RESPIRATORY PHYSIOLOGY

Neural respiratory drive in obesity

J Steier^1,2, C J Jolley^1,2, J Seymour^1,2, M Roughton^2,3, M I Polkey², J Moxham¹

¹ King’s College London School of Medicine, London, UK
² Royal Brompton Hospital, London, UK
³ University College London, Cancer Trials Centre, UK

Dr J Steier, Chest Unit, King’s College London School of Medicine, Denmark Hill, London SE5 9PJ, UK; joerg.steier{at}kcl.ac.uk

Background: The load imposed on ventilation by increased body mass contributes to the respiratory symptoms caused by obesity. A study was conducted to quantify ventilatory load and respiratory drive in obesity in both the upright and supine postures.

Methods: Resting breathing when seated and supine was studied in 30 obese subjects (mean (SD) body mass index (BMI) 42.8 (8.6) kg/m²) and 30 normal subjects (mean (SD) BMI 23.6 (3.7) kg/m²), recording the electromyogram of the diaphragm (EMGdi, transoesophageal multipair electrode), gastric and oesophageal pressures.

Results: Ventilatory load and neural drive were higher in the obese group as judged by the EMGdi (21.9 (9.0) vs 8.4 (4.0)%max, p<0.001) and oesophageal pressure swings (9.6 (2.9) vs 5.3 (2.2) cm H₂O, p<0.001). The supine posture caused an increase in oesophageal pressure swings to 16.0 (5.0) cm H₂O in obese subjects (p<0.001) and to 6.9 (2.0) cm H₂O in non-obese subjects (p<0.001). The EMGdi increased in the obese group to 24.7 (8.2)%max (p<0.001) but remained the same in non-obese subjects (7.0 (3.4)%max, p = NS). Obese subjects developed intrinsic positive end-expiratory pressure (PEEPi) of 5.3 (3.6) cm H₂O when supine. Applying continuous positive airway pressure (CPAP) in a subgroup of obese subjects when supine reduced the EMGdi by 40%, inspiratory pressure swings by 25% and largely abolished PEEPi (4.1 (2.7) vs 0.8 (0.4) cm H₂O, p = 0.009).

Conclusion: Obese patients have substantially increased neural drive related to BMI and develop PEEPi when supine. CPAP abolishes PEEPi and reduces neural respiratory drive in these patients. These findings highlight the adverse respiratory consequences of obesity and have implications for the clinical management of patients, particularly where the supine posture is required.

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