The influence of tidal volume on the dynamic variables of fluid responsiveness in critically III patients

Respiratory-related variabilities in stroke volume and arterial pulse pressure (Delta%Pp) are proposed to predict fluid responsiveness. We investigated the influence of tidal volume (V-T) and adrenergic tone on these variables in mechanically ventilated patients. Cyclic changes in aortic velocity-time integrals (Delta%VTIAo, echocardiography) and Delta%Pp (catheter) were measured simultaneously before and after intravascular volume expansion, and V-T was randomly varied below and above its basal value. Intravascular volume expansion was performed by hydroxyethyl starch (100 mL, 60 s). Receiver operating characteristic curves were generated for Delta%VTIAo, Delta%Pp and left ventricle cross-sectional end-diastolic area (echocardiography), considering the change in stroke volume after intravascular volume expansion (>= 15%) as the response criterion. Covariance analysis was used to test the influence of V-T on Delta%VTIAo and Delta%Pp. Twenty-one patients were prospectively included; 9 patients (43%) were responders to intravascular volume expansion. Delta%VTIAo. and Delta%Pp were higher in responders compared with nonresponders. Predictive values of Delta%VTIAo and Delta%Pp were similar (threshold: 20.4% and 10.0%, respectively) and higher than that of left ventricle cross-sectional end-diastolic area at the appropriate level of V-T. Delta%Pp was slightly correlated with norepinephrine dosage. Delta%Pp increased with the increase in the level of V-T both before and after intravascular volume expansion, contrasting with an unexpected stability of Delta%VTIAo. In conclusion, Delta%VTIAo and Delta%Pp are good predictors of intravascular fluid responsiveness but the divergent evolution of these two variables when V-T was increased needs further explanation.