Postural effects on pulmonary gas exchange abnormalities in severe obesity before and after bariatric surgery

BACKGROUND: We hypothesized that in morbid obesity, pulmonary gas exchange abnormalities will worsen when supine and that bariatric surgery (BS) will mitigate this effect. METHODS: Gas exchange was investigated in 19 morbidly obese and 8 non-obese, age-matched control females, spontaneously breathing ambient air, both upright and supine, before and one year after BS. RESULTS: In control non-obese individuals, no postural changes in arterial blood gases (ABGs) were observed. While obese subjects had more altered PaO₂, SaO2 and AaPO₂ values than controls (P<0.05 each) when upright, the values unexpectedly remained unchanged when supine. This was also the case in the subset of 6 normoxemic obese but the remaining 13 hypoxemic individuals actually improved ABGs when supine: PaO₂ (by +2.7±1.3 mmHg, P=0.06), SaO₂ (by +1.5±0.6%), pH (by +0.01±0.01) and AaPO₂ (by-3.4±1.4 mmHg); and cardiac output increased (by +0.4±0.2 Lmin-1) (P<0.05 each). After BS, PaO₂ (from 75.5±2.4 to 89.4±2.4 mmHg), AaPO₂ (from 27.0±2.0 to 15.4±2.1 mmHg) (P<0.05 each), and pulmonary gas exchange were improved compared to before BSwhen upright, but ABGs worsened when supine (PaO₂, by-4.6±1.7 mmHg; AaPO₂, by +4.2±1.6 mmHg) (P<0.05 each). CONCLUSIONS: Before BS, ABGs are not altered in normoxemic obese subjects moving from upright to supine, even improving in those with hypoxemia when supine. After successful BS, pulmonary gas exchange improved when upright in all subjects but ABGs deteriorated when supine. However, the important clinical observation is the lack of gas exchange deterioration when supine, which may have implications for critical care and anesthesia settings.