Timing ofthe OS and the S2OS Interval

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The OS will be expected to occur at the end ofthe isovolumic phase of relaxation. The latter has an average duration of at least 60 ms. The S2-OS interval then must be expected to be at least 50 ms or longer. In general the OS may occur anywhere between 50 and 110 ms after S2. The OS has been reported to follow A2 by a delay ranging from 30 to 150 ms (56). The interval will depend on the level ofthe aortic pressure, the rate ofisovolumic relaxation, and the left atrial v wave pressure peak. Of these three, the most important determinant is the level of the peak left atrial pressure. Thus, if the left atrial v wave is higher, then the OS will occur earlier than when the left atrial v wave is lower (Fig. 27A).

Supine

Supine

500 ms

Fig. 28. Digital display of a magnetic audio recording from a patient with mitral stenosis taken at the lower left parasternal region showing the variation in the S2-OS interval between supine and standing position. The S2-OS interval is slightly longer on standing than when supine. The fall in the left atrial pressure caused by the upright posture makes the OS come later after S2. The A2-P2 split, however, will either narrow considerably or become a single S2 on assuming an upright position.

500 ms

Fig. 28. Digital display of a magnetic audio recording from a patient with mitral stenosis taken at the lower left parasternal region showing the variation in the S2-OS interval between supine and standing position. The S2-OS interval is slightly longer on standing than when supine. The fall in the left atrial pressure caused by the upright posture makes the OS come later after S2. The A2-P2 split, however, will either narrow considerably or become a single S2 on assuming an upright position.

Because the height of the left atrial pressure is indirectly related to the severity of the mitral stenosis, the more severe the stenosis, the higher will be the left atrial pressure and the earlier the OS will occur after the S2 (Fig. 27A). The S2-OS interval can be either short, close to 60-70 ms; medium, close to 80-100 ms; or long, 100-120 ms.

A short S2-OS interval can be simulated by the syllables, Lubbb...pa..da when said as fast as one can. It can be medium and may simulate the syllables, Lubbb...pa ta when said as fast as one can. When it is late it will be mimicked by Lubbb...pa pa, again said as rapidly as possible.

It can also be easily visualized that the extent of the excursion of the leaflet before its sudden tensing because of the tethering will vary according to the severity of the mitral stenosis. The more severe the stenosis, the less will be the extent of the excursion and therefore the earlier will be the OS (Fig. 27B).

Because besides the left atrial pressure, the aortic pressure and the rate of isovolumic relaxation control the S2-OS interval, this interval may not always accurately predict the severity of mitral stenosis, especially in elderly patients and in the presence of hypertension. This interval will vary in atrial fibrillation because ofthe varying diastolic lengths. The longer cycles will be followed by lower left atrial pressure because of longer time for left atrial emptying, and the next systole will also be more likely to have a lower left atrial pressure. Thus, following long diastoles the S2-OS interval will be longer. In the presence of low cardiac output and a very large left atrium, one may have a long S2-OS interval even with significant mitral stenosis. In view of these confounding factors, a short S2-OS interval may be more helpful than a wide S2-OS interval in predicting the degree of mitral stenosis.

Maneuvers that make the left atrial pressures fall, such as making the patient stand up from a supine position, will make the OS come later (Fig. 28). The reason for this is the decreased venous return that occurs with assuming the erect posture, which will lower the left atrial pressure.

Following supine exercise, the S2-OS interval shortens because of rising left atrial pressure. Postexercise S2-OS interval of less than 60 ms will be suggestive of significant mitral stenosis (56).

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