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Validity of the 6-minute walk test for assessing heart rate recovery after an exercise-based cardiac rehabilitation programme
Physiotherapy 92 (2006) 116–121
Validity of the 6-minute walk test for assessing heart rate recovery after an exercise-based cardiac rehabilitation programme Elved Roberts a,c,∗ , Frederick K.W. Li b , Kevin Sykes c a
Liverpool Cardiothoracic Centre, Thomas Drive, Liverpool, Merseyside L14 3PE, UK b Queen Elizabeth Hospital, Hong Kong, China c Centre for Exercise and Nutrition Science, University of Chester, Chester, UK
Abstract Objectives To compare heart rate responses in the 6-minute walk test and the treadmill exercise test before and after an exercise-based cardiac rehabilitation programme. Design Prospective cohort study. Setting Hospital-based cardiac rehabilitation programme in Hong Kong. Participants Thirty patients (mean age 62.1 ± 8.5 years, 20 males) with stable ischaemic heart disease. Interventions Eight-week exercise-based cardiac rehabilitation programme involving upper and lower limb aerobic and resistance training. Main outcome measures Six-minute walk test and treadmill exercise test before and after the exercise programme. Results Comparing parameters before and after the exercise programme, the peak heart rate in the 6-minute walk test increased (median of 105 beats per minute (bpm), interquartile range 96.8–116.5 versus 110 bpm, interquartile range 100.5–124.5, P = 0.006), while heart rate recovery improved in each 30-second interval of a 2-minute recovery period. The distance covered during the 6-minute walk increased from a mean of 486.3 m (±standard deviation 113.9 m) to 552.5 m (±standard deviation 111.9 m) (P < 0.001). Rating of perceived exertion during the 6-minute walk test decreased from a median of 14 (interquartile range 13–15) to 13 (interquartile range 13–13) (P = 0.001). Heart rate recovery following treadmill exercise testing improved during the 30-second periods from 60 to 90 seconds and from 90 to 120 seconds of recovery. Metabolic equivalents increased during treadmill testing from a median of 7.0 (interquartile range 5.8–8.6) to 8.6 (interquartile range 7–8.6) (P < 0.001). Conclusions Both treadmill exercise and 6-minute walk tests demonstrated improvements in heart rate recovery and increases in achieved workload following exercise training. This suggests that the 6-minute walk test is a valid tool to assess heart rate recovery following such a programme. © 2005 Chartered Society of Physiotherapy. Published by Elsevier Ltd. All rights reserved. Keywords: Six-minute walk; Treadmill testing; Ischaemic heart disease; Cardiac rehabilitation; Heart rate recovery
Introduction Treadmill exercise test data have confirmed that heart rate recovery is a marker of physical fitness and exercise capacity, that abnormal heart rate recovery is a strong predictor of mortality in both asymptomatic individuals [1–3] and cardiac patients [4,5], and that exercise training improves heart rate recovery in cardiac patients [6]. The 6-minute walk test has ∗ Corresponding author. Present address: Centre for Exercise and Nutrition Science, University of Chester, Parkgate Road, Chester CH1 4BJ, UK. Tel.: +44 151 228 1616 (pager 782); fax: +44 151 288 2254. E-mail address: [email protected] (E. Roberts).
