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 (Vt) 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 Vt 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 Vt 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 Vt of 5 ml/kg (7±0.7%) and SVV at Vt of 15 ml/kg (21 ± 2.5%) differed significantly from that at Vt of 10 ml/kg (15±2.1%). SVV was correlated significantly with the magnitude of Vt. After volume resuscitation SVV at the respective Vt was significantly reduced; further, SVV at Vt of 5 ml/kg-1 (5.3±0.6%) and 15 ml/kg (16.2±2.0%) differed significantly from that at Vt of 10 ml/kg (10.2±1.0%). SVV and depth of Vt 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.
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