Effects of the prone position on gas exchange and hemodynamics in severe acute respiratory distress syndrome

Objectives: To address the following issues regarding the use of prone posi tion ventilation in patients with severe acute respiratory distress syndrom e (ARDS): a) response rate; b) magnitude and duration of improved oxygenati on in responders during a 12-hr trial and the consequences of returning to the supine position; c) effects of the prone position on gas exchange and h emodynamics; d) consequences of oxygenation in nonresponders; and e) effect s of repeated prone position trials. Design: Prospective, nonrandomized interventional study. Setting: Medical intensive care unit, university tertiary care center. Patients: Nineteen consecutive, mechanically ventilated patients (age 45 +/ - 20 yrs, mean +/- so) with ARDS and severe hypoxemia, defined as PaO2/FIO2 of less than or equal to 150 with FIO2 of greater than or equal to 0.6 per sisting for greater than or equal to 24 hrs, and a pulmonary artery occlusi on pressure of <18 mm Hg. Interventions: Patients were turned prone for 2 hrs. Nonresponders were ret urned supine, but responders were maintained prone for 12 hrs before being returned to the supine position. The procedure was repeated on a daily basi s in all patients, until inclusion criteria were no longer met or the patie nts died. Measurements and Main Results: Hemodynamic, blood gas, and gas exchange mea surements were performed at the following time points: a) baseline supine; b) after 30 mins prone; and c) after 120 mins prone. Additional measurement s for nonresponders were taken after 30 mins supine. For responders, additi onal measurements were taken after 12 hrs prone and 30 mins supine. Patient s were considered responders if an increase in PaO2 of greater than or equa l to 10 torr (greater than or equal to 1.3 kPa), or increase in the PaO2/FI O2 ratio of greater than or equal to 20 occurred within 120 mins. Eleven (5 7%) patients responded to the prone position. There was no difference in in itial baseline parameters between responders and nonresponders. After 30 mi ns, the prone position in responders increased PaO2 and decreased calculate d venous admixture ((Q)over dot va/(Q)over dot t). This improvement was the maximal obtained, and was maintained throughout the 12-hr prone period. Af ter 12 hrs prone, mean FIO2 had been lowered from 0.85 +/- 0.16 to 0.66 +/- 0.18 (p < .05). Thirty minutes after the patients were returned supine, Pa O2, PaO2/FIO2, and (Q)over dot va/(Q)over dot t were not different from 12- hr prone values, and were improved in comparison with baseline supine value s. There was no worsening of gas exchange or hemodynamics in nonresponders. After the initial trial, a total of 28 additional episodes of prone positi on ventilation were performed in nine of the 19 patients. Of the 24 additio nal episodes in the responders, there was a response in 17 (71%) of 24 epis odes. In the four additional episodes in nonresponders, there was a respons e in only one (25%) of four episodes. Response was accompanied by the same beneficial effects on gas exchange and (Q)over dot va/(Q)over dot t and abs ence of effect on hemodynamics as in the initial trial. There was no worsen ing in gas exchange or hemodynamics in nonresponder trials. Conclusions: Based on the data from this study, the prone position can impr ove oxygenation in severely hypoxemic ARDS patients without deleterious eff ects on hemodynamics. This beneficial effect does not immediately disappear on return to the su pine position. In our patients, an absence of response to this technique was not accompanied by worsening hypoxemia or hemodynami c instability. Repeated daily trials in the prone position should be consid ered in the management of ARDS patients with severe hypoxemia.