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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Competing interests: Professor Yuanming Luo, a close collaborator from the State Key National Laboratory of Respiratory Disease, Guangzhou Medical College, China, has developed the multipair catheter used for the measurement of the diaphragm electromyogram. No patent is pending.
Funding: JS is the recipient of a long-term Research Fellowship of the European Respiratory Society (No 18).
Ethics approval: The study was approved by the King’s College Hospital local research ethics committee and informed consent was obtained from each patient.
▸ Additional data are published online only at http://thorax.bmj.com/content/vol64/issue8
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