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Assisted ventilation for chronic neuromuscular disorders
  1. J D ROAD
  1. Respiratory Division
  2. Vancouver Hospital
  3. Vancouver, BC
  4. V5Z 3J5 Canada

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The provision of long term ventilatory support to patients with chronic neuromuscular disorders is a major logistic, technical, and clinical challenge. In this issue of ThoraxBaydur and colleagues share their experiences with ventilatory support in this context.1 Much of their experience reflects the advances in technology that have evolved over the last six decades. Their initial experience was with negative pressure ventilation and later with positive pressure ventilation. Positive pressure ventilation has evolved from an invasive interface (the tracheostomy) to, in many situations, a non-invasive interface. Although negative pressure ventilation served their early patients well in the post-poliomyelitis epidemic, their experience with non-invasive ventilation—mouth or nasal intermittent positive pressure ventilation (MNIPPV)—indicates that, in both poliomyelitis and Duchenne's muscular dystrophy, MNIPPV is more effective.

The slow decline in forced vital capacity (FVC) over 24 years in the poliomyelitis group receiving negative pressure ventilation is remarkable. In the 25 patients studied the FVC declined by an average of 5 ml per year (0.1% predicted per year) from a mean of 800 ml to a mean of 730 ml. In the albeit shorter time interval of three years their patients with poliomyelitis receiving MNIPPV had an actual increase in vital capacity from a mean of 1134 ml to 1219 ml, a mean increase of 90 ml/year. Even if their lung function were to decline at the rate of those on negative pressure ventilation, they may not reach the point of ventilatory muscle weakness that had previously been associated with the institution of invasive ventilation (tracheostomy). Bach et al 2 found that long term ventilation of patients with a history of poliomyelitis using mouth IPPV resulted in a decline in FVC of 19 ml/year. The patients in this study had slightly lower FVC values to start with which may explain the more rapid decline in lung function, and the patients were followed for 15 years rather than 24 years as in the study by Bayduret al.

In patients with Duchenne's muscular dystrophy the decline in lung function was more rapid. At the onset of mechanical ventilation the FVC values of the patients were strikingly lower than those with poliomyelitis with a mean of 600 ml falling to 533 ml (60 ml/year) over the two years that they received treatment with MNIPPV. The decline in FVC in these patients is similar to that reported previously.3 This decline in lung function from an already low initial value resulted in a tracheostomy in 10 of the 15 patients, in most cases because of their inability to ventilate adequately with non-invasive techniques. Patients with negligible vital capacity and without bulbar involvement have been managed with non-invasive ventilation.4 The experience of Bayduret al is similar to that of Legeret al 5 who also found that a tracheostomy became necessary in many of their patients with Duchenne's muscular dystrophy.

Can we manage such patients with profound ventilatory muscle weakness completely non-invasively in a routine fashion? The challenge of managing these patients 24 hours a day with non-invasive devices is significant. Patient motivation and the availability of an infrastructure to provide the appropriate interfaces with the ventilator appear critical. The use of interfaces applied to the mouth appear more important as weakness progresses.4 Failure to achieve control of ventilation often precipitates tracheostomy to secure breathing. Future improvements in technology and increased familiarity with the available techniques should facilitate the application of non-invasive ventilation in these patients.

The concept of initiating ventilation earlier in an attempt to prevent the inevitable decline in lung function, particularly in patients with Duchenne's muscular dystrophy, is laudable. Unfortunately the only randomised controlled clinical trial to assess the effectiveness of early intervention in Duchenne's muscular dystrophy had a negative result.6 The idea that intermittently resting overstimulated muscle units in patients with poliomyelitis or preventing muscle injury induced by overexertion in patients with muscular dystrophy7 ,8 is a sound one. The fact that the respiratory muscles may also be injured lends relevance to this idea.9 The finding that muscle contraction itself can result in muscle injury and subsequent cell necrosis in patients with muscular dystrophy supports the notion that intermittent rest may be beneficial. Complete rest will, of course, lead to muscle atrophy. A re-examination of the potential effects of rest on ventilatory muscle weakness in this condition will probably depend on improved usage and techniques of ventilatory assistance. At present, however, ventilation—both invasive and non-invasive—allows the prolongation of a meaningful life in patients with Duchenne's muscular dystrophy and, until a cure is found for this disease, the next obstacle confronting these individuals will be failure of the heart as a result of cardiomyopathy. In general, however, the paper by Baydur and colleagues serves to highlight the very viable option of assisted ventilation, invasive or non-invasive, in patients with chronic neuromuscular disease and chest wall disorders.10 This group of patients can clearly benefit from this intervention.