Waist-to-hip ratio is associated with pulmonary gas exhange in the morbidly obese

Backround: Morbidly obese individuals (ie, body mass index [BMI], ≥ 40 kg/m²) may have a pulmonary gas exchange impairment due to the large fat mass surrounding their abdomen. Purpose: To examine the effect of the waist-to-hip (W/H) ratio on pulmonary gas exchange in the morbidly obese. Methods: Twenty-five morbidly obese individuals (mean [± SD] age, 39 ± 10 years; mean BMI, 49 ± 7 kg/m²; mean body fat, 50 ± 6%; mean waist circumference, 135 ± 15 cm; mean W/H ratio, 0.97 ± 0.11) scheduled for bariatric surgery were recruited. Arterial blood was sampled in duplicate after 5 min of rest sitting upright. Results: The mean PaO₂ at rest was 88 ± 7 mm Hg (range, 72 to 108 mm Hg), the alveolar-arterial oxygen pressure difference (P[A-a]O₂) was 19 ± 9 mm Hg (range, 1 to 37 mm Hg), and the PaCO₂ was 38 ± 3 mm Hg (range, 32 to 44 mm Hg). Linear regression showed that 32% and 36%, respectively, of the variance in the P(A-a)O₂ and PaO ₂ were explained by the W/H ratio (p < 0.004 for both). As well, 20% of the variance in PaCO₂ was explained by the W/H ratio (p = 0.02). Men had larger W/H ratios (p < 0.01) and poorer gas exchange (p = 0.08) compared to women (mean difference: PaO₂, -7 mm Hg; P[A-a]O₂, 6 mm Hg). Conclusion: Morbidly obese men showed a trend to have poorer pulmonary gas exchange compared to morbidly obese women, and a significant part of the blood gas status in these patients is associated with the W/H ratio.