Korotkoff sounds

Tom Cunningham is not convinced Korotkoff sounds are caused by turbulent blood flow. He looks into how those important everyday sounds reach your stethoscope

Measuring blood pressure is one of the first practical skills that medical students are taught. Throughout our careers we will do this simple procedure countless times. We know that we listen for noises and these correlate to the systolic and diastolic pressure. But have you ever wondered what causes these sounds? I was taught that the Korotkoff sounds are due to turbulent blood flow. But I was not convinced and decided to look into it further.

In 1905 during a conference at the Imperial Medical Academy in St Petersburg, Dr Nicolai Korotkoff announced a new method to determine blood pressure.¹ He placed a stethoscope over the brachial artery and inflated a rubber cuff around the upper arm. Korotkoff found that as the cuff deflated he heard different noises--snapping, murmur-like noises, and muffled tones.²

Since then several different theories for what causes Korotkoff sounds have been suggested: the cavitation theory,³ the arterial wall theory,¹ the turbulence theory,⁴ the transmission of heart sounds theory,⁵ and the water hammer theory.⁶

Cavitation theory

This was first described by Malcolm⁷ in 1957 and most recently by Venet and colleagues³ in 2000. Cavitation occurs when a sudden drop in pressure causes dissolved gases in a liquid to convert into their gaseous form. When measuring blood pressure, the cuff is inflated so that the brachial artery is completely occluded and leaves only a tiny film of blood between the walls of the artery. When the cuff deflates the artery becomes patent, causing a large drop in pressure. Dissolved gases directly under the cuff revert to their gaseous form producing cavities (bubbles) (see fig 1). As blood from the heart enters the brachial artery the cavities collapse and the energy released is converted as sound.

Fig 1 Cavitation theory. As the cuff deflates, the artery widens and pressure drops. The bubbles that form burst, producing sound

Cavitation theory was supported by Ernest and colleagues, who suggested that Korotkoff sounds are produced before the blood reaches its maximum velocity.⁸ Cavitation does not, however, explain the reason for the different tones heard through the stethoscope. And Bruns also gave evidence to suggest that cavitation only occurred at pressures of about 350 mm Hg, which are unlikely in clinical practice.⁹

Arterial wall theory

"Any membrane passing from flaccidity to sudden overstretching always makes a sound", concluded Rouanet in 1832 in relation to the atrioventricular valves in the heart.¹⁰ This can also be applied to blood vessels. As the cuff is inflated the brachial artery beneath collapses (fig 2). The membrane becomes lax as blood is emptied from the artery due to pressure from the cuff. As the cuff deflates, the artery refills with blood causing the membrane to overstretch. This sudden stretching gives rise to sound.

Fig 2 Arterial wall theory. Pressure from the cuff causes the artery to empty; the membrane is flaccid. When the cuff deflates, the artery refills with blood causing the membrane to suddenly overstretch. The sudden stretching gives rise to a sound

Evidence from Ur and Gordon showed that a sharp sound recorded with a microphone corresponded to the transient pressure drop and also confirmed that changing different properties such as the length of the artery or the material of the tube would affect the quality of the sound produced.¹¹ However Chungcharoen replaced an arterial segment with a glass tube and managed to produce noises similar to Korotkoff sounds, challenging the hypothesis that the sounds are produced by movements of the artery wall.⁴

Turbulence theory

Lange and Hecht were the first to describe turbulence theory (see fig 3).¹² They noticed that there was no pressure phenomenon at the source of the sound at the instant it is produced. This theory proposes that sounds are produced because of turbulent blood flow. As the cuff deflates, blood from the heart enters the patent artery. When the artery is compressed, the blood proximal to the cuff has a considerable amount of potential energy. As the cuff deflates, blood from the heart enters the artery under the cuff (a system of low energy). This system of low energy absorbs the energy of the blood slowing the blood down and causing turbulent blood flow. The energy lost in this process could be significant enough to be recognised as sound energy as described by Chungcharoen.⁴ Esnest and colleagues also thought that turbulent blood flow produced sound but attributed it to the murmur-like tones rather than the tapping tones.⁸ Their evidence was that blood flow at the time the tapping sound was emitted was negligible or just beginning to accelerate.

Conversely Bruns thought that turbulence could not be responsible for the sounds⁹: "If turbulence caused murmurs, all murmurs would have the same frequency."

Other possibilities

Other suggestions for the origin of Korotkoff sounds are the water hammer and transmission of heart sounds theories. These theories have been consistently disproved by various studies.

Other authors, such as Ur and Gordon, think that Korotkoff sounds are due to a combination of the concepts mentioned above.¹¹ They believe that the oscillation in the arterial wall and the turbulence theory cause the tapping tones and murmur-like tones respectively.

Which is right?

The cavitation theory, the arterial wall theory and the turbulence theory are the three most popular theories that have been put forward to explain the origin of Korotkoff sounds. There is evidence to support elements of each of these theories, but further investigations would have to be completed to formally establish the cause of the sounds. The experiments I evaluated used collapsible tubes or non-living arteries, which limited the reliability of the results. Subsequent experiments would therefore have to be carried out on willing patients. Any volunteers?

Fig 3 Turbulence theory. Blood proximal to the cuff has a considerable amount of potential energy. When the cuff deflates, the blood enters the artery under the cuff (a system of low energy). This system of low energy absorbs the energy of the blood slowing the blood down and causing turbulent blood flow

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