PULSE oximetry is commonly used to assess chronic respiratory failure in patients with neuromuscular disease (NMD). While chronic respiratory failure is a leading cause of morbidity and mortality in these patients, early diagnosis and intervention are crucial. Current guidelines recommend arterial blood gas (ABG) measurement when oxygen saturation (SpO2) is ≤94%, excluding lung disease. However, some patients with NMD, particularly those with motor neuron disease, exhibit hypercapnia despite normal oxygen levels (SpO2 ≥95%).
A recent study conducted a single-center retrospective audit, conducted on room-air ABGs from stable hypercapnic chronic respiratory failure patients, spanning from 1990–2020. The study excluded patients with parenchymal lung disease and categorized the remaining patients into three groups: non-NMD, other NMD, and motor neuron disease.
The study found that among 297 ABGs from 180 patients with extrinsic restrictive lung disease, none of the patients without NMD showed hypercapnia with normoxia. In contrast, 54% of patients with other NMD and 36% of motor neuron disease patients displayed this condition (Chi-squared: 61.33; p<0.001). The proposed mechanism involves a higher calculated respiratory quotient in patients with NMD. Mixed-model analysis revealed significantly higher respiratory quotients in motor neuron disease and other patients with NMD compared to patients without NMD (estimated marginal mean: 0.99, 95% CI: 0.94–1.03; 0.86, 95% CI: 0.76–0.96; 0.73, 95% CI: 0.63–0.83, respectively; p<0.001).
The study concluded that hypercapnia cannot be ruled out by normal oximetry in patients with NMD, potentially due to an elevated respiratory quotient. The researchers stressed that new comprehensive diagnostic and monitoring approaches are needed to accurately detect respiratory insufficiency in patients with NMD.
Aleksandra Zurowska, EMJ
Reference
Gray E et al. Hypercapnia is not excluded by normoxia in neuromuscular disease patients: implications for oximetry. ERJ Open Res. 2024. [Epub ahead of print].
