The effects of passive leg raising may be detected by the plethysmographic oxygen saturation signal in critically ill patients

BackgroundA passive leg raising (PLR) test is positive if the cardiac index (CI) increased by >10%, but it requires a direct measurement of CI. On the oxygen saturation plethysmographic signal, the perfusion index (PI) is the ratio between the pulsatile and the non-pulsatile portions. We hypothesised that the changes in PI could predict a positive PLR test and thus preload responsiveness in a totally non-invasive way.MethodsIn patients with acute circulatory failure, we measured PI (Radical-7) and CI (PiCCO2) before and during a PLR test and, if decided, before and after volume expansion (500-mL saline).ResultsThree patients were excluded because the plethysmography signal was absent and 3 other ones because it was unstable. Eventually, 72 patients were analysed. In 34 patients with a positive PLR test (increase in CI10%), CI and PI increased during PLR by 2110% and 5453%, respectively. In the 38 patients with a negative PLR test, PI did not significantly change during PLR. In 26 patients in whom volume expansion was performed, CI and PI increased by 28 +/- 14% and 53 +/- 63%, respectively. The correlation between the PI and CI changes for all interventions was significant (r=0.64, p<0.001). During the PLR test, if PI increased by >9%, a positive response of CI (10%) was diagnosed with a sensitivity of 91 (76-98%) and a specificity of 79 (63-90%) (area under the receiver operating characteristics curve 0.89 (0.80-0.95), p<0.0001).Conclusion An increase in PI during PLR by 9% accurately detects a positive response of the PLR test.Trial registration ID RCB 2016-A00959-42. Registered 27 June 2016.