a range of applications in characterising exercise response in both cardiac and non-cardiac patients. It has also been proposed as a valid measure of functional capacity after a cardiac rehabilitation programme [7], and has been shown to approximate a sub-maximal treadmill exercise test at near 80% maximal oxygen uptake (VO2max ) in terms of achieved workload [8]. This test has also been validated against symptom-limited treadmill testing in cardiac rehabilitation patients [9] and in elderly patients with coronary artery disease [10]. However, heart rate recovery in the 6-minute walk test has not been validated against heart rate recovery in the treadmill exercise test following a moderate-intensity cardiac rehabilitation exercise programme. If the 6-minute walk test could be shown to reli-
0031-9406/$ – see front matter © 2005 Chartered Society of Physiotherapy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.physio.2005.06.005
E. Roberts et al. / Physiotherapy 92 (2006) 116–121
ably quantify heart rate recovery in patients undergoing an exercise-based cardiac rehabilitation programme, this simple and effective measurement tool could be used to monitor response to physical training instead of other more complicated tests of physical capacity. Furthermore, as it is known that heart rate recovery following treadmill testing has prognostic significance, and that the 6-minute walk test is capable of prognostic stratification in other conditions [11], it might be expected that prognostic information could be derived from the 6-minute walk test in cardiac rehabilitation patients. The aim of this study was to compare heart rate recovery in treadmill exercise and 6-minute walk tests in a cohort of cardiac patients undergoing an exercise-based rehabilitation programme. Methods Thirty-eight patients who had been medically stabilised following hospitalisation with an acute coronary syndrome or for coronary revascularisation were recruited prospectively, and all gave written informed consent (Fig. 1). Eligibility criteria included age >30 years and stabilised acute coronary syndrome/coronary artery disease. Exclusion criteria included permanent pacemaker or internal cardioverter/defibrillator, left bundle branch block or electrocardiographic pre-excitation, New York Heart Association Class IV heart failure or echocardiographic ejection fraction <30%, valvular heart disease, hypertrophic cardiomyopathy,
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revascularisation during the study period, inability to participate in either the exercise programme or the treadmill or 6-minute walk tests, and clinical deterioration during the study period. Eight patients were excluded: two were lost to follow-up, four were revascularised during the study period, and two developed acute coronary syndromes during the study period. All patients underwent clinical, electrocardiographic and echocardiographic screening to ensure suitability for a moderate-intensity exercise programme. Treadmill exercise and 6-minute walk tests were conducted at the beginning of the programme and after completion of 8 weeks of aerobic exercise involving upper and lower limbs. There was no control group as the exercise programme itself was not the primary variable under investigation. Treadmill exercise tests Submaximal treadmill exercise testing was carried out under the supervision of a physician, with continuous electrocardiographic monitoring, using the QUINTONTM exercise testing system and the Bruce or Cornell protocols according to standard guidelines and depending on individual suitability for each patient [12,13]. The test was terminated at 85% of age-appropriate target heart rate, or at the request of the patient because of symptoms. Blood pressure was monitored every 3 minutes and at peak exertion. Heart rate was recorded at 30, 60, 90 and 120 seconds into recovery. Metabolic equivalents (METs) were recorded for each case. Six-minute walk tests The same physiotherapist conducted this test on each occasion, and in accordance with published guidelines [14]. The explanation of the test was standardised and no encouragement was given after starting the walk, as it is known that this can improve performance [15]. Heart rate was measured continuously (SPACELAB 1050TM cardiac monitor Spacelabs Medical, Inc., Issaquah, Washington, USA 980277018) during the walk and was recorded at 30, 60, 90 and 120 seconds into recovery. The distance achieved and ratings of perceived exertion (RPEs) were recorded in each case. Exercise programme This was an 8-week aerobic training programme consisting of a combination of cycling, stepping, arm cranking, treadmill use and rowing exercises carried out at least three times per week for 20 minutes at a time. Data analysis
Fig. 1. Study outline, illustrating inclusions and exclusions.
Parametric data are expressed as mean ± standard deviation. Non-parametric data are expressed as median + interquartile range, where appropriate. Pre- and post-training comparisons were made for percentage heart rate recovery at 30-second intervals in treadmill and 6-minute walk tests.
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Pre- and post-training comparisons were made for walking distance and RPEs during the 6-minute walk test. For treadmill testing, pre- and post-training comparisons were made for METs at peak exertion. Comparison of parametric data was by means of the paired t-test, while comparison of nonparametric data was by means of Wilcoxon Signed Ranks test.
Results
gramme. The fact that peak heart rate was higher after the second test might suggest greater cardiac work following the training programme, but could simply be due to patients having greater confidence to exert themselves more after a period of stabilisation following a cardiac event. Interestingly, despite this increase in workload, the perceived work rate actually fell significantly. Comparison of heart rate recovery profiles
Subjects Thirty subjects were included in the study (20 males, 10 females, mean age 62.1 ± 8.5 years). Fourteen patients had suffered myocardial infarction and 16 patients had undergone revascularisation for chronic angina pectoris (12 percutaneous coronary angioplasty/stenting, four coronary artery bypass surgery). Thirteen subjects were diabetic, 21 were hypertensive, 25 were hyperlipidaemic, 16 were smokers at the time, and 16 were on beta-blockers. All patients were on antiplatelet and statin therapy. Treadmill tests (Table 1) These data demonstrate a reduction in resting heart rate and statistically significant improvements in heart rate recovery in the 30-second periods from 60 to 90 seconds and 90 to 120 seconds into recovery. Patients were able to sustain higher workloads in METs after the training programme. Six-minute walk tests (Table 2) These findings demonstrate the capacity for higher workload in terms of walking distance after the training pro-
The heart rate profiles following each test were very similar, although treadmill test heart rates were consistently higher than 6-minute walk test heart rates (Figs. 2 and 3). The profiles of percentage heart rate recovery are slightly different (Figs. 4 and 5). The profile for the first 6-minute walk test is similar to that for the pre- and post-training treadmill tests, albeit with lower actual percentages in the 6-minute walk test due to its relatively low workload. The profile for the posttraining 6-minute walk test is different to other profiles in that it starts with a percentage recovery similar in magnitude to treadmill test percentages, and then falls to low percentage recovery by 60–90 seconds. This is probably because peak heart rates achieved in the post-training 6-minute walk test were higher than pre-training peak heart rates, resulting in greater scope for early heart rate recovery.
