Prone position affects stroke volume variation performance in predicting fluid responsiveness in neurosurgical patients.

BACKGROUND: Stroke volume variation (SVV) during mechanical ventilation predicts preload responsiveness. We hypothesized that the prone position would alter the performance of this dynamic indicator.

METHODS: Two parallel groups of ventilated neurosurgical patients with low tidal volume (6-8 ml.kg-1) were studied before surgical incision. SVV was measured at T0, T15 and T30 min during a fluid volume expansion (250 mL hetastarch 6% over 30 min) with patients in either the supine (N.=29; Supine group) or prone position (N.=23; Prone group). Fluid responsiveness was defined as an increase in the stroke volume index (SVI) of ≥20% at T30. Receiver-operating characteristics (ROC) curves were generated for SVV.

RESULTS: Prone positioning significantly increased SVV. Volume expansion in the Prone group increased SVI but led to a decline in SVV from 16% (12-22; median, 25-75th percentile) at T0 to 9% (8-13%) at T30. These effects on SVI and SVV were more pronounced compared to those obtained in the Supine group (P ≤0.05). Fluid responsiveness was predicted by SVV >12% at T0 (sensitivity 88%, specificity 62%) in the Supine group. In the Prone group, the area under the ROC curve of SVV (0.53; 95% confidence interval 0.27-0.79) did not allow the determination of a threshold SVV value.

CONCLUSION: In ventilated patients with low tidal volume, a prone position may have a direct effect on the heart that alters the performance of SVV in predicting fluid responsiveness. External factor such as prone position renders difficult the interpretation of SVV as a dynamic indicator of cardiac preload.