Korotkoff sounds do not provide an accurate estimate of systolic and diastolic pressure in sphygmomanometry and cannot be used as a calibration standard for nibp devices

Objective: Noninvasive blood pressure (NIBP) devices are calibrated against validated auscultation sphygmomanometers using Korotkoff sounds. This study aimed to investigate the timing of Korotkoff sounds in relation to pulse appearance in the brachial artery and values of intra-arterial blood pressure. Design and method: Experiments were carried out on 15 participants, (14 males, 64.3 ±10.4 years; one female, 86 yo), undergoing coronary angiography. A conventional (oscillometric) BP cuff, with a microphone for Korotkoff sounds, was placed on the upper arm. Intra-arterial brachial artery pressure (IABP) was measured below the cuff with a fluid-filled catheter inserted via the radial artery and an external transducer. Finger photoplethysmography was used to measure brachial pulse wave velocity (PWV). Korotkoff sounds were processed electronically and custom algorithms identified the cuff pressure (CP) at which the first and last Korotkoff sounds were heard. PWV and max slope of the IABP pressure pulse were recorded to estimate arterial stiffness. Results: The brachial artery closed at a CP of 132.0±17.1 mmHg. Systolic (SBP), and diastolic (DBP) pressure were 147.6±14.3 and 72.7±10.1mmHg; mean pressure (MP, 100.1±10.4 mmHg) was similar to MP derived from the peak of the oscillogram (98.5±13.6 mmHg). Difference between IABP and CP recorded at first and last occurrence of Korotkoff sounds were, SBP: 18.9±8.3 (range 2-29) mmHg, DBP: 4.0±4.3 (range -1.6 -12.4) mmHg. Multiple linear regression showed that differences increase with arterial stiffness and can be compensated for using a prediction equation for calculation of true SBP (tSBP) using heart rate (HR) and Korotkoff derived systolic (SBPk) and diastolic (DBPk), at least over the limited age range for this study cohort. Conclusions: SBP and DBP derived from the onset and termination of Korotkoff sounds can underestimate IABP by up to 29 mmHg. Since Korotkoff sounds are the recommended method mandated by the universal standard for the validation of blood pressure measuring devices, these errors are propagated through to all NIBP measurement devices irrespective of whether they use auscultatory or oscillometric methods thus explaining the well known differences recorded between NIBP measurements and intra-arterial BP measurements.