Influence of prone positioning on the measurement of transpulmonary thermodilution-derived variables in critically ill patients

Background: Patients with respiratory failure undergoing prone positioning (PP) are often monitored by the transpulmonary thermodilution (TPTD) technique. However, it remains unclear whether the measurement of TPTD-derived variables is influenced by PP. We investigated the effects of 135 PP on the accuracy of TPTD-derived variables and their changes over an 8-10 h period of time. Methods: We studied 16 mechanically ventilated patients who underwent PP for acute respiratory distress syndrome or acute lung injury and received hemodynamic monitoring by the TPTD technique. Measurements of extravascular lung water index (EVLWI), global end-diastolic volume index (GEDVI), ejection fraction corrected GEDVI (cGEDVI), pulmonary vascular permeability index (PVPI) and hemodynamic variables were obtained 10 min before and after positioning and repositioning. One-way analysis of variance and Friedman's test with Student-Newman-Keuls method for all pairwise multiple comparisons were used for statistical analysis. Results: EVLWI increased after proning (12.7 +/- 4.7 vs. 14.8 +/- 7.8 ml/kg) and remained elevated until end of prone positioning (15.1 +/- 7.2 vs. 12.8 +/- 4.9 ml/kg) with P < 0.05 for difference between respective time points. After proning, GEDVI remained unchanged (571 +/- 153 vs. 593 +/- 152 ml/m(2)). At the end of prone positioning GEDVI was 610 +/- 155 ml/m(2) and decreased after returning to supine to 553 +/- 142 ml/m(2). Proning increased cGEDVI from 525 +/- 136 to 570 +/- 117 ml/m(2) and repositioning decreased cGEDVI from 558 +/- 116 to 496 +/- 121 ml/m(2). No significant changes in PVPI were observed during the study period. Conclusions: EVLWI and GEDVI measurements are possibly influenced by prone positioning. In spite of statistical significance, the differences in EVLWI and GEDVI are low and presumably of no clinical relevance.