Pulse pressure variations to predict fluid responsiveness: influence of tidal volume

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Abstract
To evaluate the influence of tidal volume on the capacity of pulse pressure variation (ΔPP) to predict fluid responsiveness. Prospective interventional study. A 31-bed university hospital medico-surgical ICU. Sixty mechanically ventilated critically ill patients requiring fluid challenge, separated according to their tidal volume. Fluid challenge with either 1,000 ml crystalloids or 500 ml colloids. Complete hemodynamic measurements including ΔPP were obtained before and after fluid challenge. Tidal volume was lower than 7 ml/kg in 26 patients, between 7–8 ml/kg in 9 patients, and greater than 8 ml/kg in 27 patients. ROC curve analysis was used to evaluate the predictive value of ΔPP at different tidal volume thresholds, and 8 ml/kg best identified different behaviors. Overall, the cardiac index increased from 2.66 (2.00–3.47) to 3.04 (2.44–3.96) l/min m2 ( P <0.001). It increased by more than 15% in 33 patients (fluid responders). Pulmonary artery occluded pressure was lower and ΔPP higher in responders than in non-responders, but fluid responsiveness was better predicted with ΔPP (ROC curve area 0.76±0.06) than with pulmonary artery occluded pressure (0.71±0.07) and right atrial (0.56±0.08) pressures. Despite similar response to fluid challenge in low (<8 ml/kg) and high tidal volume groups, the percent of correct classification of a 12% ΔPP was 51% in the low tidal volume group and 88% in the high tidal volume group. ΔPP is a reliable predictor of fluid responsiveness in mechanically ventilated patients only when tidal volume is at least 8 ml/kg.