In Reply:-We are aware of the issues raised by both correspondents regarding the amplification potential of currently used pulse oximeters and also that the relationships between flow in different regions of an extremity, blood pressure, and the pulsatile flow required to generate a pulse oximeter response are not simple. Research is currently underway at this institution to investigate some of these matters. However, even with the limitations of pulse oximetry in this circumstance, our technique identifies events (loss of a previously present waveform) that imply markedly decreased perfusion and require a response from the monitoring anesthesiologist (in our experience, increasing the blood pressure returns the waveform). That we identify these events would seem to us better than identifying no events at all-the situation if no monitor was used.

Dr. Sessler proposes an alternative monitoring method involving the use of a blood pressure cuff on the ankle. We would be interested in hearing how this fares when used in practice.

What is required, of course, is a clinical study documenting the incidence of complications attributable to poor perfusion during surgery in the exaggerated lithotomy position and whether that incidence is influenced by the use of monitoring; until this is done, we merely speculate.

James Y. Findlay, M.B.Ch.B., F.R.C.A.

Department of Anesthesiology; Mayo Foundation; 200 First Street SW; Rochester, Minnesota 55905

(Accepted for publication December 9, 1997.)