The S3 Versus The Opening Snap

1. What is the difference between the A2-OS interval and the S2-S3 interval?

ANS: The A2-OS interval is rarely more than 100 ms (0.10 s), whereas the shortest A2-S3 interval is usually 120 ms (0.12 s). The difference between 100 and 120 ms is merely the difference between saying "pa-pa" as quickly as possible and saying it at a normal speaking rate.

2. How does the S3 of a "tumor plop" differ from the usual S3?

ANS: A tumor plop is the early diastolic sound produced when a left (or right) atrial myxoma attached to a stalk prolapses through the mitral (or tricuspid) valve in diastole. This early diastolic sound can occur at the time one would expect either an OS or an S3. It also differs from an S3 by being more easily heard at the left lower sternal border than expected. (An S3 is not usually heard at the left lower sternal border unless it is extremely loud.)

3. How does the site of best auscultation help to distinguish an S3 from an OS?

ANS: An OS is commonly loudest at some point between the apex and the left sternal border, whereas the S3 is almost always loudest at the apex unless it is a right-sided S3 or due to pericardial constriction.

Note: In constrictive pericarditis, the early S3 or pericardial knock, like the OS, may be loudest at the left sternal border, possibly because the RV also contributes to the early S3 in this condition.

4. Why is the loud S3 of pericardial constriction called a pericardial knock?

ANS: Because it is very loud and there was controversy about calling it an S3.

Note: The pericardial knock is not present in tamponade because early rapid expansion of the ventricle is markedly blunted by the fluid.

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