Sampling problems are inevitable when selecting representative data from each sleep stage, particularly from periods of wakefulness and REM sleep. The heterogeneous nature of stage REM, comprising tonic and phasic components and often, in these subjects, brief arousal periods associated with oxygen desaturation, further limits interpretation of its ventilatory data. The heterogeneity is reflected in the measured coefficient of variation of all our measurements and is also seen in normal subjects. There have been several attempts to quantify ventilation and its components in normal subjects nonin-vasively during wakefulness and sleep. The RIP data of Stradling et al were obtained by MLR from restrained sleeping adult healthy males and so offer an appropriate comparison to the present results. Patients with Duchenne MD have severe generalized respiratory weakness usually without selective diaphragm involvement and, in our subjects, without obesity or significant scoliosis. Here
There have been few reports of quantified ventilatory analysis during sleep in patients with neuromuscular disease and we know of no other such data in Duchenne MD, with or without daytime somnolence or C02 retention. Patients with selective diaphragm weakness typically show a rapid shallow breathing pattern when awake, the frequency of ventilation falling toward normal in sleep. A similar awake breathing pattern is observed in interstitial lung disease but the severity of their sleeping ventilatory impairment depends clearly on the inclusion or exclusion of snorers from the study group, emphasizing the likely importance of upper airway resistance changes to ventilatory morbidity in sleep in these subjects.
Extrathoracic resistance changes normally seen in sleep may account for important differences in sleeping ventilation in neuromuscular disease compared with normal, but such data are not available. Airway resistance changes in sleep may have a proportionately greater influence on inspiratory timing (eg, Ti) and Ve when the inspiratory musculature is weak, especially when REM sleep further exposes the respiratory muscle dysfunction. The large fall in Vi/Ti during sleep in these subjects compared with normal subjects, which might normally reflect a fall in central ventilatory drive, may also be attributed to increased inspiratory loading in sleep.