By Philip Schiffman, M.D.
Although ALS does not affect the lungs themselves, the indirect effects on the lungs are profound. The lung is a passive organ in which air is pumped in and out. Blood flowing through the lung picks up oxygen from the fresh air inhaled into it. Carbon dioxide leaves that same blood when air is exhaled from the lung out of the body.
The diaphragm and other muscles which pump the air in and out are skeletal muscles. These muscles grow progressively weak during the course of ALS. When their strength decreases to a critical level, the act of breathing itself will cause the respiratory muscles to “fatigue”, causing further weakness. Eventually, the respiratory muscles become so weak that they cannot sustain an adequate level of breathing and respiratory failure ensues.
Deterioration of the respiratory muscles also impairs the ability to cough and clear secretions. If lung secretions from a minor respiratory infection are not cleared, breathing becomes more difficult, oxygen levels fall, and pneumonia may develop.
Progressive weakness of the swallowing muscles indirectly affects lung function. Improper swallowing may lead to aspiration of food or secretions into the respiratory tract. This, coupled with a weak cough, may directly lead to respiratory failure or recurrent pulmonary infections.
Serial testing of pulmonary function is an important part of the management of a patient with ALS. Soon after diagnosis, a baseline set of breathing tests should be performed. We perform a series of tests that measure respiratory muscle strength including a vital capacity (VC), a maximum voluntary ventilation (MVV), and maximum respiratory pressures (PiMAX and PeMAX). We do a flow volume loop, which is helpful in evaluating the patency of the upper airway. Some patients with ALS develop pharyngeal muscle weakness which can lead to partial closure of the upper airway causing positional shortness of breath. If a patient has a history or symptoms of underlying lung disease (e.g., cough, wheezing, asthma, emphysema), a more complete pulmonary function test is performed. The respiratory muscles must work harder in a patient who has specific lung disease in addition to ALS. This can lead to earlier respiratory failure. Identifying such lung diseases early and providing proper treatment for them are important parts of total ALS management.
After this initial pulmonary evaluation, the flow loop, VC, MVV, and respiratory pressures are repeated at regular intervals. The interval between tests can be as soon as one month, or as long as six months, depending upon the amount of respiratory weakness and the speed at which the disease is progressing. The sensation of shortness of breath is usually an important symptom in identifying compromised lungs. Most patients will develop shortness of breath on exertion before they develop shortness of breath at rest. This is because the body requires more oxygen and produces more carbon dioxide when it is active, than when it is at rest. We have noticed that this finding is often absent in patients with ALS. Overall weakness may limit the patient’s physical activity, preventing them from developing exertional shortness of breath. Such patients, who have not experienced shortness of breath, may be surprised to find out they have already developed significant respiratory muscle weakness.
Serial measurements of pulmonary function help to delineate progression of disease, formulate a prognosis, evaluate efficacy of treatments (especially new drugs and protocol drugs), and time interventions before critical weakness of the respiratory muscles ensues. The tests are an objective measure of respiratory muscle function and, because measurements are made on a linear scale, are helpful in judging the overall progression of the disease.
Additional measurements of pulmonary function that may be made are pulse oximetry and arterial blood gas levels. Pulse oximetry allows for a non-invasive measurement of the level of oxygen in a patient’s blood. This is a painless test that is very useful both in patients who are short of breath, and in those not short of breath, but suffering significant respiratory muscle weakness. Arterial blood gas measurement determines levels of oxygen, carbon dioxide, and degree of acid in the blood. Carbon dioxide rises late in the course of ALS when respiratory muscle weakness begins to lead to respiratory insufficiency. This test requires the removal of blood from an artery in the wrist.
Dr. Philip Schiffman is pulmonary/internal medicine consultant to the
Neuromuscular & ALS Center.