Protective effect of prone posture against hypergravity-induced arterial hypoxaemia in humans

Abstract

Patients with acute respiratory distress syndrome have increased lung tissue weight and therefore an increased hydrostatic pressure gradient down the lung. Also, they have a better arterial oxygenation in prone (face down) than in supine (face up) posture. We hypothesized that this effect of the direction of gravity also existed in healthy humans, when increased hydrostatic gradients were induced by hypergravity. Ten healthy subjects were studied in a human centrifuge while exposed to 1 or 5 G in anterio-posterior (supine) or posterio-anterior (prone) direction. We measured blood gases using remote-controlled sampling and gas exchange by mass spectrometry. Hypergravity led to marked impairments of arterial oxygenation in both postures and more so in supine posture. At 5 G, the arterial oxygen saturation was 84.6 +/- 1.2 % (mean +/- S.E.M.) in supine and 89.7 +/- 1.4 % in prone posture (P < 0.001 for supine vs. prone). Ventilation and alveolar PO2 were increased at 5 G and did not differ between postures. The alveolar-to-arterial PO2 difference increased at 5 G to 8.0 +/- 0.2 kPa and 6.6 +/- 0.3 kPa in supine and prone postures (P = 0.003). Arterial oxygenation was less impaired in prone during hypergravity due to a better-preserved alveolo-arterial oxygen transport. We speculate that mammals have developed a cardiopulmonary structure that favours function with the gravitational vector in the posterio-anterior direction.

Effects of gravity on lung diffusing capacity and cardiac output in prone and supine humans

“…Accordingly, there was a marked reduction in DL CO at 5 G in both postures, even though DL CO was lower in the supine posture than in the prone. This finding is in concordance with our laboratory's previous finding of a more marked hypergravity-induced arterial hypoxemia in the supine posture than in the prone due to a more severe mismatch between the distribution of ventilation and perfusion in the supine posture (35). In the same study, we also reasoned that interstitial pulmonary edema and alveolar hypoventilation are not likely to have influenced our results in hypergravity (35).…”

“…Even though the recumbent position improves overall tolerance to hypergravity in terms of signs of cerebral hypoperfusion (45), we reasoned that, not only lung function, but also central circulation would be severely affected by hypergravity in the horizontal posture, even though there is a somewhat lower hydrostatic distance from the dependent to the nondependent parts of the thorax in the horizontal compared with upright posture. Furthermore, we hypothesized that the hypergravity-induced decrease in DL CO over the alveolar membrane would be more pronounced in the supine than in the prone posture, which is analogous to our laboratory's previous findings with alveolar-to-arterial O 2 transfer (35). Finally, we hypothesized that, if DL CO were to be more impaired by hypergravity in the supine than in the prone posture, this would be accompanied by impairments of its major determinants, such as alveolar volume (VA) (36,43) and homogeneity of pulmonary-capillary blood volume distribution (31).…”

“…The probe was held in place by an elastic bandage, and the ear lobe was pretreated with capsaicin ointment for vasodilatation to ensure a satisfactory pulse oximetry signal. Our laboratory (35) has have previously shown an excellent agreement between this technique and concomitant arterial samples. An accelerometer was positioned on the support surface perpendicular to the rotational radius and the support surface.…”

“…The experimental protocol used in the present study was approved by the Ethics Committee of Karolinska Institutet. In a previous paper, our laboratory (35) has reported data on alveoloarterial oxygen differences in prone and supine subjects exposed to hypergravity. There was an overlap of 6 out of 10 subjects compared with the present study, and the experiments were performed on separate occasions.…”

“…The exact mechanisms by which this improvement of arterial oxygenation occurs have not yet been determined (23). We recently showed that the improved gas exchange effect of prone positioning also exists in healthy subjects using a model of acute lung insufficiency with gravity (G)-induced ventilation-perfusion mismatch (35). Common denominators between the patients and the hypergravity condition are an increased hydrostatic pressure gradient down the lung (14,37,38) and an increased weight of the heart (18,46).…”