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NPPV in Acutely Ill COPD Patients with Varying Levels of Consciousness
Abstract & Commentary
By Dean R. Hess, PhD, RRT, Assistant Director, Respiratory Care, Massachusetts General Hospital Department of Anesthesiology, Harvard Medical School, is Associate Editor for Critical Care Alert.
Dr. Hess reports no financial relationship relating to this field of study.
Synopsis: NPPV can be successfully applied to many patients with mild-to-moderately altered levels of consciousness during an exacerbation of COPD.
Source: Scala R, et al. Noninvasive positive pressure ventilation in patients with acute exacerbations of COPD and varying levels of consciousness. Chest. 2005;128:1657-1666.
In this study, scala and colleagues compared the clinical outcomes of patients with acute respiratory failure due to COPD exacerbations and different degrees of altered levels of consciousness. It was a 5-year case-control study with prospective data collection conducted in a respiratory monitoring unit. Of 153 consecutive COPD patients requiring noninvasive positive-pressure ventilation (NPPV) for acute respiratory failure, 80 were divided into 4 groups, which were carefully matched for the main physiologic variables according to the level of consciousness assessed with the Kelly-Matthay score, in which 1 is normal (control subjects) and 6 is severely impaired. Changes from baseline arterial blood gas levels and Kelly score, the rate and causes of NPPV failure, the rate of nosocomial pneumonia, and the 90-day mortality rate were compared.
NPPV significantly improved arterial blood gases and Kelly score in all groups after 1 to 2 hours. NPPV failure was 15% for Kelly score 1, 25% for Kelly score 2, 30% for Kelly score 3, and 45% for Kelly score > 3. Mortality at 90 days was 20% for Kelly score 1, 35% for Kelly score 2, 35% for Kelly score 3, and 50% for Kelly score > 3. Using multivariate analysis, the acute non-respiratory component of the APACHE III score and baseline pH independently predicted baseline Kelly score. After 1 to 2 h of NPPV, changes in the Kelly score were associated with changes in pH, but no such correlation was found with PaCO2.
Scala et al concluded that NPPV may be successfully applied to patients experiencing COPD exacerbations with a milder, altered level of consciousness. The rate of failure in patients with severely altered level of consciousness (ie, Kelly score > 3) was higher, although better than expected, suggesting that an initial and cautious attempt of NPPV may be performed even in this group of patients.
Use of NPPV in patients with altered levels of consciousness is controversial. Often stated is concern related to the risk of aspiration of gastric contents should the patient vomit. Moreover, agitated patients are typically intolerant of NPPV. Most of the randomized controlled studies of NPPV have excluded a priori patients with an altered level of consciousness. Under the umbrella of "altered level of consciousness," a heterogeneous variety of different degrees of encephalopathy are typically included, ranging from coma to agitation or confusion. In patients with COPD, this is often related to the PaCO2 (hypercapnic encephalopathy). A unique aspect of this study is use of the Kelly-Matthay scale to assess the level of consciousness, which has been specifically designed for critically ill patients requiring mechanical ventilation.
In this study, the application of NPPV was associated with a rapid improvement in gas exchange and neurologic status in the majority of patients, irrespective of the severity of the altered level of consciousness. In patients with mild-to-moderately altered levels of consciousness (Kelly score 3), NPPV was highly successful in terms of clinical outcomes. Patients with a severely altered level of consciousness had an increased but not a dramatically high rate of NPPV failure and 90-day mortality. In this subset of patients, cardiovascular events were more frequently the cause of NPPV failure.
Interestingly, pulmonary aspiration was not observed in any of the subjects. My observation is that aspiration is uncommon in patients receiving NPPV. It is often discussed but seldom seen—perhaps because NPPV is not offered to patients at potential risk for aspiration. Over the course of my career, I have seen many patients with pulmonary aspiration—only a few of whom were receiving NPPV at the time of aspiration. I would venture to guess that I have seen as many patients with gross aspiration past the cuff on an artificial airway as I have seen patients with gross aspiration during NPPV. It is interesting to note that the degree of respiratory acidosis did not fully explain the changes in Kelly score and that other factors, like non-pulmonary acute organ dysfunction, seemed to play an important role.
These data suggest that, during an exacerbation of COPD, NPPV can be successfully applied to many patients with mild-to-moderately altered levels of consciousness. Patients with severely altered levels of consciousness may also benefit from NPPV. However, close monitoring is needed in these patients and prompt intubation should be available if the patient does not respond rapidly. The changes in mental status after a brief trial of NPPV are not exclusively associated with changes in PaCO2, but rather with acute non-respiratory organ impairment. This suggests that level of consciousness may be a more important monitor of success that changes in arterial blood gases.
It should be noted that this study was limited to patients with COPD exacerbation and hypercapnia. Caution is urged when extrapolating these results to the application of NPPV in other patient populations with altered levels of consciousness.