Cardiac cycle and heart sounds
The Cardiac cycle
The cardiac cycle refers to the sequence of events that occur during one complete heartbeat. It involves the contraction (systole) and relaxation (diastole) of the heart chambers, allowing for blood to be pumped and circulated throughout the body. The cardiac cycle consists of the following phases:
- Atrial Contraction (Atrial Systole): The cardiac cycle begins with atrial contraction, during which the atria contract to push blood into the ventricles. This phase is represented by the P wave on an electrocardiogram (ECG).
- Ventricular Contraction (Ventricular Systole):
a. Isovolumetric Contraction: Initially, the ventricles contract, causing the closure of the atrioventricular (AV) valves (mitral and tricuspid valves). This phase is isovolumetric because all heart valves are closed, and no blood is being ejected.
b. Ventricular Ejection: As ventricular pressure exceeds that of the arteries, the semilunar valves (aortic and pulmonary valves) open, allowing blood to be ejected from the ventricles into the respective arteries (aorta and pulmonary artery). - Ventricular Relaxation (Ventricular Diastole):
a. Isovolumetric Relaxation: After ventricular ejection, the ventricles relax, causing the closure of the semilunar valves. This phase is isovolumetric as all heart valves are closed, and no blood enters the ventricles.
b. Ventricular Filling: As ventricular pressure drops below that of the atria, the AV valves open, allowing blood to passively flow from the atria into the ventricles. This is known as passive ventricular filling (early diastole). Additionally, atrial contraction (late diastole) contributes to the completion of ventricular filling.
Heart sounds
Heart sounds, often heard through a stethoscope, are associated with specific events during the cardiac cycle. The two primary heart sounds are:
- S1 (First Heart Sound):
- Timing: Occurs during ventricular systole (specifically during isovolumetric contraction).
- Cause: Closure of the AV valves (mitral and tricuspid valves) due to ventricular contraction.
- Sound: “Lub” sound.
- S2 (Second Heart Sound):
- Timing: Occurs during ventricular diastole (specifically during isovolumetric relaxation).
- Cause: Closure of the semilunar valves (aortic and pulmonary valves) due to ventricular relaxation.
- Sound: “Dub” sound.
In addition to S1 and S2, there may be additional heart sounds heard under certain conditions, such as heart murmurs or abnormal valve function. These additional sounds can provide further diagnostic information about the heart’s structure and function.
Understanding the cardiac cycle and heart sounds is crucial for evaluating cardiac health, diagnosing abnormalities, and assessing the effectiveness of treatment in various cardiovascular conditions. It is important to consult with a healthcare professional, such as a cardiologist, for a thorough evaluation and interpretation of cardiac cycle events and heart sounds.