Discussion These data show that submaximal treadmill exercise testing and the 6-minute walk test both demonstrate increased exercise capacity and improved heart rate recovery in a cardiac rehabilitation setting. The intensity of the 6-minute walk
Table 1 Treadmill tests: Comparison of pre and post training heart rates at rest, at peak, and during recovery phase
Resting HR (median + IQR) (bpm) Peak HR (median + IQR) (bpm) HR recovery at 30 seconds (median + IQR) (%) HR recovery at 60 seconds (median + IQR) (%) HR recovery at 90 seconds (median + IQR) (%) HR recovery at 120 seconds (median + IQR) (%) Metabolic equivalents (median + IQR)
Before training
After training
P-value
85 (67 to 92) 129 (116 to 144) 3 (1 to 6) 4 (2 to 7) 3 (2 to 5) 3 (2 to 5) 7 (6 to 9)
81 (61 to 89) 138 (116 to 146) 4 (2 to 7) 5 (3 to 8) 4 (3 to 7) 4 (3 to 6) 9 (7 to 9)
0.034 NS 0.066 0.056 0.025 0.002 <0.001
HR, heart rate; IQR, interquartile range; bpm, beats per minute; NS, not significant. Table 2 Six-minute walk tests: Comparison of pre and post training heart rates at rest, at peak, and during recovery phase
Rest HR (median + IQR) (bpm) Peak HR (median + IQR) (bpm) HR recovery at 30 seconds (median + IQR) (%) HR recovery at 60 seconds (median + IQR) (%) HR recovery at 90 seconds (median + IQR) (%) HR recovery at 120 seconds (median + IQR) (%) Distance covered (m) (mean ± standard deviation) Rating of perceived exertion (median + IQR)
Before training
After training
P-value
81 (71 to 88) 105 (96 to 117) 3 (1 to 7) 3 (2 to 4) 2 (1 to 4) 2 (1 to 3) 486 (±114) 14 (13 to 15)
82 (73 to 82) 110 (101 to 125) 4 (3 to 8) 4 (2 to 5) 2 (2 to 5) 2 (1 to 4) 553 (±112) 13 (13 to 13)
NS 0.005 0.001 0.026 0.043 0.020 <0.001 0.001
HR, heart rate; IQR, interquartile range; bpm, beats per minute; NS, not significant.
E. Roberts et al. / Physiotherapy 92 (2006) 116–121
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Fig. 2. Heart rates pre- and post-training (treadmill test, boxplots representing median value, interquartile range and absolute range).
test appeared to be lower than that of the submaximal treadmill test, as evidenced by the relatively low peak heart rate in the former. In fact, heart rates were consistently lower at all points for the 6-minute walk test. Previous data have suggested that 6-minute walk tests equate to treadmill test intensities at approximately 85% of target heart rate and 80% of VO2max ; a typical intensity for submaximal treadmill exercise testing [8]. Perhaps the fact that all patients in the present study had previously suffered symptoms of myocardial ischaemia explains some reluctance to increase their physical effort beyond a certain threshold in what is essentially a voluntarily intensive test. This might result in heart rates significantly lower than 85% of agepredicted target. In addition, previous findings were derived from healthy elderly subjects who had practiced the 6-minute
walk test on two previous occasions. Indeed, in these studies, the first two walks were characterised by significantly lower peak heart rate and subsequent absolute heart rate recovery values, confirming other data suggesting that a learning effect may occur with the 6-minute walk [9,15]. While this might be taken as evidence of a need to undertake practice 6-minute walk tests prior to a comparison between this and treadmill tests, it should be noted that there is also a learning effect associated with treadmill testing [16]. This means it could actually be misleading to compare second or subsequent 6minute walk tests with first-time treadmill tests. It is clear from the treadmill data that peak workload increased following the training programme, with METs rising from a median of 7 (interquartile range 5.8–8.6) to 8.6 (interquartile range 7–8.6). The equivalent in the 6-minute
Fig. 3. Heart rates pre- and post-training (6-minute walk, boxplots representing median value, interquartile range and absolute range).
