Korotkoff sounds are distinct noises heard through a stethoscope during the non-invasive measurement of blood pressure, a method known as auscultation. These sounds were first described in 1905 by Russian physician Nikolai Korotkoff, who realized they corresponded to different pressures within the artery as a blood pressure cuff was gradually deflated. The presence and characteristics of these audible events are directly tied to the temporary changes in blood flow dynamics within the brachial artery of the arm.
Defining the Sounds of Turbulent Flow
The sounds heard during blood pressure measurement result from changes in how blood moves through the artery, not the normal sounds of the heart contracting. In a healthy, unobstructed artery, blood flow is typically “laminar,” moving silently in smooth, parallel layers. When a blood pressure cuff is inflated around the upper arm, it compresses and occludes the brachial artery, completely stopping the flow of blood. As the pressure is released, the partial compression creates a narrow opening, disrupting the laminar flow and causing the blood to become “turbulent.” Turbulent blood flow is noisy, and these vibrations against the arterial wall are transmitted as the Korotkoff sounds.
How Korotkoff Sounds Determine Blood Pressure Readings
Blood pressure measurement relies on correlating specific sound events with the cuff’s pressure reading. The first sound heard, Korotkoff Phase 1 (K1), marks the moment the cuff pressure matches the highest pressure generated by the heart. This clear tapping noise indicates the Systolic Pressure. As the cuff continues to deflate, the sounds change in quality and eventually disappear entirely. The point at which the sounds vanish, Korotkoff Phase 5 (K5), corresponds to the Diastolic Pressure, signifying that the external cuff pressure is low enough to allow the artery to remain fully open and restore silent, laminar blood flow.
The Five Distinct Phases (K1 through K5)
The sequence of audible Korotkoff sounds is divided into five distinct phases, reflecting the changing state of arterial compression:
- Phase 1 (K1) is the first sound of clear, repetitive tapping heard for at least two consecutive beats, marking the systolic pressure. This tapping is caused by the initial spurts of blood pushing past the partially occluded artery.
- Phase 2 (K2) follows, where the tapping sounds soften and acquire a distinct swishing or murmuring quality.
- Phase 3 (K3) sees the return of sharper, crisper sounds that often become louder than those heard in Phase 1.
- Phase 4 (K4) is characterized by an abrupt muffling of the sounds, taking on a soft, blowing quality.
- Phase 5 (K5) is the complete disappearance of all sounds, which is the standard indicator for diastolic pressure.
Common Variations and Measurement Challenges
Several factors can complicate the accurate interpretation of Korotkoff sounds. One significant challenge is the “Auscultatory Gap,” a temporary period where the sounds disappear entirely between Phase 1 and Phase 2, only to reappear at a lower pressure. If a healthcare provider inflates the cuff only high enough to reach the gap, they may mistakenly record a systolic pressure that is significantly lower than the patient’s true pressure. The presence of an Auscultatory Gap is often associated with conditions like arterial stiffness or hypertension. To prevent this underestimation, the clinician must first palpate the radial pulse, inflate the cuff high enough to obliterate the pulse, then continue inflating another 20 to 40 mmHg higher before deflation; other factors like an incorrectly sized cuff, excessive environmental noise, or underlying cardiovascular conditions can also make the sounds faint or difficult to hear.