1.4.1 The cardiac cycle
The cardiac cycle can be divided into distinct periods determined by electrical and mechanical events.
Systole is the period during which the ventricles develop pressure to drive blood into the great arteries. It can be divided into three intervals.
The pre-ejection period includes both electromechanical delay and isovolucontraction.
- Electromechanical delay Electromechanical delay is the time taken for the electrical stimulus to result in activation of the ventricular muscle.
- Isovolumetric contraction The period of isovolumetric contraction is the time when the ventricles have begun to contract, but the volume of the chambers has not yet changed. It occurs immediately after the period of electromechanical delay, following electrical stimulation of the ventricles. Duringthis period, intraventricular pressure increases until it is sufficient to open the semilunar valves and eject blood into the great arteries. The AV valves have already closed at the onset of isovolumetric contraction, although at slow heart rates they may have slightly re-opened and the increasing ventricular pressure forces them closed.
- The ejection period Once the semilunar valves have opened, the ventricles can eject the forward stroke volume into the systemic circulation. There is a short period during which the velocity of blood flow accelerates to a peak, after which there is a gradual decline until the point at which the aortic and pulmonary artery pressures are sufficiently high to prevent further ejection of blood. In the aorta, the majority of blood flow occurs during the first third of the ejection period. In the pulmonary artery the peak velocity of outflow occurs slightly later in systole.
The duration of electromechanical systole is the sum of the pre-ejection period and the ejection period. The length of this period depends on loading conditions and contractility, and is often greater for pulmonary flow.
The force of contraction largely develops from a reduction in the diameter of the ventricles, rather than a shortening of their length. However, the AV ring is drawn towards the ventricles, resulting in a fall in pressure in the atria which draws in blood from the great veins.
Diastole is the period during which the filling of the ventricles occurs. It can be divided into four intervals. Blood flow principally occurs during two of these periods.
8 Cardiac Anatomy and Physiology
The events of the cardiac cycle are summarised in Figure 1.5.
- Isovolumetric relaxation At the end of systole, the semi-lunar valves shut and the ventricles relax, resulting in a fall in the intraventricular pressure, initially with no change in the chamber volume. This is an active process, known as the period of isovolumetric relaxation. It ends when the pressure in the venfalls to below that in the atria and the AV valves open.
- Early diastolic filling At resting heart rates, the majority of the filling of the ventricles occurs during this ‘passive’ period, when the blood stored in the atrial ‘priming’ chambers flows rapidly into the ventricles. The period ends when the elastic properties of the ventricle prevent further filling and the intravenpressure rises above that in the atria. In fact, relaxation of myocardial cells is an energy consuming process and normal myocardial function is required for normal relaxation and filling.
- Diastasis In the horse, at resting heart rates, diastasis is the longest period in diastole. During this phase only a small amount of blood flows from the atria, but this makes a negligible contribution to ventricular filling, the atria acting as conduits for venous return.
- Atrial contraction The second period of diastole during which there is significant blood flow is when the ventricles are actively filled by blood from atrial contraction. Atrial contraction has a ‘pump-priming’ action by increasing the ventricular pressure immediately prior to systole. This increases the strength of ventricular contraction. In the horse, atrial contraction provides only a small proportion of diastolic filling at resting heart rates. However, at higher heart rates, or in animals with abnormal diastolic filling, it is a much more significant component of filling and is essential to maintain preload.