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Fig. 4. Percentage heart rate recovery pre- and post-training (treadmill test, boxplots representing median value, interquartile range and absolute range). % = HR change during previous 30s/HR at beginning of the previous 30s × 100.
walk test was an increase in distance walked from a mean of 486.3 m (standard deviation 113.9 m) before training to 552.5 m (standard deviation 111.9 m) after training. However, caution is needed in attempting to explain the increased workload. It is not possible to know the proportions of this improvement due to cardiac changes compared with respiratory, metabolic, peripheral vascular and skeletal muscle changes. The increased peak heart rate between the pre- and post-training 6-minute walk tests could imply training-related increases in cardiac capacity, although the possibility that this was due to non-cardiac functional improvements cannot be discounted. It is also possible that workload increases could be related to patients feeling confident to exert themselves to a greater degree on the second test, with the training
programme having played a minimal part in physical conditioning. However, this conclusion is not supported by the fact that RPEs actually decreased in the second tests despite increases in walking distance and heart rate. The RPEs in the 6-minute walk test generally corresponded with exertion of the relatively low level required in symptom-limited exercise in the early stages following myocardial infarction [17]. This finding, along with low heart rates compared with those in the treadmill tests, further supports the notion that patients self-limited their exercise intensity in the 6-minute walk test. Conversely, although patients were able to request termination of treadmill testing at any point, they may have exerted themselves beyond the intensity at which they felt comfortable. Prior practice of the 6-minute
Fig. 5. Percentage heart rate recovery pre- and post-training (6-minute walk, boxplots representing median value, interquartile range and absolute range). % = HR change during previous 30s/HR at beginning of the previous 30s × 100.
E. Roberts et al. / Physiotherapy 92 (2006) 116–121
walk test could have resulted in greater correlation. Perhaps patients could have achieved an even higher workload had they been asked to exercise to a pre-determined rating of perceived exertion. These findings suggest that the 6-minute walk test may be a valid tool to assess improved exercise capacity and heart rate recovery in a cohort of patients undergoing exercisebased cardiac rehabilitation. It was not possible to quantify the effect of the training programme itself in this study as there was no control group. If these results are confirmed in larger cohorts, the 6-minute walk test could become one of a number of standard tests to evaluate the above parameters.
Limitations A full metabolic analysis of the effects of each exercise test, based on parameters derived from haemodynamic and expired air analysis, would have provided extremely useful information. However, such detailed assessment was beyond the scope of this study. The guidelines referred to for this study state that ‘a practice test is not needed in most clinical settings but should be considered’ [14]. A practice 6-minute walk test is often thought to be necessary prior to measuring parameters for the sake of scientific enquiry. The cohort in this study did not have an opportunity to practice the walk. However, they did not have the opportunity to practice the treadmill test either, where unaccustomed stride length and cadence do mean that a learning effect can occur [16]. Ideally, all patients would have had practice treadmill and 6-minute walk tests. The fact that they did not is a limitation but still allows comparison between the two tests to be made, as the patients were essentially novices at both. Approximately half of the group had suffered myocardial infarction, while the other half had undergone revascularisation prior to being enrolled in the programme. While this reflects clinical practice, it should be noted that results for the group as a whole do not necessarily translate proportionately to the constituent groups.
Key message The six-minute walk test is a valid tool to assess Heart rate recovery in a cardiac rehabilitation setting.
Ethical approval: Queen Elizabeth Hospital Ethics Committee, Hong Kong and the Research Coordination Team, Centre for Exercise and Nutrition Science, University College Chester, UK.
