The presence of clinical symptoms and/or physiologic markers of hypoventilation are useful in identifying clinical severity as it relates to therapeutic decision-making with regard to initiation of nocturnal NIPPV. In the course of a typical progressive disease, two successive steps occur more or less rapidly: (i) nocturnal hypoventilation reversible during wake associated with none or a few clinical symptoms; and (ii) nocturnal and daylight hypoventilation associated with clinical symptoms, which show a low respiratory reserve and should be considered an unstable state with increased susceptibility to life-threatening acute ventilatory failure that may be triggered by what may otherwise be trivial additional factors (41,42). A sleep study continuously recording CO2 (end-tidal EtCO2 or transcutaneous TcCO2) and/or oxygen saturation (SpO2) is required to document nocturnal hypoventilation, which may occur throughout all sleep stages but in some cases exclusively during rapid eye movement (REM) sleep. Daytime hypoventilation is defined by an abnormally elevated partial pressure of arterial carbon dioxide (PaCO2), a high-serum bicarbonate level and a relatively normal pH with associated reduction of partial pressure of arterial oxygen (PaO2). Chronic daytime hypoventilation is an important indicator invariably associated with sleep-related hypoventilation. Thus, in the presence of diurnal hypoventilation, the reason for overnight recording is only to rule out obstructive or central apnea. Clinical symptoms indicating consequences of hypoventilation (Table 1) must be carefully evaluated since even when modest, they are important for the appreciation of disease severity and prognosis and to indicate NIPPV. Pulmonary function tests help define and quantify the ventilatory-respiratory disease but have low predictive values for chronic sleep-related hypoventilation in individual patients except in those with neu-romuscular disease. Indeed, in Duchenne muscular dystrophy, hypoventilation appears only during REM sleep, all night, or during the daytime when supine inspira-tory vital capacity is < 40%, < 25%, and < 12%, respectively (41). Similarly a peak cough flow < 160 L/min, related to expiratory muscle deficit, means an increased risk of accumulation of secretions that may worsen hypoventilation and trigger acute failure (18,43-46). It is crucial to notice that isolated reduced PaO2 does not mean hypoventilation but only a mismatching of ventilation and perfusion adequately compensated or even overcompensated (low PaCO2), which will not require support by mechanical ventilation but only by supplemental oxygen.
TABLE 1 Clinical Features Frequently Associated with Alveolar Hypoventilation
Shortness of breath during activities of daily living in the absence of paralysis
Orthopnea in patients with disordered diaphragmatic dysfunction
Nocturnal or early morning headaches
Daytime fatigue, drowsiness and sleepiness, loss of energy
Decrease in intellectual performance
Loss of appetite and weight loss
Appearance of recurrent complications: respiratory infections Clinical signs of cor pulmonale
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