The influence of prone positioning on the accuracy of calibrated and uncalibrated pulse contour-derived cardiac index measurements

BACKGROUND: Patients with lung failure who undergo prone positioning often receive extended hemodynamic monitoring. We investigated the influence of modified prone positioning (135°) on the accuracy of pulse contour-derived calibrated cardiac index (CIPC) and uncalibrated cardiac index (CIVIG) in this patient population with transpulmonary thermodilution (TPTD) as reference technique.

METHODS: We studied 16 critically ill and mechanically ventilated patients (11 men, 5 women, aged 20-71 years) with acute lung injury or acute respiratory distress syndrome. Patients were monitored by TPTD with an integrated calibrated pulse contour technique (PiCCO®) and by uncalibrated pulse contour analysis (FloTrac/Vigileo™). Before prone positioning, cardiac index (given in L·min(-1)·m(-2)) was measured by TPTD (CITPTD) and CIPC was calibrated. After positioning, CIPC and CIVIG were read from the monitor and CITPTD was measured. After 8 to 10 hours, prone positioning was completed and measurements were performed analogously. Bland-Altman analysis based on a random-effects model was used to calculate limits of agreement (LOA) and percentage errors. Polar plots were used for trend analysis.

RESULTS: Supine CITPTD was 3.3 ± 0.9 (mean ± SD) and CIVIG was 3.1 ± 0.8. After proning, CIPC was 3.5 ± 0.8, CIVIG 3.3 ± 0.8, and CITPTD 3.6 ± 0.8. Before repositioning, CITPTD was 3.5 ± 0.7 and CIVIG 3.3 ± 1.0. After repositioning, CITPTD was 3.1 ± 0.7, CIPC 3.3 ± 0.7, and CIVIG 2.9 ± 0.6. Mean bias pooled for proning and repositioning was -0.1 (LOA -0.7 to 0.6) for CIPC (percentage error 19%) and 0.3 (LOA -1.3 to 1.9) for CIVIG (percentage error 48%). Changes in CI were too small for trending analysis.

CONCLUSION: Although calibrated CI measurements are only marginally influenced by prone positioning, according to the criteria of Critchley and Critchley, uncalibrated CI values show a degree of error, too high to be considered clinically acceptable.