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References [1] Cole CR, Blackstone EH, Pashkow FJ, Snader CE, Lauer MS. Heartrate recovery immediately after exercise as a predictor of mortality. N Engl J Med 1999;341:1351–7. [2] Cole CR, Foody JM, Blackstone EH, Lauer MS. Heart rate recovery after submaximal exercise testing as a predictor of mortality in a cardiovascularly healthy cohort. Ann Intern Med 2000;132:552– 5. [3] Messinger-Rapport B, Pothier Snader CE, Blackstone EH, Yu D, Lauer MS. Value of exercise capacity and heart rate recovery in older people. J Am Geriatr Soc 2003;51:63–8. [4] Nishime EO, Cole CR, Blackstone EH, Pashkow FJ, Lauer MS. Heart rate recovery and treadmill exercise score as predictors of mortality in patients referred for exercise ECG. JAMA 2000;284: 1392–8. [5] Shetler K, Marcus R, Froelicher VF, Vora S, Kalisetti D, Prakash M, et al. Heart rate recovery: validation and methodologic issues. J Am Coll Cardiol 2001;38:1980–7. [6] Tiukinhoy S, Beohar N, Hsie M. Improvement in heart rate recovery after cardiac rehabilitation. J Cardiopulmon Rehabil 2003;23:84–7. [7] Wright DJ, Khan KM, Gossage EM, Saltissi S. Assessment of a low-intensity cardiac rehabilitation programme using the six-minute walk test. Clin Rehabil 2001;15:119–24. [8] Kervio G, Carre F, Ville NS. Reliability and intensity of the sixminute walk test in healthy elderly subjects. Med Sci Sports Exerc 2003;35:169–74. [9] Hamilton DM, Haennel RG. Validity and reliability of the 6-minute walk test in a cardiac rehabilitation population. J Cardiopulm Rehabil 2000;20:156–64. [10] Gayda M, Temfemo A, Choquet D, Ahmaidi S. Cardiorespiratory requirements and reproducibility of the six-minute walk test in elderly patients with coronary artery disease. Arch Phys Med Rehabil 2004;85:1538–43. [11] Rostagno C, Olivo G, Comeglio M, Boddi V, Banchelli M, Galanti G, et al. Prognostic value of 6-minute walk corridor test in patients with mild to moderate heart failure: comparison with other methods of functional evaluation. Eur J Heart Fail 2003;5:247–52. [12] Gibbons RJ, Balady GJ, Beasley JW, Bricker JT, Duvernoy WF, Froelicher VF, et al. ACC/AHA guidelines for exercise testing: executive summary. A report of the American College of Cardiology/American Heart Association Task Force on Practice Guidelines (Committee on Exercise Testing). Circulation 1997;96:345–54. [13] Gibbons RJ, Balady GJ, Bricker JT, Chaitman BR, Fletcher GF, Froelicher VF, et al. ACC/AHA 2002 guideline update for exercise testing: summary article: a report of the American College of Cardiology/American Heart Association Task Force on Practice Guidelines (Committee to Update the 1997 Exercise Testing Guidelines). Circulation 2002;106:1883–92. [14] ATS statement: guidelines for the six-minute walk test. Am J Respir Crit Care Med 2002;166:111–7. [15] Guyatt GH, Pugsley SO, Sullivan MJ, Thompson PJ, Berman L, Jones NL, et al. Effect of encouragement on walking test performance. Thorax 1984;39:818–22. [16] Tonino RP, Driscoll PA. Reliability of maximal and submaximal parameters of treadmill testing for the measurement of physical training in older persons. J Gerontol 1988;43:M101–4. [17] Jensen-Urstad K, Samad BA, Bouvier F, Hulting J, Hojer J, Ruiz H, et al. Prognostic value of symptom limited versus low level exercise stress test before discharge in patients with myocardial infarction treated with thrombolytics. Heart 1999;82:199–203.
Validity of the 6-minute walk test for assessing heart rate recovery after an exercise-based cardiac rehabilitation programme Elved Roberts a,c,∗ , Frederick K.W. Li b , Kevin Sykes c a
Liverpool Cardiothoracic Centre, Thomas Drive, Liverpool, Merseyside L14 3PE, UK b Queen Elizabeth Hospital, Hong Kong, China c Centre for Exercise and Nutrition Science, University of Chester, Chester, UK
Abstract Objectives To compare heart rate responses in the 6-minute walk test and the treadmill exercise test before and after an exercise-based cardiac rehabilitation programme. Design Prospective cohort study. Setting Hospital-based cardiac rehabilitation programme in Hong Kong. Participants Thirty patients (mean age 62.1 ± 8.5 years, 20 males) with stable ischaemic heart disease. Interventions Eight-week exercise-based cardiac rehabilitation programme involving upper and lower limb aerobic and resistance training. Main outcome measures Six-minute walk test and treadmill exercise test before and after the exercise programme. Results Comparing parameters before and after the exercise programme, the peak heart rate in the 6-minute walk test increased (median of 105 beats per minute (bpm), interquartile range 96.8–116.5 versus 110 bpm, interquartile range 100.5–124.5, P = 0.006), while heart rate recovery improved in each 30-second interval of a 2-minute recovery period. The distance covered during the 6-minute walk increased from a mean of 486.3 m (±standard deviation 113.9 m) to 552.5 m (±standard deviation 111.9 m) (P < 0.001). Rating of perceived exertion during the 6-minute walk test decreased from a median of 14 (interquartile range 13–15) to 13 (interquartile range 13–13) (P = 0.001). Heart rate recovery following treadmill exercise testing improved during the 30-second periods from 60 to 90 seconds and from 90 to 120 seconds of recovery. Metabolic equivalents increased during treadmill testing from a median of 7.0 (interquartile range 5.8–8.6) to 8.6 (interquartile range 7–8.6) (P < 0.001). Conclusions Both treadmill exercise and 6-minute walk tests demonstrated improvements in heart rate recovery and increases in achieved workload following exercise training. This suggests that the 6-minute walk test is a valid tool to assess heart rate recovery following such a programme. © 2005 Chartered Society of Physiotherapy. Published by Elsevier Ltd. All rights reserved. Keywords: Six-minute walk; Treadmill testing; Ischaemic heart disease; Cardiac rehabilitation; Heart rate recovery
