Photoplethysmographic and pulse pressure variations during abdominal surgery

BACKGROUND: Respiratory variations in pulse pressure (ΔPP) predict fluid responsiveness during mechanical ventilation. Variations in pulse oximetry plethysmography amplitude (ΔPOP) are proposed as a non-invasive alternative. Large variations in ΔPOP and poor agreement between ΔPP and ΔPOP are found in intensive care unit patients. General anaesthesia is suggested to reduce variability of ΔPOP and improve agreement between the variables. We evaluated the variability of the agreement between and the diagnostic values of ΔPP and ΔPOP during ongoing open abdominal surgery. The variability of diagnostic methods in specific clinical conditions is important, as this reflects the stability over time during which clinical decisions are made. METHODS: Observational study during open abdominal surgery in general anaesthesia. ΔPP and ΔPOP were calculated semi-automatically from recording periods of approximately 5 min both before and after fluid challenges. Fluid responsiveness was evaluated by changes in stroke volume (oesophageal Doppler) after 250 ml colloid. RESULTS: Thirty-four fluid challenges were performed in 25 patients. Variance both within registration periods and between patients were significantly larger for ΔPOP than for ΔPP (54.1% vs. 22.1% and 69.6% vs. 22.6%, respectively, both P < 0.001). Limits of agreement with a regression-based correction were ± 13.9%. Areas under receiver operating characteristics curves for fluid responsiveness were 0.67 for ΔPP and 0.72 for ΔPOP. CONCLUSIONS: Analysis of raw signals during open abdominal surgery documents that the variance of ΔPOP is larger than of ΔPP, with wide limits of agreement between ΔPP and ΔPOP. The diagnostic values of ΔPP and ΔPOP are relatively poor.