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Does COPD Worsen Outcomes When Mechanical Ventilation Is Required for Other Reasons?
Abstract & Commentary
By David J. Pierson, MD, Professor, Pulmonary and Critical Care Medicine, Harborview Medical Center, University of Washington, Seattle.
This article originally appeared in the June 2011 issue of Critical Care Alert. It was peer reviewed by William Thompson, MD. Dr. Thompson is Associate Professor of Medicine, University of Washington, Seattle. Drs. Pierson and Thompson report no financial relationships relevant to this field of study.
Synopsis: In a cohort of mechanically ventilated patients admitted with a variety of acute diagnoses, those diagnosed with COPD (but not in exacerbation) had higher ICU mortality but no difference in risk for ventilator-associated pneumonia as compared to patients without the diagnosis of COPD.
Source: Rodríguez A, et al. Impact of non-exacerbated COPD on mortality in critically ill patients. Chest 2011; March 10. [Epub ahead of print.]
Rodríguez and colleagues report on a prospectively enrolled cohort of 235 consecutive patients admitted to two ICUs in Spain who required mechanical ventilation for at least 48 hours and had indications for ventilatory support other than respiratory infection or acute exacerbation of chronic obstructive pulmonary disease (COPD). The goal was to determine whether the existence of underlying COPD worsened survival of the critical illness and also whether having non-exacerbated COPD predisposed patients to ventilator-associated pneumonia (VAP).
The authors used a standard definition of COPD as characterized by chronic airflow obstruction, but they made the diagnosis using "clinical criteria" (not further described), medical records, and/or evidence of hyperinflation on chest radiograph if the patients had not had pulmonary function testing. The diagnosis of VAP required compatible findings on chest radiograph plus either purulent sputum, a leukocyte count exceeding 10,000/mL (or 20% higher than the admission value), or fever; microbiologic criteria were not used.
The 235 patients included 60 (26%) diagnosed with COPD and 175 (74%) without this diagnosis. Patients in the COPD group were more often admitted for medical (vs. surgical or trauma) reasons; they were also older, had more comorbidities, and had higher APACHE II scores, all these differences from the non-COPD group being statistically significant. Overall ICU mortality was 26% and was higher in the COPD group (37%) than in the non-COPD group (23%; P < 0.05). The magnitude of the difference (14%) was attributed to the presence of COPD in the former group. Duration of mechanical ventilation in survivors was not different in the two groups.
The incidence of VAP as defined in this study was 11.9/1000 ventilation days in the COPD group and 16.0/1000 ventilation days in the non-COPD group (P = 0.40). By multivariate analysis, the statistically significant predictors of increased mortality were the presence of COPD, being in shock on admission to the ICU, and having a medical (vs. surgical/trauma) diagnosis. The authors conclude that patients with underlying but non-exacerbated COPD have higher ICU mortality than patients without COPD, but no increased risk for developing VAP during their ICU stay.
Three aspects of this study diminish the confidence with which its findings can be accepted. First, although it was generally in line with ICU practice in my experience, the diagnosis of COPD was imprecise and likely inaccurate in at least some patients. Airflow obstruction was acknowledged as the primary defining characteristic of the disorder, but in only 11 of the 60 "COPD" patients could severity be determined according to the criteria of the Global Initiative for Chronic Obstructive Lung Disease. This means that only one-sixth of the patients in the COPD group had had spirometry to confirm the diagnosis and determine severity, and that in the others the diagnosis was based on vaguely stated clinical grounds. Because the history, physical exam, and chest X-ray are much less reliable for detecting the presence of COPD than spirometry, especially when it is only moderate in severity, the possibility exists that at least some of the "COPD" patients did not in fact have this disorder or, alternatively, that if they all had COPD it was most likely severe.
The second weakness is that the patients assigned to the COPD group differed in important ways from those in the non-COPD group, apart from the presumed presence of COPD. They were older and had more comorbid conditions such as diabetes and cardiomyopathy. Their illness severity was greater on ICU admission, by APACHE II scores, and they were more likely to be admitted because of medical illness as compared to surgery or trauma. These important potential confounders make it more difficult to ascribe the observed outcome differences to the presence of COPD, despite at least some of them failing to be independent predictors by multivariate analysis.
Third, the diagnostic criteria used for VAP purely clinical without the use of microbiologic data were not as robust as those used in the most rigorous studies of this entity. In the methods it is stated that all patients had quantitative cultures performed on endotracheal aspirates, but the results were not used in diagnosing VAP.
These drawbacks notwithstanding, this study supports the concept that the presence of COPD as a background comorbidity in patients requiring mechanical ventilation for reasons other than an acute exacerbation predisposes patients to worse outcomes than individuals not having COPD, all other conditions being equal. It also supports the concept that VAP is primarily a complication of endotracheal intubation, and that underlying COPD is not a major determinant of its acquisition in the context of critical illness.