S4 gallop

S4 occurs when the atria contract to push blood into a ventricle with decreased compliance (increased stiffness), creating vibrations audible just before S1, producing a 'Ta-Lub-Dub' rhythm (atrial gallop). It reflects diastolic dysfunction rather than systolic failure and is never heard in atrial fibrillation because it requires atrial contraction against a closed valve. Common causes include left ventricular hypertrophy secondary to chronic hypertension, aortic stenosis, hypertrophic cardiomyopathy, and ischemic heart disease/prior MI with reduced compliance. It can also be a normal finding in trained athletes and older adults due to age-related stiffening of the ventricle. Distinguish from S3, which occurs in early diastole during rapid ventricular filling and is associated with volume overload states (e.g., systolic heart failure, mitral regurgitation) or can be physiologic in young healthy people and pregnancy. On exam, S4 is best appreciated with the bell of the stethoscope lightly applied at the cardiac apex with patient in left lateral decubitus position, coinciding with palpation of the point of maximal impulse. Clinically, an S4 in the setting of chest pain suggests reduced compliance from ischemia or infarction. It's an important auscultatory clue tested in USMLE Step 1/2 for diastolic dysfunction and left ventricular hypertrophy.