Hypercapnia nocturnal

Although a Cochrane meta-analysis (73) does not recommend the institution of NIV for symptomatic sleep hypopneas, a prospective study from Mellies et al. (74) and a randomized controlled trial by Ward et al. (72) support the indication of NIV for patients with NMD and nocturnal hypercapnia. The consensus conference of the American College of Chest Physicians on MV beyond the ICU recommended that patients who develop symptomatic nocturnal hypercapnia, even in the absence of daytime hypercapnia, are candidates for long-term ventilatory assistance (75). 

Nocturnal NIV is indicated when hypercapnia develops. Vianello and colleagues (108) report a life expectancy of less than one year once diurnal hypercapnia develops. Many studies of NIV in DMD note its effect on increasing survival (103,109,110), which, in one study, increased to a median of 73% at five years (110). In an RCT of normocapnic, asymptomatic, patients with a VC 20-50% predicted, survival did not increase, although... 

Garray SM, Turino GM, Goldring RM. Sustained reversal of chronic hypercapnia in patients with alveolar hypoventilation syndromes. Long term maintenance with non-invasive nocturnal mechanical ventilation. Am J Med 1981 70 269-274. 

Approximately 10-15% of patients with OSAS present with daytime CRF along with hypoxia, hypercapnia, and PH (2). When these patients present with a stable respiratory status, their nocturnal pattern of predominant OSA leads to a discussion on the use of... 

Mechanical ventilation (MV) with bi-level NIV must be considered in the presence of nocturnal hypoventilation, especially if accompanied by cor pulmonale, nocturnal arryth-mias, morning headache, impaired cognitive function, or reduced daytime vigilance. Right heart failure is also frequently present. The obese sleepy patient (27) with chronic hypoxia and hypercapnia is the typical presentation of OHS. To adapt to NIV, it is important to identify the nocturnal respiratory pattern of such patients. 

A previous study (14) has suggested that nocturnal MTV could be used as an interim measure in subjects with severe OSA and hypercapnia until ventilatory decompensation is reversed (possibly by alterations in ventilatory drive and ventilatory responses to hypercapnia and hypoxia) and CPAP therapy can then be used long term. Others (30) have shown that a proportion of patients may be switched over to CPAP once respiratory failure has been controlled. CPAP therapy from the start, rather than bi-level ventilation followed by CPAP, may be just as effective (particularly improving sleep architecture and arousals) and potentially more cost-effective in patients with OHS, even if blood gases are not corrected immediately. 

Nocturnal hypoventilation, which persists during the day in obese patients with nocturnal apneas and hypoventilation may be associated with COPD, defining the overlap syndrome (3). In 264 OSAS patients (32), 30 had coexistent COPD, with hypoxemia in 57%, hypercapnia in 27%, and PH in 37% of them. In the other 234 pure OSAS patients, only 26% were hypoxic, 8.5% were hypercapnic, and 11 % had PH at baseline. The same schedule may be proposed for OHS, but long-term studies are also warranted to confirm the initial choice and continuation of nCPAP or bi-level NIV. 

Bi-level NIV may be used as a first-line treatment, with supplemental oxygen (27). Expiratory airway pressure is titrated to control h5q)opneas and apneas, and inspiratory airway pressure is added to control Paco2. If bi-level NIV fails, nasal volume ventilation may be used (29). In many patients with OHS and predominant OSA, once hypercapnia has improved (which may take several weeks) nCPAP may be used (29). Thirteen obese patients (n = 13) with a BMI > 35, aged 28-69 years with severe OSAS and hypercapnia (8.2 0.3 kPa) and failing to respond to initial CPAP therapy, were treated via a nasal nocturnal volume-cycled ventilator, which was tolerated by all patients. Significant improvements in daytime arterial blood gas levels were obtained after 7 to 18 days of nasal intermittent positive pressure ventilation (29) in 10 of the 13 patients three months later, 12 of the 13 patients could be converted to nCPAP therapy and one patient remained on NIV. In another study (37), the same results were observed after three months of home nocturnal bi-level NIV in seven patients, three of whom had severe obesity. 

These improvements in nocturnal alveolar hypoventilation translate into a decrease in dirrmal hypercapnia among patients with OSA and NMD (38,39). This benefit may be due to the combined effects of several interrelated processes. Reduced cerebrospinal fluid bicarbonate concentration resets the ventilatory response to CO2 and increases respiratory drive. Improved sleep quality influences the ventilatory response to CO2 and improved... 

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