Fourth

The fourth heart sound (S4) occurs in late diastole, just before S1, and corresponds to atrial contraction (the 'atrial kick') pushing blood into a stiff, hypertrophied, or noncompliant left ventricle. It is caused by vibration of the ventricular wall and valvular apparatus as blood is forced into a ventricle with reduced compliance. Because it requires atrial contraction to generate turbulent flow into a stiff ventricle, S4 is absent in atrial fibrillation (no coordinated atrial contraction). Clinically, S4 is associated with conditions that cause ventricular hypertrophy or decreased compliance: chronic hypertension (left ventricular hypertrophy), hypertrophic cardiomyopathy, aortic stenosis, and post-myocardial infarction with ventricular stiffening. It can be a normal finding in older adults due to age-related decreased compliance, but is pathologic in younger patients. S4 is best auscultated with the bell of the stethoscope (low-pitched sound) at the cardiac apex with the patient in the left lateral decubitus position, which brings the heart closer to the chest wall. Differentiating S4 from a split S1 or other extra sounds is a classic board question. Compare with S3, which occurs in early diastole during rapid ventricular filling and is associated with volume overload states (e.g., systolic heart failure, mitral regurgitation) and can be normal in children/young adults. Understanding the timing and pathophysiology of extra heart sounds is high-yield for distinguishing diastolic dysfunction (S4) from systolic dysfunction/volume overload (S3).