Which of the following is true about fouh hea sound 'S4': (A) Can be heard by the unaided ear, (B) Frequency is greater than 20 Hz, (C) Heard during ventricular filling phase, (D) Heard during ventricular ejection phase
Answer is C. 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.