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 ...

Heart sounds are the noises made as blood moves ... ventricle and closes to prevent backflow into the left atrium. S3 (gallop) is an abnormal heart sound. It is not usually present but may be ...