Basic Cardiovascular Physiology

A basic understanding of cardiac physiology is also essential to interpreting the physical finding during a cardiac exam. Each pump or beat of the heart consists of two parts or phases - diastole and systole. During diastole the ventricles are filling and the atria contract. Then during systole, the ventricles contract while the atria are relaxed and filling. A more detailed understanding of the of cardiac physiology can be obtained by examining in detail the simultaneous pressure characteristics in the aorta, left atrium (atrium) and left ventricle (ventricle) through one cardiac cycle.

For the purposes for this discussion of cardiac physiology, we will focus on the physiology associated with the heart sounds S1, S2, S3, and S4. S1 occurs near the beginning of (ventricular) systole with the closing of the tricuspid and mitral valves. The closing of these two valves with increasing pressure in the ventricles as they begin to contract should be simultaneous. Any splitting in which the closing of the two valves are heard separately should be considered pathological. S2 occurs near the end of (ventricular) systole with the closing of the pulmonary and aortic valves. The closing of these two valves occurs with beginning of backward flow in the pulmonary artery and aorta respectively as the ventricles relax. The two valves can occur simultaneously or with slight gap between them under normal physiologic circumstances. S3 occurs at the end of the rapid filling period of the ventricle during the beginning of (ventricular) diastole. An S3, if heard should occur 120-170 msec after S2. S4 occurs, if heard coincides with atrial contraction at the end of (ventricular) diastole.

Be sure you can correlate what you feel and what you hear in the cardiac exam with the underlying physiology. The top tracing is the standard EKG (electrocardiogram) with underlying blue line indicating diastole and the red line indicating systole. The second tracing indicates what you should hear in with your stethoscope, with the underlying colors indicating the cardiac physiology associated with each sound (mitral valve closure = purple (associated with S1), tricuspid valve closure = green (associated with S1), aortic valve closure = orange (associated with S2), pulmonic closure = blue (associated with S2), and if present ventricular wall tensing = yellow (associated with S3)).

The bottom tracing indicates the peripheral pulse wave felt when taking the pulse while auscultating, which is the recommended method. Notice that the pressure wave begins near the end of systole and continues through the beginning and middle of diastole. The green tabs underneath this tracing indicates when the pulse should be felt by the examiner. This technique of simultaneously listening and feeling the pulse (generally radial) is especially helpful to avoid confusion between S1 and S2. Palpation of the peripheral pulse should correspond with S2.