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CARDIOVASCULAR ANESTHESIA

How Does the Plethysmogram Derived from the Pulse Oximeter Relate to Arterial Blood Pressure in Coronary Artery Bypass Graft Patients?

Department of Anesthesiology, Yale University School of Medicine, New Haven, Connecticut

Address correspondence & reprint request to Kirk H. Shelley, MD, PhD, Department of Anesthesiology, Yale University School of Medicine, 333 Cedar Street, TMP-3, PO Box 208051, New Haven, CT 06520-8051. Address e-mail to kirk.shelley{at}yale.edu

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 extract 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 patients. 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_calc) in the next 10 patients as:

equation

where i = systolic BP, mean BP, and pulse pressure. The initial bias was too large to be clinically useful. To improve clinical applicability a period of calibration was 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 systolic BP, mean BP and pulse pressure bias values were -2.6, -1.88 and -1.28 mm Hg, and the precision values were 15.9 10.09, and 9.94 mm Hg, respectively. The present attempt to develop a clinically useful method of noninvasive BP measuring was partly successful with the requirement of a calibration period.

IMPLICATIONS: Statistical comparison was made between measured blood pressure (BP) from arterial line and calculated BP derived from ear pulse oximeter waveform in 10 patients undergoing coronary artery bypass graft surgery. Using 62,077 paired readings, the mean difference for systolic BP, mean BP, and pulse pressure between the 2 methods was -2.6, -1.88, and -1.28 mm Hg, respectively.

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