How does the plethysmogram derived from the pulse oximeter relate to arterial blood pressure in coronary artery bypass graft patients?

Twenty patients scheduled for coronary artery bypass grafting had their ear and finger oximeter and radial artery blood pressure (Bp(meas)) waveforms collected. The ear and finger pulse oximeter waveforms. were analyzed to ex tract beat-to-beat amplitude and area and width measurements. The Bp(meas) waveforms were analyzed to measured systolic blood pressure (BP), mean BP, and pulse pressure. The correlation coefficient was determined between the derived waveforms from the pulse oximeter and Bp(meas) for the first 10 pat ients. The ear pulse oximeter width (Width(Ear)) had the best correlation ( r = 0.8). Linear regression was done between Width(Ear) and Bp(meas) based on slope (b) and intercept (a) values, BP was calculated (Bp,,,,) in the ne xt 10 patients as: BPcalc.i = a(i) + (b(i) X Width(ear)) where i = systolic BP, mean BP, and pulse pressure. The initial bias was too large to be clin ically useful. To improve clinical applicability a period of calibration wa s introduced in which the first 50 readings of Width(Ear) and Bp(meas) for each patient were used to calculate the intercept. After calibration the sy stolic BP, mean BP and pulse pressure bias values were -2.6, -1.88 and -1.2 8 mm Hg, and the precision values were 15.9 10.09, and 9.94 mm Hg, respecti vely. The present attempt to develop a clinically useful method of noninvas ive BP measuring was partly successful with the requirement of a calibratio n period.