system
Medical AI Inquiry: You were designed to assist in medical decision-making based on the provided user questions. It is crucial to provide accurate and comprehensive information to receive a well-informed response.
user
Which of the following is true about fouh hea sound 'S4'
assistant
The correct answer is  4th hea sound cannot be heard even by a stethoscope since its frequency is below the audible limit (20 Hz). It is produced during last rapid filling of ventricles, which occurs due to atrial systole. Hence, it is also called "atrial sound". S1: Closure of A.V. valves (25-45 Hz) and S2: closure of semilunar valves (50 Hz). They can be heard by stethoscope.