Respiratory pathophysiological bases for mechanical ventilation in neuromuscular diseases

Advances in knowledge about the respiratory changes and mechanisms of development of respiratory failure in many neuromuscular diseases have led to an improved rationalization of the therapeutic strategies; however, many aspects, particularly those concerning ventilatory therapy, remain to be clarified. Because of the extreme heterogeneity of the neuromuscular diseases, the changes in respiratory failure and their course over time have, in part, different characteristics leading to different respiratory pathophysiological pictures. In general, a whole series of pathological events whose evolution over time causes a progressive worsening of respiratory function are based on two fundamental changes: the deficits in the strength of skeletal respiratory muscles and of those of the thoracic cage. These alterations generate progressive reduction of pulmonary volumes and changes in respiratory mechanics, so that a vicious circle is created whose final result is an increased resistive load, which is sustained by an ever less adequate pump system. In the treatment of manifest respiratory failure, there is now good agreement in the literature on the inclusion criteria defined as necessary for mechanical ventilation of hypercapnic patients. There does not seem to be room, however, for the so-called 'preventive' ventilation proposed in the past to delay the onset of respiratory failure. In this context we have suggested the possible identification of a set of patients who could benefit from mechanical ventilation at a particular point in the natural evolution of their disease towards chronic respiratory failure before it becomes manifest. The results of our study suggest that the presence of nocturnal hypoventilation, even in the absence of daytime alterations in gas exchange, should be considered as an inclusion criterion for mechanical ventilation.