Detection of fluid responsiveness by changes of perfusion index and pleth-variability index during passive leg raising in spontaneously breathing post-surgical patients: a prospective interventional study

Predicting fluid responsiveness is crucial in treating circulatory failure, as only about half of patients benefit from volume expansion by increasing cardiac output (CO). Dynamic tests like passive leg raising (PLR) are preferred over static parameters. While PLR reliably predicts fluid responsiveness, it typically requires invasive measurement of stroke volume (SV) or CO. The perfusion index (PI) and pleth variability index (PVI) are non-invasive metrics derived from oxygen saturation signals. PI has been shown to correlate with SV, and PVI has predicted fluid responsiveness in mechanically ventilated patients, but their role in spontaneously breathing patients remains unclear. This study aimed to assess whether PI and PVI could predict fluid responsiveness in post-surgical, spontaneously breathing patients during a PLR test. The hypothesis was that PI would increase and PVI would decrease in fluid responders. The prospective study included spontaneously breathing patients after major abdominal surgery in the ICU of Merano Hospital, Italy. SV and CO were measured using the FloTrac™ system, and PI and PVI were assessed using the Radical 7^® monitor. Patients were considered responders if SV increased by ≥ 10% during the PLR test. Of the 47 patients enrolled, 23 (48.9%) were fluid responders. The percentage change in PI from baseline to 60 s into the PLR test was + 41.2% in responders and + 11.3% in non-responders. A PI increase of ≥ 23% predicted responders with 70% sensitivity and 75% specificity, with an area under the ROC curve of 0.74. Twenty-two patients (47%) were inside the grey zone. PVI did not differ significantly between groups. In conclusion, PI could be a helpful non-invasive tool for predicting fluid responsiveness during a PLR test in spontaneously breathing patients, though its diagnostic accuracy appears to be moderate.