Background Screening patients regularly for evidence of respiratory failure is an important facet in the management of ALS.

Objectives To evaluated the relative value of symptom history, forced vital capacity (FVC) and day time transcutaneous carbon dioxide (Ptc CO2) monitoring in early detection of respiratory failure in ALS.

Methods This is a prospective observational cohort study consisting of 50 consecutive patients with ALS. The participants underwent 3 monthly assessments for symptoms of respiratory failure, FVC and Ptc CO₂ monitoring.

Results Symptoms of respiratory failure were the most powerful tool, alerting the physician to the possibility of respiratory failure. All the patients where respiratory failure was confirmed on overnight capnometry had symptoms of respiratory failure. Shortness of breath was the most common symptom reported by the patients, present in 74% of the patients with confirmed respiratory failure. 37% of these patients had FVC of >50% predicted and only 15% had day time hypercapnia (Ptc CO₂>6.0 kPa). None of the patients had day time hypercapnia without any other marker of respiratory failure. There was statistically significant difference between the day time Ptc CO₂ and median overnight Ptc CO₂ (p=0.0002).

Discussion This study has emphasised the importance of symptom history in detection of respiratory failure in ALS.