© 2008 American Thoracic Society doi: 10.1164/rccm.200801-181ED
Boosting the Effectiveness of Rehabilitative Exercise Training
Los Angeles Biomedical Research Institute at Harbor–UCLA
Good researchers design important studies. Great researchers design easy studies. Pulmonary rehabilitation is an important component of care for the patient with chronic obstructive pulmonary disease (COPD) (1). Exercise training is an essential (probably the essential) component of a rehabilitative program. Having decided that exercise training is important, the question becomes, How can its effectiveness be improved? Easier asked than answered, it turns out. Comparing training strategies requires that randomized trials be conducted in patient groups in which only the feature in question differs between training regimens. Because patients with COPD differ in many respects that may influence training responses, and because crossover trials are not generally appropriate (bringing the patient back to the pretraining state before applying the second training period is not practical), the sample size required to discern modest differences between training strategies is likely to be large. Because training interventions are lengthy (perhaps 3 sessions/wk for 8 wk), comparing training strategies is a major undertaking. Choosing the most relevant outcomes for study on training interventions can be problematic. Tolerated exercise duration seems a reasonable outcome, but which test to choose? Furthermore, exercise duration is clearly motivation and effort dependent. Selecting non–effort-dependent outcomes might overcome this problem, but changes in variables such as lactic acidosis threshold or isotime pulmonary ventilation might be questioned as to their clinical relevance. Discerning differences in quality of life or survival between training strategies might be attempted (and might be considered important), but sample sizes would need to be very large and/or duration of observation would have to be very long. A sterling example of how training interventions are compared is shown in the work of Pollack and his colleagues, whose studies in healthy subjects in the 1970s informs our strategies for patients with COPD (2, 3). Wondering how long training sessions had to be and how many times a week subjects had to exercise, Pollack assigned subjects to a matrix of training programs and compared their effects on peak oxygen uptake. From his studies, and the studies of others, it has been concluded that exercise sessions of at least 30 minutes' duration held at least three times a week are advisable (4). It has been assumed that these findings apply to COPD training programs; it would be prohibitively difficult to reproduce them in the COPD population. Greater inherent variability among subjects implies that very large study groups would be required to adequately make these distinctions. Studies have started to appear in which rehabilitative training strategies are compared. An early study showed that higher intensity exercise was more effective than lower intensity exercise, even when total work per session was the same (5). In a high-intensity training program, supplemental oxygen increased tolerated training intensities and resulted in superior exercise tolerance improvement in patients with COPD without clinically appreciable exercise desaturation (6). Testosterone was shown to have an additive effect on strength improvements elicited by a strength-training program (7). The bronchodilator tiotropium had additive effects on exercise tolerance gains from a rehabilitative training program (8). It is remarkable that the study groups in these investigations were rather small. The sole exception is the bronchodilator trial (supported by pharmaceutical industry funding) in which 91 subjects were studied. The success of these studies in demonstrating significant effects despite small study groups speaks either to a large effect size or perhaps, dare I say, to a little good luck! Not all such attempts have been successful. Interval training (alternation of high- and low-intensity periods during a training session) has not been shown to yield superior results as compared with constant intensity training in patients with COPD (9, 10). Heliox breathing has been shown to increase exercise tolerance, but has failed to improve rehabilitative training benefits (11). Proportional assist ventilation, by unloading the respiratory muscles, has the potential to improve exercise tolerance, but so far has not been shown to improve enhance exercise training (12). The study of Collins and colleagues (13) in this issue of the Journal (pp. 844–852) is a worthy addition to this literature. It is posited that a slower–deeper ventilatory pattern has the potential to decrease dynamic hyperinflation and thereby improve exercise tolerance in COPD. These investigators sought to determine whether incorporating sessions in which subjects received feedback that encouraged a slower–deeper breathing pattern improved exercise tolerance gains of an endurance training program. In the key comparison, constant work rate exercise endurance gains of 16 subjects who completed endurance training were compared with 17 subjects who used ventilation feedback during their training. The difference in the increase in exercise endurance between the two groups failed to achieve the prespecified level of statistical significance. Yet the results are quite encouraging. The key comparison would have been statistically significant had only these two groups been compared (a third group that underwent ventilation feedback training alone was included). Also, the way in which the primary outcome measure was incorporated may have decreased the likelihood of demonstrating a significant result. Oga and coworkers demonstrated that, for a given intervention, fractional exercise tolerance increases were larger when assessed by constant work rate duration than with incremental exercise testing or with the six-minute-walk test (14). However, a disadvantage of this test is that, if the intervention is very effective, a work rate that is above critical power before the intervention will be below the critical power after the intervention. In this case, the postintervention test will have a duration that is essentially infinite. This becomes more problematic if preintervention constant work rate test duration is long (i.e., it is only a little above critical power). A target constant work rate duration in the 4- to 7-minute range has been proposed (15). Because, in Collins and colleagues' study, pretraining duration averaged about 10 minutes, it seems plausible that, post-training, exercise duration may have been limited by boredom rather than physiologic factors in some subjects. Nonetheless, the results obtained by Collins and coworkers are quite promising. A larger trial of this strategy would seem to be a priority. Furthermore, pulmonary rehabilitation practitioners should, I think, start to consider incorporating selected strategies with the goal of boosting the effectiveness of this important therapy. FOOTNOTES Conflict of Interest Statement: R.C. has no financial relationship with a commercial entity that has an interest in the subject of this manuscript. REFERENCES
Related articles in AJRCCM:
This article has been cited by other articles:
|
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
