1. To examine the relationship between eucapnic morbid obesity and ventilatory responsiveness to chemical stimuli, we measured hypercapnic and hypoxic ventilatory responses in 29 patients (26 women, three men) before and 3–6 months after gastroplasty. No subject demonstrated resting awake hypercapnia and none suffered from sleep-disordered breathing.

2. Mean weight fell significantly (122.8 ± 21.4 vs 102.2 ± 22.8 kg, P <0.0001) and functional residual capacity rose slightly but significantly (1.94 ± 0.58 vs 2.18 ± 0.64 litres; P <0.05) after weight loss.

3. The hypercapnic ventilatory response slope fell significantly after weight loss (2.88 ± 2.27 vs 2.24 ± 1.06 litres min−1 mmHg−1, P <0.05) with a significant shift of the ventilatory response curve to the right. There were no statistically significant changes in the patterns of ventilatory response.

4. In addition, isocapnic hypoxic ventilatory response slopes, measured at two levels of carbon dioxide partial pressure, fell significantly after weight loss. These changes were accompanied by significant shifts of the ventilatory response curves to the left, such that, for a given oxygen saturation, mean ventilation was significantly lower in the less obese state. Similarly to hypercapnic responses, there were no statistically significant changes in ventilatory pattern despite the changes in overall ventilatory response.

5. We conclude that ventilatory responsiveness to chemical simuli is increased in obese subjects who maintain adequate alveolar ventilation while awake.

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