Sphygmomanometer: Korotkoff Sounds & Blood Pressure

Patient Vitals

True systolic P_S 120 mmHg

True diastolic P_D 80 mmHg

Heart rate 72 bpm

Cuff deflation rate 3 mmHg/s

Auscultation

Cuff at rest

Cuff pressure

0 mmHg

Arterial pressure

0 mmHg

Detected systolic

—

Detected diastolic

—

How it works

A brachial-artery cuff is inflated above systolic pressure, fully occluding flow, then deflated slowly while a stethoscope listens over the artery. At any instant the vessel's transmural pressure is:

P_tm(t) = P_art(t) − P_cuff(t)

P_cuff > P_S (every beat): P_tm < 0 always — the artery stays fully collapsed, no blood crosses, silent.
P_D < P_cuff < P_S: P_tm swings from negative to positive each cardiac cycle. On systole the vessel snaps open through a narrow slit; continuity (A₁v₁=A₂v₂) forces the jet to accelerate, the Reynolds number crosses the laminar→turbulent threshold, and the resulting eddies radiate the audible Korotkoff sound. First sound = systolic reading (Phase I).
P_cuff < P_D (every beat): P_tm > 0 throughout the cycle — the artery never re-collapses, flow stays laminar and silent again (Phase V). Last sound heard = diastolic reading.

The arterial waveform itself is modelled as a fast systolic upstroke followed by a slower diastolic runoff each beat, matching the asymmetric shape of a real intra-arterial pressure trace. This turbulence-onset mechanism is exactly what an oscillometric automated cuff also exploits, just measured as pressure-oscillation amplitude in the cuff bladder instead of sound.

Blood / turbulent jet Cuff bladder

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Sphygmomanometer: Korotkoff Sounds & Blood Pressure Measurement