Effect of prone position on pulmonary vascular permeability in acute respiratory distress syndrome

In early acute respiratory distress syndrome (ARDS), alteration in the pulmonary capillary permeability is associated with outcome [1]. Alteration in the relation of extravascular lung water to intrathoracic blood volume (EVLW/ITBV) derived from thermal-dye dilution curves indicates changes in the pulmonary vascular permeability. Prone positioning improves gas exchange in most patients with ARDS, however whether this improvement is related to effects on pulmonary vascular permeability has not been evaluated. This prospective pilot study was designed to investigate whether prone positioning would alter EVLW/ITBV as a measure of pulmonary vascular permeability.

Patients with ARDS on inverse ratio pressure-controlled ventilation with PEEP > 10 cmH₂O for at least 24 hours were recruited. Patients were turned prone for 18 hours. Except for FiO₂, ventilatory settings remained unchanged during the study period. Values of EVLW and ITBV were obtained using a single transpulmonary arterial thermodilution technique with a 5F-fibreoptic thermistor femoral artery catheter. Measurements of EVLW and ITBV were taken at pre-prone, 1, 2, 6, 12 and 18 hours after proning and 1 hour after supine. EVLW/ITBV while prone was normalised to pre-prone values as a baseline to illustrate differences during prone and supine. Data were expressed as mean (SEM). Repeated measures ANOVA was used for statistical analysis.

Twelve episodes of proning in 11 patients were studied. Although mean PaO₂/FiO₂ improved within 1 hour, it continued to improve during the period studied and only reached significance 12 hours after proning (17.9 ± 2.9 v 35.1 ± 4.2, P < 0.05). Mean EVLW/ITBV did not change significantly.

At least 12 hours may be needed for maximal benefit with prone positioning. Changes in pulmonary vascular permeability in ARDS do not appear to be an important mechanism to account for the improvement in gas exchange seen following prone positioning.

Table 1 Changes in EVLW/ITBV after prone positioning in ARDS