Baroceptor responses to acute rhythm changes.

There is now evidence that the response of the peripheral resistance to acute changes in the stroke output of the heart is determined by baroceptor reflexes, since it has been shown in cats (Ead et al., 1952) and in man (Sharpey-Schafer, 1953) that these receptors respond to arterial pulse pressure as well as mean pressure changes. In normal man when the effective filling pressure of the heart is reduced by the Valsalva manceuvre or the erect posture, the subsequent decrease in stroke output and arterial pulse pressure is followed by peripheral vasoconstriction. It is more difficult to increase suddenly the normal stroke output and pulse pressure; but it has been shown that imposed cough transients are followed by peripheral vasodilatation (Sharpey-Schafer, 1953), which also results from the increased pulse pressure during squatting (Sharpey-Schafer, 1956). It seemed possible that spontaneous acute changes in cardiac rhythm might provide additional evidence, since pulse pressure may alter conspicuously with sudden slowing or quickening of the heart beat.