Effect of body position on the arterial partial pressures of oxygen and carbon dioxide in spontaneously breathing, conscious dogs in an intensive care unit

Objective To evaluate the effect of body position on the arterial partial pressures of oxygen and carbon dioxide (PaO2, PaCO2), and the efficiency of pulmonary oxygen uptake as estimated by alveolar-arterial oxygen difference (A-a difference). Design Prospective, randomized, crossover study. Setting University teaching hospital, intensive care unit. Animals Twenty-one spontaneously breathing, conscious, canine patients with arterial catheters placed as part of their management strategy. Interventions Patients were placed randomly into lateral or sternal recumbency. PaO2 and PaCO2 were measured after 15 minutes in this position. Patients were then repositioned into the opposite position and after 15 minutes the parameters were remeasured. Measurements and Main Results Results presented as median (interquartile range). PaO2 was significantly higher (P=0.001) when patients were positioned in sternal, 91.2 mm Hg (86.0-96.1 mm Hg), compared with lateral recumbency, 86.4 mm Hg (73.9-90.9 mm Hg). The median change was 5.4 mm Hg (1.1-17.9 mm Hg). All 7 dogs with a PaO2 < 80 mm Hg in lateral recumbency had improved arterial oxygenation in sternal recumbency, median increase 17.4 mm Hg with a range of 3.8-29.7 mm Hg. PaCO2 levels when patients were in sternal recumbency, 30.5 mm Hg (27.3-32.7 mm Hg) were not significantly different from those in lateral recumbency, 32.2 mm Hg (28.3-36.0 mm Hg) (P=0.07). The median change was -1.9 mm Hg (-3.6-0.77 mm Hg). A-a differences were significantly lower (P=0.005) when patients were positioned in sternal recumbency, 21.7 mm Hg (17.3-27.7 mm Hg), compared with lateral recumbency, 24.6 mm Hg (20.4-36.3 mm Hg). The median change was -3.1 mm Hg (-14.6-0.9 mm Hg). Conclusions PaO2 was significantly higher when animals were positioned in sternal recumbency compared with lateral recumbency, predominantly due to improved pulmonary oxygen uptake (decreased A-a difference) rather than increased alveolar ventilation (decreased PaCO2). Patients with hypoxemia (defined as PaO2 < 80 mm Hg) in lateral recumbency may benefit from being placed in sternal recumbency. Sternal recumbency is recommended to improve oxygenation in hypoxemic patients.