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(Netscape users, click here for sound)
As mentioned in the description of the normal S1/S2 heart tones, S2 is created when the ventricles relax and pressure from the aorta and pulmonary arteries exceeds the ventricular pressures. This causes the closure of the aortic and pulmonic valves creating the combined tone we call S2. Unlike the splitting of S1 however, the splitting of S2 is a normal event, although it may not always be appreciated in the pre-hospital environment. This is due to the fact that ejection pressures are higher and depolarization occurs earlier on the left side of the heart causing the aortic valve to close slightly earlier than the pulmonic. This is most evident during inspiration. During expiration the two sounds merge and are appreciated as one sound. During inspiration the pulmonic valve closes slightly later than the aortic giving S2 it's two components. When the breath is held during inspiration the splitting also diminishes as ejection times equalize. This splitting is more often heard and easier to detect in the young. As you listen to the sound again relate it to the graph above. Notice how the S2 sound "drags" slightly, and is not as crisp as it was in the normal S2. (again the "skip" in the sound file is a result of looping it for continuous play and should not be considered part of the splitting)
Abnormal splitting of the S2 heart tone is that which does not vary with the respiratory cycle, or does not vary as expected. These situations can generally be placed into one of three categories when describing the split, and is generally indicative of an underlying disease process. The example playing on this page would be considered a wide split and is presented as such to make it easier to recognize the split.
Unexpected Splitting of Heart Tones
Wide Splitting
The split becomes wider when there is delayed activation of contraction or emptying of the right ventricle resulting in a delay in pulmonic closure. This occurs for example in right bundle branch block which splits both S1 and S2. Wide splitting of S2 also occurs when stenosis delays closure of the pulmonic valve, when pulmonary hypertension delays ventricular opening, or when mitral regurgitation induces early closure or the aortic valve. The split becomes narrower and is even eliminated or paradoxical when closure of the aortic valve is delayed, such as in left bundle branch block.
Fixed Splitting
Splitting is said to be fixed when it is unaffected by respiration. This occurs with delayed closure of the pulmonic valve when output of the right ventricle is greater than that of the left ventricle (such as occurs in large atrial septal defects, a ventricular septal defect with left to right shunting, or right ventricular failure).
Paradoxical Splitting
Paradoxical splitting occurs when the closure of the aortic valve is delayed (such as in left bundle branch block) so that pulmonic closure occurs first followed by aortic closure. In this case the interval between pulmonic and aortic closure is heard during expiration (hence the term paradoxic) and disappears during inspiration.
From: Mosby's Guide to Physical Examination, Seidel H.M., et al. Fourth ed. Mosby Inc. St Louis Mo. 1999.