The fourth heart sound (S4), also known as the “atrial gallop,” occurs just before S1 when the atria contract to force blood into the left ventricle. If the left ventricle is noncompliant ...

The fourth heart sound (S4), also known as the "atrial gallop", occurs just before S1 when the atria contract to force blood into the LV. If the LV is non-compliant and atrial contraction forces ...