Mechanisms of orthopnea in patients with advanced COPD

Background: Many patients with COPD report unpleasant respiratory sensation at rest, further amplified by adoption of supine position (orthopnea). The source of this sensation remains poorly understood. Methods: 16 symptomatic patients with advanced COPD and 16 age- and gender-matched healthy controls (CTRL) underwent pulmonary function tests and detailed sensory-mechanical measurements with diaphragm electromyography (EMGdi) and respiratory pressures in sitting and supine positions. Results: Patients (age:66±7years, BMI:25±6 Kg/m2, smoking history:52±25 pack-year, FEV1:40±18 %predicted) had lower inspiratory capacity (IC) by 44% and greater intensity of breathing difficulty (Borg scale) compared with CTRL (p<0.01). In supine (vs. sitting): CTRL expanded IC (by 0.33L) with a drop in ventilation mainly due to reduced breathing frequency (all p<0.05). In contrast, patients’ IC remained fixed and ventilation and breathing frequency, which were significantly higher (p<0.05) than CTRL, remained stable while supine. In COPD, (vs CTRL) supine position shift was associated with greater rise in both unpleasant respiratory sensation, inspiratory neural drive (EMGdi/EMGdi, max) and tidal trans-diaphragmatic pressure as % of maximum (p<0.05) [Figure]. Conclusion: In COPD, acute orthopnea while supine coincided with an abrupt increase in amplitude of diaphragm activation and force generation in the setting of a low fixed inspiratory capacity.