How Does the Plethysmogram Derived from the Pulse Oximeter Relate to Arterial Blood Pressure in Coronary Artery Bypass Graft Patients? Aymen A. Awad, MD, M. Ashraf M. Ghobashy, MD, Robert G. Stout, MD, David G. Silverman, MD, and Kirk H. Shelley, MD, PhD Department of Anesthesiology, Yale University School of Medicine, New Haven, Connecticut Twenty patients scheduled for coronary artery bypass grafting had their ear and finger oximeter and radial ar- tery 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 mea- surements. The Bp meas waveforms were analyzed to mea- sured systolic blood pressure (BP), mean BP, and pulse pressure. The correlation coefficient was determined be- tween 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: BP calc.i = a i +  b i  Width ear ) 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 suc- cessful with the requirement of a calibration period. (Anesth Analg 2001;93:1466 –71) O ver the last decade, the pulse oximeter has be- come a standard clinical monitor in both the operating room and intensive care environment. This noninvasive monitor normally provides informa- tion about arterial oxygen saturation and heart rate (1). Pulse oximetry works by using light at 2 wave- lengths (660 –940 nm). The light is transmitted through tissues, sensed by a photodetector, amplified, and pro- cessed (2). The arterial oxygen saturation and heart rate are displayed on a screen. With additional anal- ysis, it may be possible for the same device to measure other important clinical variables. A high correlation has been found between plethysmographic wave- forms and blood pressure oscillations in normal sub- jects by using spectral-domain analysis. Furthermore, both measurements respond similarly to cardiovascu- lar challenge, i.e., sympathetic activation induced by active standing (3). Despite the availability of the pulse oximeter waveform on most modern monitors, the underlying physiology is not well understood. The lack of understanding requires an empiric approach to its analysis. This article outlines an attempt to measure blood pressure (BP) noninvasively on a beat-to-beat basis by using the pulse oximeter waveform. Methods Data Collection With IRB approval, 20 patients (14 males and 6 fe- males) scheduled for elective coronary artery bypass graft surgery (CABG) at Yale-New Haven Hospital had their ear and finger oximeter and radial artery BP waveform signals collected. Patients with an intracar- diac shunt, aortic or mitral valve disease, or poor ejection fraction (30%) were excluded. Anesthesia consisted of a narcotic-based technique with inhaled anesthesia (isoflurane) as dictated by clinical practice. A Bair Hugger warming unit (Model 505; Augustine Medical, Inc., Eden Prairie, MN) was applied at the start of the procedure. Accepted for publication July 25, 2001. 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@yale.edu. ©2001 by the International Anesthesia Research Society 1466 Anesth Analg 2001;93:1466–71 0003-2999/01