Identifying Daytime Hypercapnia Using Transcutaneous Carbon Dioxide Monitoring in Patients With Amyotrophic Lateral...

Respiratory failure from hypoventilation is the most common cause of death in amyotrophic lateral sclerosis (ALS). However, ALS care rarely assesses hypercapnia, a physiologic measure of hypoventilation. We investigated the prevalence and clinical significance of daytime hypercapnia measured by transcutaneous carbon dioxide (tcCO₂) monitoring in patients with ALS.

This retrospective study included patients seen at two ALS clinics in the United States between 2012 and 2024 who had tcCO₂ measured concurrently with pulmonary function tests (PFTs), which included forced vital capacity (FVC) and, at one site, maximum inspiratory pressure (MIP). We assessed the prevalence of hypercapnia (tcCO₂ > 45 mmHg), the sensitivity and specificity of patient symptoms and PFTs for hypercapnia, and the relationship between hypercapnia and survival.

Daytime hypercapnia was present in 33/328 (10%) patients at baseline. Hypercapnia was associated with an increased rate of death (aHR 2.1, 95% CI 1.4-3.3). Orthopnea or dyspnea was 70% sensitive for hypercapnia (95% CI 51%-84%). Absolute value of MIP (|MIP|) < 60 cmH₂O was 95% sensitive (95% CI 74%-100%) and 22% specific (95% CI 16%-30%), FVC < 50% predicted was 33% sensitive (95% CI 18%-52%) and 82% specific (95% CI 78%-87%), and FVC < 80% predicted was 85% sensitive (95% CI 68%-95%) and 31% specific (95% CI 26%-36%) for hypercapnia.

TcCO₂ monitoring identified strengths and weaknesses of PFTs in identifying hypercapnia in ALS. We found high sensitivity of |MIP| < 60 cmH₂O and FVC < 80% predicted and high specificity of FVC < 50% predicted. Prospective studies should investigate the optimal clinical role for tcCO₂.