Does normoxia exclude hypercapnia in people with neuromuscular disease? A retrospective analysis of arterial blood gases

Background: Pulse oximetry is widely used in the assessment of chronic hypercapnic respiratory failure (CHRF) in people with neuromuscular disease (NMD). Guidelines suggest an arterial blood gas (ABG) is indicated if oxygen saturations (SpO2) ≤ 94% in the absence of lung disease. However, hypercapnia with normoxia (SpO2 ≥ 95%) has been observed on ABGs of patients with NMD. Aims: To determine the prevalence of hypercapnia with normoxia in people with CHRF. It was hypothesised that hypercapnia with normoxia would occur in a small proportion of NMD patients. Methods: A single-centre retrospective audit of room-air ABGs in stable CHRF patients from 1990–2020 was performed. Patients with parenchymal lung disease were excluded. Patients were grouped into three main categories: non-NMD, other-NMD and motor neurone disease (MND). Results: Three-hundred-and-six ABGs with hypercapnia from 185 patients with extrinsic restrictive lung disease were analysed. No patients with non-NMD, 65% of other-NMD and 36% of MND patients demonstrated hypercapnia with normoxia (χ2 83·61; p<0·001). The potential mechanism is proposed to be a difference in calculated respiratory quotient (RQ). Assuming the A-a gradient is normal, the calculated RQ was significantly higher in MND patients and other-NMD patients compared with non-NMD patients (estimated-marginal-mean 0·99 [95%CI 0·93–1·03]; 0·88 [95%CI 0·77–0·97]; 0·74 [95%CI 0·64—0·84] respectively; p<0·001) by mixed-model analysis. Conclusion: Hypercapnia is not excluded with normal oximetry in people with NMD and may be due to an elevated RQ. This has implications in the diagnosis and monitoring of CHRF in people with NMD with oximetry alone.