Korotkoff sounds are five distinct auditory phenomena heard when manually measuring blood pressure using a sphygmomanometer and a stethoscope. These sounds originate from the flow of blood through a partially compressed artery, unlike heart sounds which result from valve closures. They are named after Dr. Nikolai Korotkoff, a Russian military surgeon who first described the auscultatory technique in 1905. Understanding these five phases is fundamental to accurately determining a patient’s systolic and diastolic blood pressure readings.
How the Sounds Are Generated
Korotkoff sounds depend on the temporary alteration of blood flow dynamics within the brachial artery. Under normal conditions, blood flows through the arteries in a smooth, continuous pattern known as laminar flow, which is silent. A blood pressure cuff is inflated around the upper arm until the pressure exceeds the maximum pressure generated by the heart, completely occluding the brachial artery and halting blood flow.
As the cuff pressure is slowly released, blood begins to forcefully push past the point of compression during the peak of the heart’s contraction. This partial obstruction causes the flow to become chaotic and irregular, a state known as turbulent flow. Turbulent blood flow creates vibrations in the arterial wall, which are detected as Korotkoff sounds through the stethoscope placed below the cuff. The sounds persist only as long as the cuff pressure is high enough to cause this turbulent, partially restricted flow.
The Initial Phases of Sound Appearance
The first three phases mark the return of blood flow as the cuff pressure drops. The first Korotkoff sound, or K1, is the first clear, repetitive tapping sound heard as the cuff pressure falls to the level of the systolic blood pressure. This sound signifies that the pressure in the artery during the heart’s contraction is overcoming the pressure exerted by the cuff. The pressure reading at the appearance of K1 defines the patient’s systolic blood pressure.
Following K1, the sounds enter Phase 2 (K2), where the distinct tapping softens and often acquires a swishing or murmuring quality. This change occurs because the artery is still compressed, but the blood flow is less turbulent than at the moment of opening. The swishing sound reflects the longer duration and complexity of the vibrations as the blood pulses through the still-narrowed vessel segment.
Phase 3 (K3) is marked by the return of sharper, crisper tapping sounds that often increase in intensity. Despite the increased clarity, the artery remains partially compressed throughout the cardiac cycle at this stage. K2 and K3 are considered transitional phases and do not serve as primary clinical markers for systolic or diastolic pressure.
The Final Phases and Diastolic Pressure
The final two phases indicate the cuff pressure approaching and falling below the pressure within the artery during relaxation. Phase 4 (K4) is characterized by a distinct and abrupt muffling of the sounds, which take on a softer, blowing quality. This change suggests that the artery is no longer collapsing completely during the heart’s resting phase, though the vessel is still experiencing some compression. The muffling is caused by the pressure dropping low enough that the artery is only partially collapsing during the diastolic pressure trough.
Phase 5 (K5) is the point where all sounds disappear completely, which represents the standard measure for the diastolic blood pressure reading in adults. Once the cuff pressure drops below the patient’s diastolic pressure, the artery is no longer compressed during the cardiac cycle, and the blood flow immediately returns to its silent laminar state. K5 is the universal standard for diastolic pressure because it is the most reproducible point of silence.
In specific clinical situations, such as in children or pregnant women, K4 is sometimes recorded as the diastolic pressure because the sounds may persist down to zero pressure, making K5 unmeasurable. For most adults, the disappearance of sound (K5) is the recognized clinical endpoint for the lower blood pressure number. K5 remains the universally accepted standard for adult blood pressure measurements.
Clinical Considerations and Variations
A phenomenon known as the “auscultatory gap” can complicate manual blood pressure measurement and lead to inaccurate readings if undetected. This gap is a temporary disappearance of the Korotkoff sounds between K1 and K2 or K3, followed by the reappearance of sounds at a lower pressure. If the clinician does not inflate the cuff high enough, this silent interval can cause the true systolic pressure to be underestimated.
To prevent misreading the systolic pressure due to this gap, the correct technique involves first palpating the radial pulse while inflating the cuff. The cuff should be inflated at least 20 to 30 mmHg above the point where the radial pulse disappears to ensure the true K1 sound is captured upon deflation. Other factors affect the audibility and clarity of the sounds, including excessive patient movement, an improperly sized blood pressure cuff, and the rate of cuff deflation. Releasing the pressure too quickly can make it difficult to accurately pinpoint the brief moments of K1 and K5.