The fact that the findings with regard to the pulmonary function variables were inconsistent suggests several possible interpretations. The most likely explanation is that the pulmonary function variables assessed were not meaningful in the examination of the relationship. This could be true because the participants in this study were primarily only mildly to moderately impaired (95% of the sample). Therefore, the participants were not experiencing desaturation and were not limited in their oxygen delivery at submaximal levels of performance. It could also be that the measures that were taken in this study were not the ideal in terms of assessing limitations, which could influence the cerebral environment. For example, some researchers who have used Pa02 as their measure of oxygenation have found that there is a relationship between Pa02 and cognitive functioning. A third explanation for the discrepant results is that the cerebral environment may be resistant to declines in pulmonary function. This notion is supported by past research and is suggestive of a threshold model in the relationship between pulmonary function and cognition. It is also possible that the demands of the tasks are differentially related to disease state and pulmonary function so that on some tasks, there is a positive relationship, while in others there is a negative relationship. Finally, it is possible that the use of different samples for the different cognitive measures resulted in the differing results. However, random assignment was used to minimize the likelihood of systematic differences existing between the samples.
In conclusion, on a fluid intelligence task and on a speed-of-processing task, support was found for the idea that age and aerobic fitness have the predicted relationship with cognitive performance in a COPD sample. This finding adds to the growing literature suggesting that aerobic fitness may serve to minimize or slow the normal age-related declines in cognitive functioning. This finding also extends the past literature by providing evidence that aerobic fitness may lessen age-related declines in cognition even in a sample of participants with pulmonary limitations. However, it is necessary to conduct further research in this area to determine the precise relationship between these variables and cognition and to further examine the mechanisms underlying the relationship. In addition, a study is needed in which aerobic fitness is actually manipulated so that the mechanisms underlying this relationship can be more closely examined and so that a cause and effect relationship can be tested.