NBCE Exam 2026 – 400 Free Practice Questions to Pass the National Board of Chiropractic Examiners

The fourth heart sound (S4) occurs during atrial contraction, specifically when the atria contract to push blood into the ventricles just before the ventricles start to contract. This sound is associated with the rapid filling of the ventricles due to atrial contraction and the resultant vibrations of the ventricular walls.

S4 is often the result of a stiff or hypertrophied left ventricle, which may not accommodate blood flow as easily during this late phase of the cardiac cycle. This sound is particularly prominent in conditions like hypertension or left ventricular hypertrophy, where the stiffness of the ventricle can contribute to the sound's production.

The other choices refer to different phases of the cardiac cycle without specifically linking to the mechanism of S4. Ventricular contraction pertains to the events leading up to the second heart sound (S2), early diastole relates to the filling phase after ventricular contraction, while late diastole encompasses the period leading up to atrial contraction itself, but it does not specifically define the timing of S4 as well as atrial contraction does.