Which of the following is true about fourth heart 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. Fourth heart sound occurs due to the late rapid filling phase of the ventricle, which occurs due to atrial systole. Since its frequency is usually less than 20Hz, it is not heard by an unaided ear.