What are Korotkoff sounds?

Korotkoff Sounds: Definition, Phases, and Clinical Significance

Korotkoff sounds are auditory phenomena heard during blood pressure measurement that originate from a combination of turbulent blood flow and oscillations of the arterial wall when a partially occluded artery is gradually released from compression. 1

Mechanism and Origin

Recent research using ultrafast ultrasound imaging has revealed that Korotkoff sounds are not actually sound waves emerging from the brachial artery as traditionally thought, but rather shear vibrations conveyed in surrounding tissues by nonlinear pulse wave propagation. When these shear vibrations reach the stethoscope, they correlate with the audible Korotkoff sounds. 2

The Five Phases of Korotkoff Sounds

Korotkoff sounds are traditionally classified into five distinct phases during blood pressure measurement:

1. Phase I: Appearance of clear tapping sounds corresponding to the appearance of a palpable pulse. This marks systolic blood pressure.
2. Phase II: Sounds become softer and longer.
3. Phase III: Sounds become crisper and louder.
4. Phase IV: Sounds become muffled and softer.
5. Phase V: Sounds disappear completely. This marks diastolic blood pressure. 1

Clinical Significance

• Systolic Pressure: The onset of Phase I corresponds to systolic pressure but tends to underestimate the true intra-arterial systolic pressure.
• Diastolic Pressure: The disappearance of sounds (Phase V) corresponds to diastolic pressure but tends to occur before the true intra-arterial diastolic pressure. 1
• Phases II and III: No specific clinical significance has been attached to these phases. 1

Accuracy and Limitations

• The Korotkoff sound method tends to give values for systolic pressure that are lower than true intra-arterial pressure, and diastolic values that are higher.
• The discrepancy between auscultatory and intra-arterial measurements can be as much as 25 mmHg in some individuals. 1
• There has been historical disagreement about whether Phase IV or V should be used for recording diastolic pressure, but current consensus favors Phase V. 1

Special Considerations

Auscultatory Gap

• In older patients with wide pulse pressure, Korotkoff sounds may become temporarily inaudible between systolic and diastolic pressure, then reappear as cuff deflation continues.
• This phenomenon, known as the auscultatory gap, is most likely to occur in subjects with target organ damage. 1
• Management tip: The gap can often be eliminated by elevating the arm overhead for 30 seconds before inflating the cuff, then bringing the arm to the usual position. This reduces vascular volume in the limb and improves inflow to enhance the Korotkoff sounds. 1, 3

Special Populations

Phase V may not be reliable in:

• Pregnant women
• Patients with arteriovenous fistulas (e.g., for hemodialysis)
• Patients with aortic insufficiency 1

In these cases, Phase IV may need to be used instead.

Variability and Quality

• Research shows significant variability in Korotkoff sounds, with 80.7% of beats at systolic BP and 69.5% at diastolic BP being analyzed as true Korotkoff sounds. 4
• Greater within-subject variability is observed at systolic (21.4%) and diastolic (28.9%) points. 4
• The quality of Korotkoff sounds is better when using the bell of the stethoscope placed over the brachial artery pulse rather than the diaphragm placed over the cubital fossa. 5

Modern Applications

Visual assessment of Korotkoff sounds can improve the accuracy of automated blood pressure monitors. When poor-quality Korotkoff sound readings are excluded, measurement accuracy improves significantly. 6

Understanding the mechanisms and limitations of Korotkoff sounds is essential for accurate blood pressure measurement, which remains fundamental to cardiovascular risk assessment and management.