Introduction Treadmill exercise test data have confirmed that heart rate recovery is a marker of physical fitness and exercise capacity, that abnormal heart rate recovery is a strong predictor of mortality in both asymptomatic individuals [1–3] and cardiac patients [4,5], and that exercise training improves heart rate recovery in cardiac patients [6]. The 6-minute walk test has ∗ Corresponding author. Present address: Centre for Exercise and Nutrition Science, University of Chester, Parkgate Road, Chester CH1 4BJ, UK. Tel.: +44 151 228 1616 (pager 782); fax: +44 151 288 2254. E-mail address: [email protected] (E. Roberts).
a range of applications in characterising exercise response in both cardiac and non-cardiac patients. It has also been proposed as a valid measure of functional capacity after a cardiac rehabilitation programme [7], and has been shown to approximate a sub-maximal treadmill exercise test at near 80% maximal oxygen uptake (VO2max ) in terms of achieved workload [8]. This test has also been validated against symptom-limited treadmill testing in cardiac rehabilitation patients [9] and in elderly patients with coronary artery disease [10]. However, heart rate recovery in the 6-minute walk test has not been validated against heart rate recovery in the treadmill exercise test following a moderate-intensity cardiac rehabilitation exercise programme. If the 6-minute walk test could be shown to reli-
0031-9406/$ – see front matter © 2005 Chartered Society of Physiotherapy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.physio.2005.06.005
E. Roberts et al. / Physiotherapy 92 (2006) 116–121
ably quantify heart rate recovery in patients undergoing an exercise-based cardiac rehabilitation programme, this simple and effective measurement tool could be used to monitor response to physical training instead of other more complicated tests of physical capacity. Furthermore, as it is known that heart rate recovery following treadmill testing has prognostic significance, and that the 6-minute walk test is capable of prognostic stratification in other conditions [11], it might be expected that prognostic information could be derived from the 6-minute walk test in cardiac rehabilitation patients. The aim of this study was to compare heart rate recovery in treadmill exercise and 6-minute walk tests in a cohort of cardiac patients undergoing an exercise-based rehabilitation programme. Methods Thirty-eight patients who had been medically stabilised following hospitalisation with an acute coronary syndrome or for coronary revascularisation were recruited prospectively, and all gave written informed consent (Fig. 1). Eligibility criteria included age >30 years and stabilised acute coronary syndrome/coronary artery disease. Exclusion criteria included permanent pacemaker or internal cardioverter/defibrillator, left bundle branch block or electrocardiographic pre-excitation, New York Heart Association Class IV heart failure or echocardiographic ejection fraction <30%, valvular heart disease, hypertrophic cardiomyopathy,
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revascularisation during the study period, inability to participate in either the exercise programme or the treadmill or 6-minute walk tests, and clinical deterioration during the study period. Eight patients were excluded: two were lost to follow-up, four were revascularised during the study period, and two developed acute coronary syndromes during the study period. All patients underwent clinical, electrocardiographic and echocardiographic screening to ensure suitability for a moderate-intensity exercise programme. Treadmill exercise and 6-minute walk tests were conducted at the beginning of the programme and after completion of 8 weeks of aerobic exercise involving upper and lower limbs. There was no control group as the exercise programme itself was not the primary variable under investigation. Treadmill exercise tests Submaximal treadmill exercise testing was carried out under the supervision of a physician, with continuous electrocardiographic monitoring, using the QUINTONTM exercise testing system and the Bruce or Cornell protocols according to standard guidelines and depending on individual suitability for each patient [12,13]. The test was terminated at 85% of age-appropriate target heart rate, or at the request of the patient because of symptoms. Blood pressure was monitored every 3 minutes and at peak exertion. Heart rate was recorded at 30, 60, 90 and 120 seconds into recovery. Metabolic equivalents (METs) were recorded for each case. Six-minute walk tests The same physiotherapist conducted this test on each occasion, and in accordance with published guidelines [14]. The explanation of the test was standardised and no encouragement was given after starting the walk, as it is known that this can improve performance [15]. Heart rate was measured continuously (SPACELAB 1050TM cardiac monitor Spacelabs Medical, Inc., Issaquah, Washington, USA 980277018) during the walk and was recorded at 30, 60, 90 and 120 seconds into recovery. The distance achieved and ratings of perceived exertion (RPEs) were recorded in each case. Exercise programme This was an 8-week aerobic training programme consisting of a combination of cycling, stepping, arm cranking, treadmill use and rowing exercises carried out at least three times per week for 20 minutes at a time. Data analysis
Fig. 1. Study outline, illustrating inclusions and exclusions.
