From the Research
The atria contract before S1, during late diastole, and do not contract during S1 or S2 heart sounds. The cardiac cycle is a highly coordinated process, and understanding the timing of atrial contraction is crucial for interpreting heart sounds and diagnosing cardiac abnormalities. Atrial contraction occurs when the P wave appears on an ECG, pushing the final amount of blood into the ventricles. Then, the ventricles contract, marked by S1, which is produced by the closure of the mitral and tricuspid valves at the beginning of ventricular systole, after atrial contraction has already completed. S2 occurs at the end of ventricular systole when the aortic and pulmonary valves close, by which time the atria are in a relaxed state, passively filling with blood returning from the systemic and pulmonary circulations 1. The sequence of the cardiac cycle is: atrial contraction, followed by ventricular contraction (marked by S1), and then ventricular relaxation (marked by S2), and this cycle repeats. This understanding is essential for correctly interpreting heart sounds and diagnosing cardiac abnormalities, and is supported by the most recent evidence on cardiac function and arrhythmia management 1. Key points to consider include:
- Atrial contraction occurs before S1
- S1 is produced by the closure of the mitral and tricuspid valves
- S2 occurs at the end of ventricular systole
- The atria are relaxed during S1 and S2
- The cardiac cycle follows a specific sequence: atrial contraction, ventricular contraction, and ventricular relaxation.