To the Editor:-Findlay et al. recommended using toe pulse oximetry for patients in exaggerated lithotomy position, claiming it provides “reassuring documentation throughout the procedure that distal perfusion is being achieved.” Despite their disclaimer that pulse oximetry “cannot guarantee adequate perfusion in all parts of the limb,” we are concerned that their recommendation may lead some to a false sense of confidence.
The algorithm in pulse oximeters is designed exclusively to determine oxyhemoglobin saturation, not blood flow. Although pulsatile flow is necessary for the pulse oximeter proper function, Lawson et al. have shown that pulse oximeters are sensitive enough to continue to function with less than 10% normal flow. Ten percent normal flow to a toe tells us little about the flow to the muscle masses of the lower extremity and the potential for a compartment syndrome. 
Further, Graham et al. demonstrated with replanted and revascularized fingers that the sudden loss of the plethysmographic signal may indicate arterial occlusion and that a continuous slow decrease in pulse oximetric saturation may indicate diminished tissue perfusion as a result of venous occlusion. However, in the case of successfully replanted toes, they were never able to obtain a pulse oximeter reading.
Therefore, with the pulse oximeter on a toe, there may be adequate flow with no pulse oximeter reading or markedly reduced flow with satisfactory pulse oximetric output. We do not find this at all reassuring.
We would consider a sudden change in the toe pulse oximeter reading or a large discrepancy between the toe and finger pulse oximetry values as cause for concern and further investigation, but we do not find normal pulse oximetry reassuring regarding flow and perfusion. Unfortunately, we still await a reliable, noninvasive, widely available monitor of tissue perfusion.
Juraj Sprung, M.D., Ph.D.
Department of General Anesthesiology; The Cleveland Clinic Foundation; 9500 Euclid Avenue; Cleveland, Ohio 44195
Denis Bourke, M.D.
Chief, Anesthesiology Service; Department of Anesthesiology; University of Maryland and Baltimore Veterans Affairs Medical Center; 10 North Greene Street; Baltimore, Maryland 21201
(Accepted for publication December 9, 1997.)