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Charlottesville, VA
Dr. Arsura is Professor of Clinical Medicine, University of Virginia, School of Medicine.
Correspondence to: Edward Arsura, MD, Professor of Clinical Medicine, University of Virginia, School of Medicine, Charlottesville, VA 22908; e-mail: edward.arsura{at}med.va.gov
Stroke is the leading cause of serious long-term disability in adults. In the United States, > 700,000 people experience a stroke annually, and approximately 4 million Americans are currently alive after experiencing a cerebrovascular event.1 Two thirds of individuals require rehabilitation, and the majority of stroke survivors have residual disability, with equal proportions having mild, moderate, or severe impairment.
All stroke survivors expect to receive appropriate interventions directed at improving outcomes and preventing a recurrent event. In addition to addressing hypertension, predisposing cardiac conditions, and hypercoagulable states, secondary prevention should be directed toward improving other risk factors that contribute to the development of atherosclerosis, including lifestyle modifications. Over the past 2 decades, exercise capacity and activity status have become well-established predictors of cardiovascular and overall mortality.23 In both healthy subjects and those with cardiovascular disease, peak exercise capacity is a stronger predictor of an increased risk of death than other clinical variables or established risk factors. In one trial,4 the risk of death from any cause in subjects whose exercise capacity was < 5 metabolic equivalents was roughly double that of subjects whose exercise capacity was > 8 metabolic equivalents.
Exercise capacity 1 month after a stroke is compromised by approximately 30%.56 The population of individuals who are alive after a stroke is more likely to have a sedentary lifestyle, which leads to progressive deconditioning and worsening cardiovascular fitness. Unfortunately, this deterioration occurs in those who intuitively have the greatest need for the optimization of cardiovascular fitness. Poor cardiorespiratory fitness has been linked to a higher risk of stroke and stroke mortality.7
In order to assess and monitor cardiovascular fitness, a practical, reliable, and valid tool needs to be utilized. The measurement of maximal oxygen uptake (
O2max) [ie, the greatest amount of oxygen a person can utilize from inspired air while performing dynamic exercise involving a large portion of the total muscle mass8) is considered to be the best measure of cardiorespiratory fitness and exercise capacity. In patients with paretic limbs, the determination of O2max has shortcomings related to issues with tolerability of the test, as well as technical and training concerns. Measurements of cardiorespiratory fitness such as the 6-min walk test (6MWT) or cycle ergometry might be suitable substitutes for the determination of O2max. However, these tests were initially studied in populations of individuals who differ from those with stroke and have not been extensively studied in this group of individuals.
The article by Pang and colleagues in the February 2005 issue of CHEST (see page 495) helps us to better select the most appropriate alternative to O2max for the assessment of cardiorespiratory fitness in stroke survivors. Central to the importance of their study is its contribution to our understanding of the impact that stroke has in decreasing cardiorespiratory fitness.
The authors studied 63 single-stroke survivors who had experienced a cerebrovascular event > 1 year previously. The study participants were subjects who were able to walk independently with or without a walking aid. The investigators performed the following two exercise tests: the 6MWT; and cycle ergometry. To establish whether there was a relationship between O2max and the two more easily executed assessments of cardiorespiratory fitness, O2max was measured during both tests.
In contrast to what has been demonstrated in nonstroke populations, Pang et al found a nonsignificant, low correlation between 6MWT and O2max values in the chronic stroke population. The explanation for this finding lies in stroke-specific impairments that lead to poor 6MWT performance. Using stepwise multiple regression analysis, the authors were able to quantify the role of each of the assessed variables. The most important variable was functional balance, as assessed by the Berg balance score. This factor accounted for over two thirds of the variance in performance. Other determinants of lesser importance included knee extension strength, reduction in lean muscle mass in the paretic leg, and the degree of lower extremity spasticity. The 6MWT was normalized to leg length, and additional factors that could affect the 6MWT (ie, age, gender, time elapsed since stroke, and percentage of body fat) were evaluated in the regression model. The significance of their observations is that the 6MWT is not a good indicator of cardiorespiratory fitness in individuals with chronic stroke.
In healthy populations, the O2max determined during cycle ergometry is modestly lower than that achieved during an exercise treadmill test.9 In addition, cycle ergometry may be unsuitable for elderly, frail, and severely limited patients, and a clinician might expect that the 6MWT more accurately reflects cardiorespiratory fitness.9 However, in the chronic stroke population, cycle ergometry (mean [± SD] O2max, 22.0 ± 4.8 mL/kg/min) actually provides a better estimate of O2max than the 6MWT (mean O2max, 14.7 ± 3.3 mL/kg/min), and it is a more reliable marker of cardiorespiratory fitness. It is assumed that the difficulties that the study population had with balance, strength, and spasticity were better accommodated by cycle ergometry. In line with this assumption is the mean age-predicted maximum heart rate, which was 91.8 ± 10.7% compared with 65.1 ± 8.9%, respectively, using cycle ergometry and 6MWT. The level of perceived exertion, as measured by the 16-point Borg rating of perceived exertion scale, was also significantly greater during cycle ergometry. The authors discuss the fact that an exercise treadmill test with harness support minimizes the requirement for maintaining balance and may be an alternative to cycle ergometry. This qualification is intuitive, and further comparison of the two tests is warranted. However, the current study seems to indicate that cycle ergometry is the better assessment modality and perhaps the preferred training method for improving cardiorespiratory fitness in the population of patients with chronic stroke and residual disability.
One of the most significant findings in the study (and perhaps the true bottom line for all of us who concern ourselves with the care of patients discussed in the article by Pang et al) is the exceedingly poor cardiorespiratory fitness of these individuals. When compared to norms in age-matched healthy populations, the O2max in the study population was approximately 20 to 25% lower and in the region of the 10th percentile. The mean percentage of body fat is increased in this population, and the lean body mass is decreased. This situation is especially critical considering that the study subjects were all at a higher level of function compared to the stroke population in general.
Now that we have been educated and given the appropriate tools, we have the imperative to develop programs for patients that can lead to improved fitness. Appropriate trials that build on the data in the article by Pang et al to determine the impact of fitness enhancement on pertinent outcomes should be encouraged.
References
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