Influence of prone positioning on measurement of extravascular lung water and pulmonary vascular permeability indexes by transpulmonary thermodilution

Abstract

Introduction: The transpulmonary thermodilution technique enables measurement of the extravascular lung water index (EVLWI), which has been described in experimental and clinical studies to be accurate when compared with the reference method (that is, transpulmonary double indicator technique) [1,2]. In this study, the influence of prone positioning on the reliability of the measurement of the EVLWI and the pulmonary vascular permeability index (PVPI) - as a measure of capillary function - by transpulmonary thermodilution was assessed. Methods: We prospectively studied 12 consecutive critically ill patients (eight male, four female, age 20 to 64 years) receiving mechanical ventilation, who for clinical indication due to severe chest trauma or acute respiratory distress syndrome underwent modified prone positioning and extended hemodynamic monitoring by the transpulmonary thermodilution technique (PiCCO®; Pulsion Medical Systems AG). In all patients, an arterial thermistor catheter (A. femoralis) was placed and connected to a monitor (PiCCO®plus, version 7.0 nonUS). Measurements of cardiac output, intrathoracic blood volume (ITBV), EVLWI and PVPI (five central venous bolus injections, 15 ml NaCl, <8°C) were performed 10 minutes before and after turning the patients into a modified prone position (135°) from a supine position. No changes in respirator settings and fluid status were made. The statistical analysis was performed by linear regression and according to Bland-Altman. Results: The range for the EVLWI was 5.0 to 21.0 and 5.0 to 22.7 ml/kg at the two time points. Linear regression analysis revealed r = 0.95 (mean bias 0.5 ml/kg, one standard deviation 1.4 ml/kg). Furthermore, the PVPI ranged from 1.9 to 5.3 before and 1.7 to 5.8 after prone positioning. Correlation coefficient was r = 0.96 (mean bias 0.04, one standard deviation 0.35). The linear regression analysis with respect to the ITBV at the two time points showed r = 0.96, respectively. Conclusions: Our results suggest that measurement of extravascular lung water and pulmonary vascular permeability indexes by the transpulmonary thermodilution technique is not influenced by prone positioning.