Patients with neuromuscular diseases who are developing progressive respiratory failure due to respiratory muscle weakness will die unless mechanical ventilation is used. The rate of progression is often hard to predict. Some patients seem suddenly to experience life-threatening hypercapnic respiratory failure. They may not have been aware of gradually increasing symptoms and signs, particularly since they are often not physically active and are often not being regularly monitored with simple pulmonary function tests.
Administering oxygen does not provide assistance to the weakening respiratory muscles, but gives both the patient and the doctor the false impression that appropriate treatment is being provided. While in fact hypoventilation is mistaken for an oxygen transfer problem. Indeed, administering oxygen can mask the problem. Also there is a danger of causing respiratory depression by giving oxygen. Oxygen is not the treatment for hypoventilation. It will improve the SaO2, but not the hypoventilation and may increase the danger of dying of sudden respiratory failure.
In hypercapnic respiratory failure due to hypoventilation, the SaO2 falls due to the rise of the CO2. The alveoli in the lungs (tiny gas exchange units) should clear most of the CO2 out with each breath. Instead, with hypoventilation, CO2 accumulates and thus there is decreased room in the alveoli for oxygen. When mechanical ventilation using room air is provided, it lowers the CO2 in the alveoli, corrects the SaO2 and rests the respiratory muscles. The ventilator should be adjusted to achieve a normal SaO2, on room air. If oxygen is being administered, one cannot use noninvasive oximetry to tell whether enough assisted ventilation is being provided; repeated arterial blood gas specimens (ABGs) would be needed.
When there is respiratory failure in neuromuscular patients (ALS, postpolio, SMA, muscular dystrophy, etc.) who have no additional pulmonary disease that impairs oxygen transfer, the ventilator set-up is adjusted to:
E.A.Oppenheimer, MD, FCCP, Pulmonary Medicine, Southern California Permanente Medical Group, 4950 Sunset Boulevard, Los Angeles, California 90027-5822 (firstname.lastname@example.org).
Bach, J.R. (1999). Guide to the evaluation and management of neuromuscular disease. Philadelphia, PA: Hanley & Belfus.
Gay, PC., & Edmonds, L.C. (1995). Severe hypercapnia after low-flow oxygen therapy in patients with neuromuscular disease and diaphragmatic dysfunction. Mayo Clinic Proceedings, 70(4), 327-330.
Hsu, A., & Staats, B. (1998). "Postpolio" sequelae and sleep-related disordered breathing. Mayo Clinic Proceedings, 73, 216-224.
Krachman, S., & Criner, G.J. (1998). Hypoventilation syndromes. Clinics in Chest Medicine, 19(l),139-155.
Editor's Note: The International Ventilator Users Network office continually hears of people with post-polio or other neuromuscular diseases and conditions being inappropriately prescribed O2 therapy. This anecdotal evidence (confirmed by the Mayo Clinic case series) supports the need for accurate information from the physicians most expert in the pulmonary aspects of neuromuscular disease to be disseminated more widely to alert people to the reasons why they should be wary of O2 therapy.