INDIRECT MEASUREMENT OF MEAN BLOOD-PRESSURE IN THE ANESTHETIZED PATIENT

Noninvasive measurement of mean pressure makes use of a principle first reported by Marey. Using a liquid-filled occluding chamber around an artery, he found that intra-arterial pulsations were transmitted to the liquid in the chamber. Subsequent investigators attempted to define the relationship between the amplitude of the externally measured pressure oscillation and the pressure within the vessel. Several investigators suggested that the occlusive pressure yielding maximum oscillation was very close to diastolic pressure. Subsequently, it was suggested that maximum oscillation occurred above diastolic pressure, perhaps above mean pressure. Two separate groups, using liquid-filled and air-filled chambers, independently reported that the minimum occlusive pressure for maximum oscillation was equal to mean arterial pressure. This conclusion was supported in more sophisticated studies, examining both isolated arterial segments and intact arteries in extremities encircled by compressible air bladders. Unfortunately, none of these studies report data permitting statistical analysis of the accuracy and reliability of the reported correlation. All of the reported techniques were technically difficult. We used special computer-assisted instrumentation for validation of the indirect technique presented. However, a commercial instrument that automatically determines mean blood pressure using this technique is now available.