The exact mechanism of origin of Korotkoff sounds is not completely established. They have been thought to result from turbulence of flow coming through the partially occluded artery. This is thought to be supported by the following: they become muffled (Phase IV) and eventually disappear (Phase V) generally when the flow resumes in diastole, i.e., when the cuff is deflated below the diastolic pressure, allowing the artery to be open throughout the cardiac cycle. In addition, before they become muffled and completely disappear, they may sound like a short bruit (initially as soft bruit in Phase II and louder bruit as in Phase III).
However, they do not always sound like murmurs and often present as sharp sounds. The Waterhammer theory was suggested (not to be confused with the water-hammer pulse in aortic regurgitation) to explain the presence of distinct sounds (2). The sounds are thought to be produced by the deceleration of high-velocity flow coming through the vessel as it opens up from an occluded state against the stationary column of blood distal to the occlusion. The intensity ofthe sounds, however, may vary, being loud and persistent to low diastolic pressures or throughout diastole in certain situations such as in aortic regurgitation, in children, and in pregnancy. In aortic regurgitation, the stroke volume tends to be large with increased velocity of ejection. Children generally have hyperkinetic circulation, and pregnant women tend to have a high cardiac output with increased sympathetic tone. On the other hand, Korotkoff sounds tend to be poor in low-output states.
When an oversized cuff is used it may lead to underestimation of the systolic blood pressure. It has been shown that the state of the distal vasculature in the limb where the blood pressure is being measured affects the intensity of the Korotkoff sounds. Vasodilatation makes the sounds louder, and vasoconstriction softer (3,4). When the peripheral resistance in the arm distal to the cuff was changed by interventions such as heating, cooling, and induction of reactive hyperemia, the amplitude of the Korotkoff sounds appeared to change. Thus, these effects may lead to either over- or underestimation of both systolic and diastolic pressures (5). In fact, when the Korotkoff sounds are poorly heard, they are best augmented by raising the arm (decreasing venous distension) and having the patient open and close his or her fist on the side where the pressure is being measured a few times (6,7). This is thought to increase the forearm flow.
Others have proposed that the pressure pulse wave itself may be the source of the Korotkoff sounds. The sounds may be attributed to "shock waves" where the flow velocity of blood in the narrowed segment may exceed the pulse wave propagation velocity and give rise to vibrations in the audible frequency range. This would be analogous to the sonic boom heard when the speed of a jet plane reaches and surpasses the speed of sound.
It is also possible that the Korotkoff sounds are related to energy (vibrations) that results from sudden termination of the pressure pulse wave at the site of the inflated cuff, which leads to partial occlusion of the vessel, causing this to become a terminating site favoring reflection. Between the systolic peak and the diastolic pressures during which time the Korotkoff sounds are present there is a considerable degree of termination and reflection together with beginning onward transmission of the pressure pulse wave. Once the cuff pressure is lowered below the diastolic pressure, there is no more termination or reflection at the site of the cuff application and the pressure pulse is further transmitted along the artery. Therefore, there is no sound to be heard. This may be supported by the fact that when an oversized cuff is used, it may lead to underestimation of the systolic blood pressure. In conditions where the Korotkoff sounds are loud and last longer to low levels of diastolic pressure such as aortic regurgitation, there is a more rapid and higher launching momentum to the pressure pulse wave because of the large stroke volume, which is ejected with a rapid velocity. Thus, there may be more energy for dissipation at the termination sites. In significant aortic regurgitation, one often feels
"pistol shots" over the femoral arteries, which are also often sites of reflection because of bifurcation. On the other hand, this mechanism does not explain why the Korotkoff sounds are heard distal to the cuff and not proximal to it.
Tavel et al. had previously shown using invasive pressure measurements that the Korotkoff sounds correspond to the steepened portion of the anacrotic limb of the pressure pulse (8). More recently Drzewiecki et al. have proposed an alternate origin for the production of the Korotkoff sounds. They relate it to the distortion of the pressure pulse under the cuff in the narrowed segment and a change in both pressure and flow distal to the cuff resulting in a nonlinear pattern of pressure-flow relationship. This results in a steeper pulse slope distal to the cuff with higher-frequency harmonics content of this steeper pulse slope reaching the audible frequency range. This interesting theory was proposed based on a mathematical model representing the structures involved, which was able to predict the range of features of the Korotkoff sounds previously reported (9). However, it does not satisfactorily explain why in some people the Korotkoff sounds persist even when the cuff is fully deflated.
It is clear from the short description above that the origin of the Korotkoff sounds is far from established.
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