Parametric data are expressed as mean ± standard deviation. Non-parametric data are expressed as median + interquartile range, where appropriate. Pre- and post-training comparisons were made for percentage heart rate recovery at 30-second intervals in treadmill and 6-minute walk tests.
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E. Roberts et al. / Physiotherapy 92 (2006) 116–121
Pre- and post-training comparisons were made for walking distance and RPEs during the 6-minute walk test. For treadmill testing, pre- and post-training comparisons were made for METs at peak exertion. Comparison of parametric data was by means of the paired t-test, while comparison of nonparametric data was by means of Wilcoxon Signed Ranks test.
Results
gramme. The fact that peak heart rate was higher after the second test might suggest greater cardiac work following the training programme, but could simply be due to patients having greater confidence to exert themselves more after a period of stabilisation following a cardiac event. Interestingly, despite this increase in workload, the perceived work rate actually fell significantly. Comparison of heart rate recovery profiles
Subjects Thirty subjects were included in the study (20 males, 10 females, mean age 62.1 ± 8.5 years). Fourteen patients had suffered myocardial infarction and 16 patients had undergone revascularisation for chronic angina pectoris (12 percutaneous coronary angioplasty/stenting, four coronary artery bypass surgery). Thirteen subjects were diabetic, 21 were hypertensive, 25 were hyperlipidaemic, 16 were smokers at the time, and 16 were on beta-blockers. All patients were on antiplatelet and statin therapy. Treadmill tests (Table 1) These data demonstrate a reduction in resting heart rate and statistically significant improvements in heart rate recovery in the 30-second periods from 60 to 90 seconds and 90 to 120 seconds into recovery. Patients were able to sustain higher workloads in METs after the training programme. Six-minute walk tests (Table 2) These findings demonstrate the capacity for higher workload in terms of walking distance after the training pro-
The heart rate profiles following each test were very similar, although treadmill test heart rates were consistently higher than 6-minute walk test heart rates (Figs. 2 and 3). The profiles of percentage heart rate recovery are slightly different (Figs. 4 and 5). The profile for the first 6-minute walk test is similar to that for the pre- and post-training treadmill tests, albeit with lower actual percentages in the 6-minute walk test due to its relatively low workload. The profile for the posttraining 6-minute walk test is different to other profiles in that it starts with a percentage recovery similar in magnitude to treadmill test percentages, and then falls to low percentage recovery by 60–90 seconds. This is probably because peak heart rates achieved in the post-training 6-minute walk test were higher than pre-training peak heart rates, resulting in greater scope for early heart rate recovery.
