Cardiac Cycle MCQ — NEET PG Practice Question | NEETPGAI
Cardiac Cycle
medium
heart-pulse Physiology
During the cardiac cycle, the most common site of occurrence of the third heart sound (S3) in healthy individuals is at which phase?
A. Isovolumetric relaxation phase
B. Late diastole during atrial contraction
C. Early systole
D. Early diastole during rapid ventricular filling
Explanation
Timing and Origin of S3
Key Point
The third heart sound (S3) occurs in early diastole, specifically during the rapid ventricular filling phase, approximately 120–150 ms after the aortic component of the second heart sound (A2).
Physiological Basis
S3 is produced by:
1.
Abrupt deceleration of blood flow as the ventricle reaches its elastic limit
2.
Vibration of the ventricular wall and mitral apparatus
3.
Occurs when the rate of ventricular filling is maximal
High-YieldNEET PG
S3 is a normal finding in children and young adults (< 30 years) due to high compliance and rapid early diastolic filling. It becomes pathological in older adults when associated with ventricular dysfunction or volume overload.
Cardiac Cycle Phases
Table
Phase
Timing
S3 Occurrence?
Early systole (isovolumetric contraction)
0–40 ms
No
Early diastole (rapid filling)
120–150 ms after A2
Yes (S3)
Mid-to-late diastole
Slow filling phase
No
Late diastole (atrial systole)
Produces S4 if abnormal
No
Clinical Pearl
A physiologic S3 is best heard at the apex with the patient in the left lateral decubitus position using the bell of the stethoscope. It is a low-frequency sound (20–50 Hz).
