Femororadial Pressure-delta and Thermoregulation

Abstract

Professor, Department of Anesthesia, The Bowman Gray School of Medicine, Medical Center Boulevard, Winston-Salem, North Carolina 27157-1009.To the Editor:--Hynson et al. [1]attempted to elucidate the mechanism of the aortoradial pressure difference (A-R delta) sometimes seen at the conclusion of cardiopulmonary bypass (CPB). They measured femoral and radial arterial pressures and finger, skin, and forearm blood flows in six healthy 20-40-yr-old volunteers exposed first to cold temperature (21 degrees Celsius) until shivering; second, to warm air (42 degrees Celsius) until profuse sweating; third, to cold temperature; and fourth, to anesthesia with nitrous oxide-propofol. The authors concluded that thermoregulation and anesthesia produced the post-CPB A-R delta by increasing "upper-extremity blood flow."Although thermoregulation may play an indirect role in the A-R delta during anesthesia, this study in young, healthy volunteers, does not provide the data needed to suggest that conclusion. The typical patient affected by post-CPB A-R delta is older than 52 yr with various degrees of vascular disease, has undergone coronary artery bypass grafting (CABG), [2-6]and CPB, and, in most instances, has been cooled temporarily to a core temperature of 28 degrees Celsius. In adolescents and young adults, the peripheral (brachial, radial, femoral) arterial pulse pressure is about 50% greater than that measured in the ascending aorta, whereas in elderly subjects, the two measurements are virtually identical. [7]Thus, the two patient populations bear little resemblance to each other.The authors intended to measure forearm blood flow (FBF) but do not mention whether the circulation to the hand was excluded during the FBF measurement. I suspect that the authors never measured FBF properly. This is supported by their finding that propofol/nitrous oxide anesthesia increased FBF. Other investigators, well acquainted with plethysmography, find that both anesthetics decrease FBF, [8]but that nitrous oxide increases total hand blood flow (HBF). [9]Failure to exclude the hand circulation during the FBF measurement also would explain the large increase in FBF during sweating. Body heating produces a large increase in HBF and only a mild increase in FBF. [10]A second potential problem concerns the authors' technique for assessment of finger blood flow (fbf) with venous occlusion plethysmography. The authors state that the fbf was measured "as previously described." [11]In the study quoted, two requirements to accurately measure flows by venous occlusion plethysmography were ignored: the venous occluding cuff should be in close proximity to the plethysmograph (in the study referred to by Rubinstein and Sessler, [11]the plethysmograph covered the last phalanx, and the venous tourniquet was at the base of the finger, leaving a large venous bed to be filled before blood flow could be detected); and flow should be calculated during the first few seconds after the occlusion. [12,13]Burch [12]demonstrated, using both calculations and real flow measurements, how critical it is to include only the first three pulsations after the occlusion. However, Hynson et al. compared high and slow flows as described by Rubenstein and Sessler. [11]The latter authors traced high flows during the first 5 s after venous occlusion and slow flows after an interval of approximately 7 s after venous occlusion. In Figure 1, I have traced (dotted line) the slope of the high flow in a comparable time scale with the slow flow. It can be seen that any real difference was smaller than what they reported; however, neither tracing reflect real fbfs.In conclusion, this study does not provide any useful data by which the cause of the post-CPB A-R delta can be determined, because the wrong population was studied and the wrong measurements were performed.Alfredo L. Pauca, M.D., Professor, Department of Anesthesia, The Bowman Gray School of Medicine, Medical Center Boulevard, Winston-Salem, North Carolina 27157-1009.(Accepted for publication July 3, 1995.)