Which phase of the Korotkoff sounds corresponds to the systolic blood pressure reading?

Systolic Blood Pressure Corresponds to Phase 1 of Korotkoff Sounds

Phase 1 (the onset of the first audible sound) is documented as the systolic blood pressure reading. 1

The Standard for Blood Pressure Measurement

According to the American Heart Association guidelines on blood pressure measurement, there is clear agreement that the onset of Phase I corresponds to systolic pressure 1. This represents the point during cuff deflation when blood first begins to flow turbulently through the partially compressed artery, creating the first audible tapping sounds.

Understanding the Korotkoff Phases

The Korotkoff sounds originate from a combination of:

• Turbulent blood flow
• Oscillations of the arterial wall 1

While five phases of Korotkoff sounds exist, no clinical significance has been attached to phases II and III 1. The clinically relevant phases are:

• Phase I (systolic): First appearance of clear tapping sounds
• Phase V (diastolic): Complete disappearance of sounds 1

Important Clinical Caveat

Phase I tends to underestimate the true systolic pressure when compared to direct intra-arterial measurement 1. The discrepancy between auscultatory and true intra-arterial pressure can be substantial—potentially as much as 25 mm Hg in some individuals 1. Recent research confirms that blood flow under the cuff does not begin exactly when cuff pressure falls just below systolic pressure, and this delay can lead to underestimation of systolic blood pressure by up to 24 mmHg 2.

Practical Application

When measuring blood pressure:

• Record the cuff pressure at the first audible Korotkoff sound as systolic pressure
• Be aware of the auscultatory gap in older patients with wide pulse pressure, where sounds may become inaudible between systolic and diastolic readings 1
• This gap can be eliminated by elevating the arm overhead for 30 seconds before inflating the cuff 1

The reliability of Phase I for systolic pressure determination has been validated in children as young as 1 month, where K1 provides a reliable estimate of systolic blood pressure 3.