Where is a fourth heart sound (S4) typically heard in neonates?

The fourth heart sound (S4), also known as the atrial sound, is typically associated with the end of diastole and reflects vibrations caused by the atrial contraction as it forces blood into a stiff or hypertrophied ventricle. In neonates, this sound is best heard at the apex of the heart due to the anatomical position and the heart's proximity to the chest wall in this area.

Localization of heart sounds can vary by age, and in the case of infants, their smaller thoracic cavity allows for more pronounced sounds in the apex region, making it an appropriate spot for auscultation. The apex corresponds to the area where the left ventricle is more accessible to the chest wall, which accentuates the audibility of S4 when present.

Other regions such as the base of the heart may pick up different heart sounds or murmurs, while the left sternal border and right atrium are not typically associated with the S4 sound in neonates. Hence, identifying S4 at the apex is vital for understanding cardiac function in this population.