diastole. In this case, shortening of diastole – because of tachycardia or loss of atrial systole due to the development of atrial fibrillation – can cause marked hemodynamic compromise.

A related situation occurs when active relaxation is protracted (e.g., owing to myocardial ischemia or left ventricular hypertrophy). On the other hand, in some circumstances (e.g., restrictive cardiomyopathy) a larger proportion of diastolic filling occurs early on in diastole. In this case, cardiac output may be improved with moderate tachycardia.

Each cycle is begun by the spontaneous production of an action potential in the SA-node (sino-atrial node). This action potential is carried throughout the atria to the AV-node; then spread to the Purkinje fibres. These fibres move up to the A-V bundle and into the ventricles. At the apex, the blood travels into the Purkinje fibres.

How many times does a person’s heartbeat?

During an average lifetime, the heart beats approximately 2.5 billion times and pumps out about 300 million litres of blood. The heart is very sensitive to the changing needs of the body and cardiac output can vary from as little as 5,041ml (during rest) per minute to as much as 35 litres of blood per minute in peak exercise.

The cardiac cycle has been defined as a sequence of alternating contractions and relaxations of the atria and ventricles in order to be able to pump blood throughout the body. It begins at the start of one heartbeat and finishes at the beginning of another. The process starts as soon as the fourth gestational week when the heart first starts to contract.

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