Cardiac Cycle and Heart Sounds
Introduction:
At rest, the healthy adult heart beats at a rate of 60–80 beats per minute (b.p.m.). During each heartbeat, or cardiac cycle, the heart contracts (systole) and then relaxes (diastole).
Stages of the Cardiac Cycle:
Taking 74 b.p.m. as an example, each cardiac cycle lasts about 0.8 seconds and consists of:
- Atrial systole: Contraction of the atria.
- Ventricular systole: Contraction of the ventricles.
- Complete cardiac diastole: Relaxation of the atria and ventricles.
Sequence of Events in the Cardiac Cycle:
- The period when the atria are filling is chosen as the starting point for convenience.
- The superior and inferior vena cava transport deoxygenated blood into the right atrium, while the four pulmonary veins bring oxygenated blood into the left atrium.
- The atrio-ventricular (AV) valves are open, allowing passive blood flow to the ventricles.
- The SA node triggers a wave of contraction that spreads over both atria, completing ventricular filling (atrial systole 0.1 s).
- When the electrical impulse reaches the AV node, conduction slows, allowing the atria to finish emptying before ventricular contraction begins.
- The AV node triggers an impulse that spreads through the AV bundle, bundle branches, and Purkinje fibers, producing ventricular contraction (ventricular systole 0.3 s) and pumping blood into the pulmonary artery and aorta.
- High ventricular pressure closes the AV valves, preventing backflow into the atria.
- Complete cardiac diastole (0.4 s) follows, during which atria and ventricles relax, myocardium recovers, and atria refill for the next cycle.
- Heart valves open and close based on chamber pressures: AV valves open during atrial filling and systole; pulmonary and aortic valves open when ventricular pressure exceeds arterial pressure.
- The sequence of valve opening and closing ensures unidirectional blood flow.
Heart Sounds:
Heart sounds are audible using a stethoscope on the chest wall, slightly below the left nipple. There are four heart sounds, though the first two are most prominent:
- First sound (“lub”): Loud, caused by closure of the atrioventricular valves at the start of ventricular systole.
- Second sound (“dup”): Softer, caused by closure of the aortic and pulmonary valves during ventricular diastole.
Detailed Notes:
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