Section 8: Respiratory Diseases

8.2 Chronic Obstructive Pulmonary Disease

COPD and Driving Literature Review

As with asthma, there are no studies available that have examined explicitly the relationship between COPD and motor vehicle crashes. Because of the paucity of research regarding COPD and crashes, decisions regarding driving competence for individuals with COPD, as with many other chronic conditions, must be based on an evaluation of the effects of the condition on driving competence (e.g., direct driving performance measures) rather than on relative risk data.

Cognitive impairment, resulting from chronic hypoxemia, is the primary issue for driving competency in individuals with COPD. A number of researchers have examined cognitive performance in individuals with COPD (Fix, Golden, Daughton, Kass, and Bell, 1982; Grant, Prigatano, Heaton, McSweeny, Adams, and Timms, 1982; Huppert, 1982; Incalzi et al., 1997; Isoaho, Puolijoki, Huhti, and Laippala, 1996; Incalzi, Chiappini, Fuso, Torrice, Gemma, and Pistelli, 1998; Kozora, Filley, Julian, and Cullum, 1999; Prigatano et al., 1983; Stuss, Peterkin, Guzman, Guzman, and Troyer, 1997). In general, the overwhelming majority of studies have found cognitive deficits in individuals with COPD, with impairments greater on the more complex and demanding cognitive tasks.

Two studies worthy of further description are those conducted by Grant et al. (1982) and Prigatano et al. (1983). Both studies were large multicenter trials in United States and Canada. Each study included extensive neuropsychological and pulmonary function testing in patients with COPD and a group of non-patient demographically matched controls. The first study (Grant et al., 1982), termed the Nocturnal Oxygen Therapy Trial (NOTT), included 203 patients who were moderately to severely hypoxic (mean age = 64 years, mean PaO2 = 51 mm Hg). Results revealed that patients with COPD performed significantly worse than controls on almost all neuropsychological tests. Forty two percent of the patients exhibited moderate to severe neuropsychological test performance compared to 14 percent of controls. Higher cognitive functions were the most severely affected. Correlations between neuropsychological test performance and pulmonary function measures were, however, disappointingly low. As noted by the authors, a restriction of range on some of the key medical variables most likely accounts for the findings.

The second multicenter investigation was conducted by Prigatano et al. (1983) and was called the Intermittent Positive Pressure Breathing (IPPB) Trial. One hundred patients with COPD (mean age = 62, mean PaO2 = 66 mm Hg) underwent extensive neuropsychological and pulmonary function testing (identical with those of the Grant et al., 1982 investigation), with results compared with demographically matched controls tested on the same measures. The COPD patients in this investigation were classified as more mildly impaired than those in the Grant et al. (1982) investigation. Nevertheless, the mildly impaired patients were found to have selective neuropsychological impairments compared to controls. Again, the correlations between neuropsychological test performance and pulmonary function measures were disappointing.

In a more recent report, Grant, Prigatano, Heaton, McSweeny, Wright, and Adams (1987) merged the databases from these two large multicenter trials. Importantly, the merged database addressed some of the limitations inherent in the independent databases: notably the restriction of range in key medical variables of interest. Thus, the merged database allowed for the analysis of results from a much larger and more representative database. The sample consisted of 86 mildly hypoxic (PaO₂ > 60 mm Hg), 155 moderately hypoxic (PaO₂ = 50-59 mm Hg), and 61 severely hypoxic (PaO₂ <50 mm Hg) COPD patients and 99 age- and education-matched non-patients. Results indicated that the rate of neuropsychological deficits rose from 27 percent in those with mildly hypoxemia to 61 percent in those with severe hypoxemia. One category of neuropsychological tests (based on factor analysis) was the most effective in discriminating between the study groups. Specifically, tests reflecting perceptual learning and problem solving were effective in separating the groups: Controls and mildly hypoxic patients were similar in performance, moderately hypoxic patients were significantly worse than controls or mildly hypoxic patients, and severely hypoxic patients were the most impaired. Results from multiple regression indicated that age and PaO₂ were significant predictors of the perceptual learning and problem scores. Finally, logistic regression results indicated that there were three predictors of neuropsychological impairment versus non-impairment. Those predictors were age, education, and PaO₂. Higher age, lower levels of education, and lower PaO₂ levels were associated with impairment.

Results from Grant et al. (1987) suggest that categorization of COPD patients in terms of disease severity is an important factor for determining the presence or absence of cognitive impairment. In an earlier study, Grant et al. (1982) developed a summary index of medical disease for the patients with COPD. The Severity of Disease Index is based on five key variables (forced expiration volume, maximum exercise tolerance, heart rate, mean pulmonary artery pressure, and resting arterial oxygen saturation), and rated on a four point scale with a higher rating (4) indicative of a severely abnormal result. The overall disease severity index has a range of 5 to 20, with the upper end of the scale reflective of more severe disability. The psychometric properties of the Severity of Disease Index were not provided and perhaps are not available. This is unfortunate as this type of scale may have considerable utility for evaluating disease severity. Important next steps would be to validate the scale against the type and extent of cognitive impairment, and, for fitness-to-drive goals, to validate it against defensible driving performance measures.

Criteria for the evaluation of respiratory impairment/disability also have been published by the American Thoracic Society (1986). In their statement, the American Thoracic Society recommends that evaluation of respiratory impairment include both forced vital capacity (FVC) and forced expiratory volume in the first second (FEV1). Categorization of patients as to degree of impairment is listed in Table 22. As with the Severity of Disease Index, psychometric properties of the Ratings of Impairment were not reported. Important next steps would be to evaluate the utility of the ratings in predicting cognitive impairment, and to validate the ratings against defensible driving performance measures.

Table 22 Ratings of Impairment in Individuals with Respiratory Disorders

(based on information from The American Thoracic Society (1986). Evaluation of impairment/disability secondary to respiratory disorders. American Review of Respiratory Disorders, 133, 1205-1209)

In conclusion, although the literature is sparse, that which is available suggests that individuals with COPD are at higher risk of cognitive impairment compared to age matched controls. Clearly, the presence of cognitive impairment places the individual at-risk for motor vehicle crashes. Future research, focusing on predictors of cognitive decline in this population, is needed. For the present, decisions regarding fitness-to-drive should be made on an individual basis, with determinations of driving competence based on cognitive and/or on-road assessments.