Background Continuous positive airway pressure (CPAP) is an effective treatment for obstructive sleep apnoea (OSA). We assessed neural respiratory drive (NRD), as measured by the surface electromyogram of the parasternal intercostals (sEMG_para), during awake CPAP titration to quantify the effect of chest inflation on the load of the respiratory system.

Patients and methods Obese patients (body-mass-index, BMI >30) with confirmed obstructive sleep apnoea (OSA) were studied and NRD (sEMG_para) and the surface EMG of the external oblique (sEMG_abd) were recorded and normalised to baseline activity (awake, supine). The apnoea-hypopnoea index (AHI) and 95^th percentile of CPAP were determined in sleep studies. The patients were then studied whilst awake and breathing on CPAP (4–20 cmH₂O, increments of 2 cmH₂O/3 mins), with the modified Borg score (mBorg) recorded.

Results 15 patients (age 48 ± 10 years, 12 male, BMI 38.9 ± 5.8) suffering with moderate-severe OSA (AHI 32.2 ± 21.1/h, 95^th percentile nocturnal CPAP 14.1 ± 3.8 cmH₂O) were studied. Awake, sEMG_para declined by 15.1 ± 1.5% from baseline when CPAP was applied, with the nadir at a CPAP of 10.6 ± 3.4 cmH₂O (p = 0.026). Further increase in CPAP levels led to a rise in sEMG_para and breathlessness (mBorg at lowest sEMG_para 0.9 ± 0.8 points, at CPAP of 20 cmH₂O 2.7 ± 2.7 points, p = 0.02).

Conclusion The respiratory system is maximally offloaded with subtherapeutic CPAP levels in OSA. Levels of NRD observed at effective CPAP levels are associated with breathlessness which can impact on CPAP compliance.