Cardiac cycle

Cardiac cycle

Cardiac cycle consists of all events occurring in the heart during a systole and the following diastole. Clinically systole starts from the first heart sound and ends at the onset of the second heart sound. Diastole is between the second heart sound and the next first heart sound. Typically a cardiac cycle lasts 0.8 seconds.

Sequence of opening and closing of the valves

The sequence follows this dictum: Right sided valves open first and close late. So if you are starting with systole, mitral valve closes at the onset of systole followed by tricuspid valve. This gives the M1T1 sequence when there is a split first heart sound. The interval after the atrioventricular valve closure and the opening of the semilunar valves constitute the isovolumetric contraction time. Next the pulmonary valve opens followed by aortic valve. At the end of systole, aortic valve will close first and pulmonary valve next, giving the A2P2 sequence for a split second heart sound. Interval between the closure of the semilunar valve and the opening of the atrioventricular valves constitute the isovolumetric relaxation period time (IVRT). This is followed by the opening of the tricuspid and mitral valves in sequence.

Phases of cardiac cycle

Phases of cardiac cycle are classically described in relation to the Wiggers diagram which incorporates drawings of phonocardiogram, electrocardiogram (ECG) and pressure tracings of atrium, ventricle and aorta as well as ventricular volume curve. The diagram has been in use for over a century, with initial publication by Carl Wiggers in 1915 [1].

Isovolumic contraction phase: Isovolumic contraction phase starts at the peak of the QRS complex in the ECG. As soon as the left ventricular pressure starts rising, the mitral valve closes. After a short while when the left ventricular pressure becomes more than the aortic diastolic pressure, the aortic valve opens. The period during which both mitral and aortic valves are closed and the ventricle is in systole, is known as isovolumic contraction phase. First heart sound is heard when the mitral valve closes at the onset of isovolumic contraction phase.

Ejection phase: The blood is ejected into the aorta once the aortic valve opens. After an initial period of rapid ejection while the ventricle continues to contract, there is a period of reduced ejection when the ventricle starts relaxing. The ejection phase ends when the aortic valve closes. This event is marked by the second heart sound.

Isovolumic relaxation phase: Isovolumic relaxation phase starts with the closure of the aortic valve and ends with the opening of the mitral valve.

Rapid inflow phase: Initial part of the diastole in which there is rapid inflow of blood into the ventricle ends with the third heart sound if one is present. Ventricular volume rises steeply in the initial part of this phase.

Diastasis: Next phase in diastole is the diastasis during which there is very little change in the ventricular volume and there is almost equalization of ventricular and atrial pressures. In mitral stenosis, diastasis is absent as there is a constant gradient across the mitral valve.

Atrial systole: Towards the end of diastole, there is a further increase in atrial and ventricular pressures and ventricular volume with atrial contraction. This is how atria act as a booster for ventricular filling. This is the time of presystolic accentuation of the murmur in mitral stenosis due to the enhanced flow across the mitral valve. This is absent in atrial fibrillation. Atrial systole starts at the peak of the P wave on the electrocardiogram, accounting for the initial electromechanical delay.

Electrical systole of the heart: With respect to the ECG, systole starts at the peak of the QRS (accounting for the initial electromechanical delay). Systole ends at the end of the T waves. So both ventricular depolarization (except a small initial portion) and repolarization is complete during systole.

Reference

1. Wiggers C. Circulation in Health and Disease. Philadelphia, PA: Lea & Febiger, 1915. Cited in ‘Mitchell JR, Wang JJ. Expanding application of the Wiggers diagram to teach cardiovascular physiology. Adv Physiol Educ. 2014 Jun;38(2):170-5.’