Describe Fourth Heart Sound in details?

S₄ is generated during the atrial-filling phase. It is a low frequency event. LVS, is best heard over apex. Mechanism of S₄ generation: It occurs when augmented atrial contraction occurs against relatively stiff ventricle to generate pre systolic ventricular distension so that it can contract with greater force.

Physiological S₄ call is heart, in athletes who have physiological hypertrophy. It is rarely heard in normal subjects hence should be considered pathological unless proved otherwise.

Pathological S, is generated in any condition, which reduces ventricular compliances in LVH, acute MR, HOCM or ischaemic heart disease. Almost all patients with MI will have phonocardiographic S, unless they have MS (in which atria is unable to transmit its pressure freely to LV) or atrial infarction.

S₄ is not heard in constrictive pericarditis or tamponade because the ventricle is unable to expand at the end of diastole and also the atria may be tethered by constrictive process and therefore, not contract well. S, is not heard in patients with atrial fibrillation, as atrial booster pump is lost.