Although the above reasons support the notion that COPD patients do not tolerate NPPV as well as patients with restrictive disorders, some COPD patients chronically use NPPV for long periods of time and appear to derive physiologic and functional benefit. What makes this patient group distinct from other patient groups with COPD is currently unknown, but several hypotheses could be put forth.
First, some COPD patients have been shown to have an overlapping syndrome (ie, combination of COPD and obstructive sleep apnea syndrome) that may benefit from the use of nocturnal ventilation. This patient group may, therefore, favorably influence the beneficial effects of NPPV in chronic respiratory failure and COPD. Most studies that tend to demonstrate an improvement in symptoms and gas exchange with NPPV in severe COPD have included patients with moderate to severe hypercap-nia on implementation. Indeed, our patients had higher levels of PaCo2 than did any other patients in whom NPPV has been applied on a chronic basis.
Our results are similar to those of Meecham-Jones et al, who showed a benefit in 12 patients with severe COPD whose mean PaC02 was > 55 mm Hg. In contrast, Strumpf et al and Gay et al failed to show any benefit with NPPV in COPD patients whose average PaC02 levels were < 45 mm Hg. asthma mist asthma relief
Perhaps COPD patients with severe hypercapnia represent a subgroup of patients in whom hypoventilation is a component of their disease, and thus they benefit from chronic NPPV application.
Our data suffer from the standpoint that the study did not have an appropriate control arm of patients who only received long-term oxygen or low levels of continuous positive airway pressure. We believe that this would not have been appropriate for those patients with restrictive ventilatory disorders because many studies have now shown the beneficial effects of NPPV therapy in this patient group. Our COPD patient group, as a whole, was extremely ill (ie, COPD patients were acidemic, 40% had cor pulmonale, and one third had been intubated < 6 months prior to study entry) and had already failed maximal standard therapy. Despite the absence of an adequate control group, however, the observation that patients who were noncompliant with NPPV tend to showed a worsening in gas exchange and functional status supports our notion that NPPV had a important therapeutic role in those who complied with therapy.
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