Dr. Lam Siu Pui, Integrated Medical Services, Kwong Wah and Wong Tai Sin Hospitals
Exercise intolerance in COPD patients has important implications on health-related quality of life, hospitalization rate and survival.1 The factors limiting exercise tolerance in COPD include central (lung dynamic, hyperinflation, reduced ventilatory reserve, increased cost of breathing) and peripheral (skeletal muscle dysfunction as a result of de-conditioning or myopathy, poor nutritional status, poor bioenergetic, metabolic acidosis). Consequently, exercise testing is increasingly being used in the functional assessment of COPD patients.
The indications of clinical exercise testing in COPD include (1) assessment of exercise capacity; (2) aid to diagnosis of causes for exercise limitation and symptoms; (3) assessment of factors contributing to exercise limitation; (4) prescription of exercise training program; (5) assessment of need for specific therapy that may improve exercise performance e.g. oxygen therapy during exercise; (6) assess response to therapy e.g. pulmonary rehabilitation.2 The possible methods of assessment of exercise capacity are field tests (six-minute and shuttle walking tests) and cardio-pulmonary exercise testing (CPET).
Six-minute Walk Test (6MWT) is a simple, inexpensive and efficient evaluation tool of functional capacity in COPD patients and is a widely reported outcome measure. Practical guideline is available and should be followed.3 Outcome parameters of the 6MWT include the distance achieved in six minutes, number of rests, trough SaO2 and oxygen supplementation required, heart rate response, rate of perceived dyspnoea (RPD) and rate of perceived exertion (RPE). Factors reducing the 6MWT distances include shorter height, female sex, impaired motivation, impaired cognition, cardio-pulmonary and musculoskeletal diseases and shorter corridors. The minimum clinically important difference in 6MWT distance was estimated at 54 meters (with 95% C.I. 37-71 meters).4 Although the 6MWT is a sub-maximal exercise test for normal subjects, it is towards a maximal test for severe COPD patients.5 The main disadvantage of the 6MWT is the measurement of global function without specific information regarding the cause of the limitation. In addition, patients with milder diseases may not fully demonstrate their exercise ability since walking on a level surface is the only activity allowed.
Incremental Shuttle Walking Test (ISWT): Subjects are required to walk around a 10-m course marked by cones placed 9 meters apart. Walking speed is regulated by pre-recorded signals on a cassette tape. The test started at an initial speed of 0.5 m/s, and increased each minute by 0.17 m/s for a maximum of 12 stages. Only standard taped instructions are given and the distance achieved is measured.6 A modified endurance shuttle walking test has been developed with distance walked at 85% of the maximum speed identified from the initial ISWT along the same 10-m shuttle course. The change in ISWT distance can be used as an outcome measure but is less well studied as compared with 6MWT and CPET.7
Cardiopulmonary exercise testing (CPET) is considered as the gold standard to study a patient's level of exercise limitation and its causes. It provides a comprehensive assessment of the integrative responses involving the pulmonary, cardiovascular, haemopoietic, neuropsychological and skeletal systems. The 2 CPET protocols most frequently used in the clinical setting are the maximal incremental and the constant work rate tests.3 Interpretation of CPET involves a systematic review of the indices of the exercise capacity [peak oxygen uptake (VO2
peak), max. work rate], cardiovascular response [HR vs. VO2, O2 pulse, anaerobic threshold (AT), VO2 vs. work rate], ventilatory response [minute ventilation (VE) / MVV, max RR, PaCO2] and gas exchange [VD/VT, VE/VCO2, PaO2, P(A-a) O2, SaO2]. The response pattern in COPD patients are exampled by (1) decreased VO2 peak (2) decreased or normal AT (3) decreased HR peak (4) normal or decreased O2 pulse and (5) increased in VE / MVV x 100, VE / VCO2 (at AT), VD / VT parameters.3
Exercise testing and predictors of outcomes
Exercise tolerance has been shown to be a good predictor of mortality in COPD patients.8-11 Both the distance and oxygen desaturation during the 6MWT had been shown to be predictors of mortality in COPD patients with FEV1 < 50% of predicted.8-9 Exercise indices, such as VO2 peak, VE/VCO2 (at AT) and SaO2, have in fact proven to be better predictors of prognosis than lung function measurements obtained at rest. 10 Patients with higher BODE scores were at higher risk for death; the hazard ratio for death from any cause and from respiratory causes per one-point increase in the BODE score was 1.34 and 1.48 respectively.11 In addition, the BODE index and the GOLD staging has been shown to be the predictors of the one year exacerbation risk in COPD.12
References
1. Nici L, Donner C, Wouters E et al. American Thoracic Society / European Respiratory Society statement on pulmonary rehabilitation. Am J. Respir Crit Care Med 2006;173:1390-1413.
2. ATS Statement : Guidelines for the Six-minute Walk Test. American Thoracic Society. Am J. Respir Crit Care Med 2002;166:111-117.
3. ATS/ACCP Statement on Cardiopulmonary Exercise Testing. American Thoracic Society. Am J. Respir Crit Care Med 2003;167:211-277.
4. Redelmeier DA, Bayoumi AM et al. Interpreting small differences in functional status: the Six Minute Walk test in chronic lung disease patients. Am J. Respir Crit Care Med 1997; 155(4):1278-82
5. Casas A,Vilaro J et al. Encouraged 6-min Walking Test Indicates Maximum Sustainable Exercise in COPD Patients. Chest 2005;128(1): 55-61
6. Singh S, Morgan D et al. Development of a shuttle waking test of disability in patients with chronic airways obstruction. Thorax 1992;47:1019-1024.
7. Singh SJ, Jones PW et al. Minimum clinically important improvement for the incremental shuttle walking test. Thorax 2008; 63:775-777.
8. Casanova C, Cote C et al. Distance and oxygen desaturation during the 6-min walk test as predictors of long-term mortality in patients with COPD. Chest 2008; 134(4): 746-52
9. Cote CG, Pinto-Plata V et al. The 6-min walk distance, peak oxygen uptake, and mortality in COPD. Chest 2007; 132 (6):1778-85
10. Oga T., Nishimura K. et al. Analysis of the factors related to mortality in chronic obstructive pulmonary disease: role of exercise capacity and health status. Am J. Respir Crit Care Med 2003;167(4):544-9
11. Celli BR, Cote CG, Marin JM, Casanova C et al. The body-mass index, airflow obstruction, dyspnoea, and exercise capacity index in chronic obstructive pulmonary disease. NEJM 2004; 350(10):1005-12
12. Faganello MM. Tanni SE et al. BODE index and GOLD staging as predictors of 1-year exacerbation risk in chronic obstructive pulmonary disease. American Journal of the Medical Sciences 2010; 339(1):10-4
