The underlying principles of Korotkoff sound (KorS) during blood pressure measurement and its waveform characteristic changes with cuff pressure and stethoscope position have not been fully understood. This study aimed to quantify the effects of cuff pressure and stethoscope position on the measured KorS waveform characteristics. Thirty healthy subjects were recruited in this study. Four stethoscopes were placed on the circumferential direction around the arm (m1, m2, m3 and m4; m1 was above the artery, and equal distance between each other), and then sequentially at three different longitudinal positions ('upper', 'middle' and 'low' part under the cuff). At each longitudinal position, three levels of static cuff pressure (high: SBP + 10 mmHg, low: DBP-10 mmHg, and medium: DBP + (SBP-DBP)/3) were applied during the recording of KorS waveform. The averaged KorS waveform was firstly computed by using an interpolation method, separately for measurements from different stethoscope locations and cuff pressures. Two quantitative indices were derived to characterize the recorded KorS waveform: intensity amplitude and high-level duration of KorS waveform. Post-hoc pairwise comparisons after analysis of variance were used to compare the waveform characteristic differences between different stethoscope locations and between cuff pressures. Variance analysis demonstrated that the effects of stethoscope circumferential and longitudinal positions and cuff pressure on the two KorS waveform indices were significant (all p < 0.001). In detail, KorS waveform recorded at cuff pressure PMEDIUM had larger intensity amplitude and shorter high-level duration than those recorded at cuff pressure PHIGH or PLOW. In most conditions, the stethoscope above the artery (m1) produced the largest RMS intensity amplitude and shortest high-level duration, while the stethoscope at the opposite location of m1 generated the smallest RMS intensity amplitude and longest high-level duration. In terms of the effect of longitudinal position, the stethoscopes below the middle of the cuff always produced KorS recordings with larger intensity amplitude and shorter high-level duration. This study has quantified and provided scientific evidence that cuff pressure, stethoscope longitudinal and circumferential positions are important factors influencing KorS waveform characteristics.
Keywords: Cuff pressure; Korotkoff sound; Stethoscope position; Waveform characteristic.