Pulse oximetry plethysmographic waveform during changes in blood volume

Systolic pressure variation (SPV) and its dDown component have been shown t o be sensitive factors in estimating intravascular volume in patients under going mechanical ventilation. In this study, ventilation-induced changes in pulse oximeter plethysmographic waveform were evaluated after removal and after reinfusion of 10% estimated blood volume. The plethysmographic wavefo rm variation (SPVplet) was measured as the difference between maximal and m inimal peaks of waveform during the ventilatory cycle, and expressed as a p ercentage of the signal amplitude during apnoea. dUP(plet) and dDown(plet) were measured as the distance between the apnoeic plateau and the maximal o r minimal plethysmographic systolic waveform, respectively. Intravascular v olume was changed by removal of 10% of estimated blood volume and followed by equal volume replacement with Haemaccel. A 10% decrease in blood volume increased SPVplet from mean 17.0 (SD 11.8)% to 31.6 (28.0)% (P=0.005) and d Down(plet) from 8.7 (5.1)% to 20.5 (12.9)% (P=0.0005) compared with baselin e. Changes in plethysmographic waveform correlated with changes in arterial SPV and dDown (r=0.85; P=0.0009). In the absence of invasive arterial pres sure monitoring, ventilation-induced waveform variability of the plethysmog raphic signal measured from pulse oximetry is a useful tool in the detectio n of mild hypovolaemia.