A series is reported of five patients with acute respiratory failure complicating neuromuscular diseases, three of whom required assisted ventilation. Initially the arterial carbon dioxide tension fell to levels below the normal range in all the patients and this change reflects alveolar hyperventilation which is probably produced by a combination of fear, small airway collapse, and reflex tachypnoea. It is only late in the development of respiratory weakness that alveolar ventilation falls and carbon dioxide tension rises to normal and then elevated levels. Several factors contribute to the reduction in arterial oxygen tension, including airway collapse and infection, and it is important to try to prevent these by correct posture, vibration, and passive deep breathing. We think that objective measurements of respiratory function are essential in the management of these patients and that the most valuable are serial measurements of vital capacity and carbon dioxide tension and the least helpful is the arterial oxygen tension. Assisted ventilation will probably be required if the vital capacity falls below 30% of that predicted for the patient or if the carbon dioxide tension rises from low to normal levels as vital capacity is falling.
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