Ventricles

The left ventricle pumps blood into the systemic circulation via the aorta. The systemic vascular resistance (SVR) is 5-7 times greater than the pulmonary vascular resistance (PVR). This makes it a high-pressure system (compared with the pulmonary vascular system) which requires a greater mechanical power output from the left ventricle (LV). The free wall of the LV and the interventricular septum form the bulk of the muscle mass in the heart. A normal LV can develop intraventricular pressures up to 300 mmHg. Coronary perfusion to the LV occurs mainly in diastole when the myocardium is relaxed.

The right ventricle receives blood from the vena cavae and coronary circulation, and pumps it via the pulmonary vasculature into the LV. Since PVR is a fraction of SVR, pulmonary arterial pressures are relatively low and the wall thickness of the right ventricle (RV) is much less than that of the LV. The RV, thus, resembles a passive conduit rather than a pump. Coronary perfusion to the RV occurs continuously during systole and diastole because of the low intraventricular and intramural pressures.

In spite of the anatomical differences, the mechanical behaviour of the RV and LV are very similar.

The cardiac cycle can be examined in detail by considering the ECG trace, intracardiac pressure and volume curves and heart valve function (Figure HE.14).

Systolic Function

Systole can be broken down into the following stages:

Figure HE.14

Cardiac cycle showing ventricular volume, ventricular pressure, aortic pressure and atrial pressure

Figure HE.14

Cardiac cycle showing ventricular volume, ventricular pressure, aortic pressure and atrial pressure

Isovolumetric ventricular contraction

• Ventricular ejection

Systole commences with a period of isovolumetric contraction initiated by the QRS complex of the ECG. During this brief period the volume of the ventricle does not change since both the AV and semilunar valves are closed. Isovolumetric contraction ends when the semilunar valve opens and ejection begins. The events during systole are described below and should be considered along with the ventricular pressure, aortic pressure and ventricular volume curves.

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