health answers:
The cuff of a sphygmomanometer is placed around a patient’s upper arm and inflated to a pressure above the patient’s systolic blood pressure, at this time there will be no sound audible. This is because the pressure in the cuff is high enough, so that it completely occludes the blood flow. It is similar to a flexible tube or pipe with fluid in it, that is being pinched shut.
If the pressure is dropped to just slightly below the patient’s systolic blood pressure, the first Korotkoff sound will be heard. Since the pressure in the cuff is now slightly below the pressure produced by the heart, some blood will be able to pass through the upper arm, when the pressure in the artery rises during systole. This blood flows in spurts as the pressure in the artery rises above the pressure in the cuff and then drops back down, resulting in turbulence that results in audible sound.
As the pressure in the cuff is allowed to fall further, thumping sounds continue to be heard as long as the pressure in the cuff is between the systolic and diastolic pressures, as the arterial pressure keeps on rising above and dropping back below the pressure in the cuff.
Eventually, as the pressure in the cuff drops further, the sounds change in quality, then become muted, then disappear altogether. Now the pressure in the cuff has dropped below the diastolic blood pressure and the cuff no longer provides any restriction to blood flow – allowing the blood flow to become smooth again with no turbulence and thus produce no further audible sound.
Korotkoff actually described 5 types of Korotkoff sounds:
1. The first Korotkoff sound is the snapping sound first heard at the systolic pressure.
2. The second sounds are the murmurs heard for most of the area between the systolic and diastolic pressures.
3. The third sound was described as a loud, crisp tapping sound.
4. The fourth sound, at pressures within 10 mmHg above the diastolic blood pressure were described as “thumping” and “muting”.
5. The fifth Korotkoff sound is silence as the cuff pressure drops below the diastolic blood pressure.
Traditionally, the systolic blood pressure is taken to be the pressure at which the first Korotkoff sound is initially heard and the diastolic blood pressure is the pressure at which the fourth Korotkoff sound is just barely audible. However, there has recently (2000 onwards) been a move towards the use of the 5th Korotkoff sound (i.e. Silence) as the diastolic blood pressure, as this has been felt to be more reproducible.
In pregnancy a fifth phase may not be identifiable, in which case the fourth is used. This change, and associated uncertainty adds to the difficulties in interpreting clinical studies of hypertension although modern trials will specify the method used.
