Listen with the bell part of your stethoscope. Hold the bell gently over the apex area of the heart. Normal S1 and S2 sounds should be the first identifiable heart sounds. S1 precedes S2 and they can be described simply as the basic "lub-dub" sounds. The basic heart sounds result from the vibrations of the left ventricular muscle, cardiac skeleton, the valves and great vessels, and the momentum and velocity of the blood as it flows through the elastic system. For our purposes, or as the result of closure of the A.V. or mitral and tricuspid valves, and the aortic and pulmonary valves respectively.
With this in mind, let us review the cardiac cycle. We will begin with diastole when blood flows into both the right and left atria through open A.V. valves. At the end of diastole, when the atria contract, increased blood flow into the ventricles causes a rise in the ventricular filling pressure, causing the mitral and tricuspid valves to snap shut, thus producing the S1 or first heart sound.
During systole the ventricles contract, exerting pressure which eventually rises higher than the pressure of the aorta and the pulmonary artery, forcing the aortic and pulmonary valves open. Eventually at the end of systole the pressure within the ventricles starts to fall as the ventricles start to eject blood. As the pressure drops below the pressure in the aorta, and pulmonary artery, the aortic and pulmonary valves snap shut, eliciting the S2 or second heart sound.
S3 - Ventricular gallop
Another valvular sound, the "opening snap" may be auscultated at the apex as a high-pitched sound. This is the opening snap of the mitral valve. Not normally audible, this sound may alert you into the existence of mitral stenosis. It usually closely follows S2. As the ventricles fill following opening of the A.V. valves, a third heart sound may be auscultated. This sound is due to the rapid filling of the ventricles and is referred to as the S3 sound or ventricular gallop. It follows S2 in the cardiac cycle and may be confused with an S2 split (dub-dub). However, remember that an S2 split should normally split on inspiration whereas an S3 will not. It is a lower pitched sound than the S2 and can best be heard with the bell. S3 sound is considered by most sources to be normal in children and young adults, but suggests ventricular dysfunction or left ventricular hypertrophy in adults over 40 years
S4 - Atrial gallop
This precedes S1 and is the result of vigorous atrial contraction. It occurs in patients with hypertension, aortic stenosis, or acute myocardial infarction. S4 has also been studied in patient's with left ventricular hypertrophy.