The medical practice of measuring blood pressure by constricting the upper arm and listening with a stethoscope to assess when the first sound and final sounds are audible dates back over 100 years ago. This method is still considered the gold standard for noninvasive blood pressure measurement. While there have been advancements in the technique for upper arm constriction, the quality of stethoscopes, and most recently fully automatic blood pressure devices – the overall process has not changed significantly. The discovery and utilization of Korotkoff sounds (KorS) are what allows physicians to check patient blood pressures and provide appropriate medical treatments. Understanding the underlying physiology and proper measurement techniques are important for quality patient care and appropriate medical therapy.

Korotkoff sounds (KorS) are the audible noises used to measure blood pressure. Throughout the short process of taking a blood pressure measurement, the sounds change a phenomenon which has undergone extensive study. Korotkoff sounds are broken down into five distinct phases, each with a distinct sound and waveform associated with the phase. Phase I has a clear tapping tone. Phase II is associated with a softening of the tapping and a swishing element. Phase III sounds like phase I but with distinct sharpening. Phase IV is noted to have abrupt muffling of the sounds, followed by Phase V which is the cessation of all sounds. When measuring blood pressure, a cuff is applied to the upper arm and inflated to the point that blood is no longer able to flow through the brachial artery. When pressure is released to point blood is again able to flow, phase I begins. As pressure continues to decrease the examiner can hear the different phases, ultimately concluding with phase V when no sound is audible. The first sound appears as the pressure approaches systolic blood pressure (SBP) and the diastolic blood pressure (DBP) measurement is by the disappearance of sound.[1] Despite 100 years of debate and research, the underlying physiology of Korotkoff sounds is not fully understood. There are several theories that are prominent in the literature trying to explain the underlying cause of Korotkoff sounds. There are two main schools of thought guiding the research of Korotkoff sounds; the sounds are a result of fluid turbulence or the result of arterial wall oscillations. Adding a layer of complexity, those who test the arterial walls are uncertain what the best method of experimentation is. For example, Dr. Charles Babbs used a primarily mathematical approach to analyze wall motion mechanics, while Drzewiecki et al. used an electrical analogy in attempts to postulate the equations governing the fluid dynamics and nonlinear compliance of arterial segments. While there are many ideas, two commonly speculated theories for the causation of the sounds heard in stethoscopes will be briefly discussed here.[2]

Dr. Nikolai Korotkov is the physician credited with discovering the sounds that carry his name. He speculated the sounds resulted from the closing and opening of the vessel. He suspected the net transmural pressure oscillating from positive to negative was responsible for the different phases of sound. Dr. Charles Babbs, conducted an experiment measuring blood pressure and the associated Korotkoff sounds, concluding the sounds were from the transition of the vessel from a buckled to fully expanded the state. The transient vibrations of the artery being the mechanism for actual sound production. The study, while not universally accepted as proof for the physiologic source of Korotkoff sounds. It was able to explain how venous engorgement downstream to the collapsed artery adds to the increased resistance, preventing artery collapse, as the cause of the muffling heard in phase IV.[2]

A study performed at Chengdu University in China demonstrated the importance of proper technique when measuring blood pressure (BP). The study concluded that different stethoscope positioning results in sounds being present sooner/later, resulting in higher/lower BP readings than the actual blood pressure. Over the last 100 years, it has been widely accepted that hearing the first sound is in direct correlation with SBP. However, there is still discourse regarding the measurement of the DBP. It has actually become known as the “diastolic dilemma” since the argument is whether the DBP should be measured during phase IV when the sound muffles or in phase V when the sound disappears completely. The position of the stethoscope also affects auscultation and influences the DBP reading by up to 10 mm Hg. The study was able to recommend stethoscope positioning which most accurately represented the DBP, via comparison to an invasive blood pressure measurement simultaneously. The study concluded the proper placement of the cuff, just above the elbow, with the stethoscope under the cuff provided the most accurate blood pressure readings. This study along with many others have demonstrated how a seemingly simple procedure can have extensive variability, dependent on multiple factors (cuff pressure, circumferential and longitudinal stethoscope positioning, and sound cutoff). Ongoing research and continued technological advancements will hopefully definitely pinpoint the true physiology responsible for Korotkoff sounds, enabling even more precise blood pressure readings and patient care.[2][1][3][4][5]