Background: The load that increased body mass imposes on ventilation contributes to the respiratory symptoms caused by obesity. We wished to quantify ventilatory load and respiratory drive in obesity, in both the upright and supine posture.
Patients&Methods: We studied resting breathing seated and supine in 30 obese (body-mass-index (BMI) 42.8(8.6)kg/m2) and 30 normal subjects (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)cmH2O, p<0.001). The supine posture caused an increase in oesophageal pressure swings to 16.0(5.0)cmH2O in obese (p<0.001) and to 6.9(2.0)cmH2O 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)cmH2O when supine. Applying continuous positive airway pressure (CPAP) in a subgroup of obese subjects, supine, reduced the EMGdi by 40%, inspiratory pressure swings by 25% and largely abolished PEEPi (4.1(2.7) vs 0.8(0.4)cmH2O, p=0.009).
Conclusion: Obese patients have substantially elevated neural drive related to BMI, and develop PEEPi when supine. CPAP abolishes PEEPi and reduces neural respiratory drive in such 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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