Influence of tidal volume on left ventricular stroke volume variation measured by pulse contour analysis in mechanically ventilated patients

Objective: Real-time measurement of stroke volume variation by arterial pulse contour analysis (SVV) is useful in predicting volume responsiveness and monitoring volume therapy in mechanically ventilated patients. This study investigated the influence of the depth of tidal volume (V-t) on SVV both during the state of fluid responsiveness and after fluid loading in mechanically ventilated patients. Design and setting: Prospective study in a university hospital, adult cardiac surgery intensive care unit. Patients and participants: 20 hemodynamically stable patients immediately after cardiac surgery. Interventions: Stepwise fluid loading using colloids until stroke volume index (SVI) did not increase by more than 10%. Before and after fluid loading V-t was varied (5, 10, and 15 ml/kg body weight) in random order. Measurements and results: Pulse contour SVV was measured before and after volume loading at the respective V-t values. Thirteen patients responded to fluid loading with an increase in SVI greater than 10%, which confirmed volume responsiveness at baseline measurements. These were included in further analysis. During volume responsiveness SVV at V-t of 5 ml/kg (7+/-0.7%) and SVV at V-t of 15 ml/kg (21+/-2.5%) differed significantly from that at V-t of 10 ml/kg (15+/-2.1%). SVV was correlated significantly with the magnitude of V-t. After volume resuscitation SVV at the respective V-t was significantly reduced; further, SVV at V-t of 5 ml/kg(-1) (5.3+/-0.6%) and 15 ml/kg (16.2+/-2.0%) differed significantly from that at V-t of 10 ml/kg (10.2+/-1.0%). SVV and depth of V-t were significantly related. Conclusions: In addition to intravascular volume status SVV is affected by the depth of tidal volume under mechanical ventilation. This influence must be regarded when using SVV for functional preload monitoring.