Discussion These data show that submaximal treadmill exercise testing and the 6-minute walk test both demonstrate increased exercise capacity and improved heart rate recovery in a cardiac rehabilitation setting. The intensity of the 6-minute walk
Table 1 Treadmill tests: Comparison of pre and post training heart rates at rest, at peak, and during recovery phase
Resting HR (median + IQR) (bpm) Peak HR (median + IQR) (bpm) HR recovery at 30 seconds (median + IQR) (%) HR recovery at 60 seconds (median + IQR) (%) HR recovery at 90 seconds (median + IQR) (%) HR recovery at 120 seconds (median + IQR) (%) Metabolic equivalents (median + IQR)
Before training
After training
P-value
85 (67 to 92) 129 (116 to 144) 3 (1 to 6) 4 (2 to 7) 3 (2 to 5) 3 (2 to 5) 7 (6 to 9)
81 (61 to 89) 138 (116 to 146) 4 (2 to 7) 5 (3 to 8) 4 (3 to 7) 4 (3 to 6) 9 (7 to 9)
0.034 NS 0.066 0.056 0.025 0.002 <0.001
HR, heart rate; IQR, interquartile range; bpm, beats per minute; NS, not significant. Table 2 Six-minute walk tests: Comparison of pre and post training heart rates at rest, at peak, and during recovery phase
Rest HR (median + IQR) (bpm) Peak HR (median + IQR) (bpm) HR recovery at 30 seconds (median + IQR) (%) HR recovery at 60 seconds (median + IQR) (%) HR recovery at 90 seconds (median + IQR) (%) HR recovery at 120 seconds (median + IQR) (%) Distance covered (m) (mean ± standard deviation) Rating of perceived exertion (median + IQR)
Before training
After training
P-value
81 (71 to 88) 105 (96 to 117) 3 (1 to 7) 3 (2 to 4) 2 (1 to 4) 2 (1 to 3) 486 (±114) 14 (13 to 15)
82 (73 to 82) 110 (101 to 125) 4 (3 to 8) 4 (2 to 5) 2 (2 to 5) 2 (1 to 4) 553 (±112) 13 (13 to 13)
NS 0.005 0.001 0.026 0.043 0.020 <0.001 0.001
HR, heart rate; IQR, interquartile range; bpm, beats per minute; NS, not significant.
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Fig. 2. Heart rates pre- and post-training (treadmill test, boxplots representing median value, interquartile range and absolute range).
test appeared to be lower than that of the submaximal treadmill test, as evidenced by the relatively low peak heart rate in the former. In fact, heart rates were consistently lower at all points for the 6-minute walk test. Previous data have suggested that 6-minute walk tests equate to treadmill test intensities at approximately 85% of target heart rate and 80% of VO2max ; a typical intensity for submaximal treadmill exercise testing [8]. Perhaps the fact that all patients in the present study had previously suffered symptoms of myocardial ischaemia explains some reluctance to increase their physical effort beyond a certain threshold in what is essentially a voluntarily intensive test. This might result in heart rates significantly lower than 85% of agepredicted target. In addition, previous findings were derived from healthy elderly subjects who had practiced the 6-minute
walk test on two previous occasions. Indeed, in these studies, the first two walks were characterised by significantly lower peak heart rate and subsequent absolute heart rate recovery values, confirming other data suggesting that a learning effect may occur with the 6-minute walk [9,15]. While this might be taken as evidence of a need to undertake practice 6-minute walk tests prior to a comparison between this and treadmill tests, it should be noted that there is also a learning effect associated with treadmill testing [16]. This means it could actually be misleading to compare second or subsequent 6minute walk tests with first-time treadmill tests. It is clear from the treadmill data that peak workload increased following the training programme, with METs rising from a median of 7 (interquartile range 5.8–8.6) to 8.6 (interquartile range 7–8.6). The equivalent in the 6-minute
Fig. 3. Heart rates pre- and post-training (6-minute walk, boxplots representing median value, interquartile range and absolute range).
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Fig. 4. Percentage heart rate recovery pre- and post-training (treadmill test, boxplots representing median value, interquartile range and absolute range). % = HR change during previous 30s/HR at beginning of the previous 30s × 100.
walk test was an increase in distance walked from a mean of 486.3 m (standard deviation 113.9 m) before training to 552.5 m (standard deviation 111.9 m) after training. However, caution is needed in attempting to explain the increased workload. It is not possible to know the proportions of this improvement due to cardiac changes compared with respiratory, metabolic, peripheral vascular and skeletal muscle changes. The increased peak heart rate between the pre- and post-training 6-minute walk tests could imply training-related increases in cardiac capacity, although the possibility that this was due to non-cardiac functional improvements cannot be discounted. It is also possible that workload increases could be related to patients feeling confident to exert themselves to a greater degree on the second test, with the training
programme having played a minimal part in physical conditioning. However, this conclusion is not supported by the fact that RPEs actually decreased in the second tests despite increases in walking distance and heart rate. The RPEs in the 6-minute walk test generally corresponded with exertion of the relatively low level required in symptom-limited exercise in the early stages following myocardial infarction [17]. This finding, along with low heart rates compared with those in the treadmill tests, further supports the notion that patients self-limited their exercise intensity in the 6-minute walk test. Conversely, although patients were able to request termination of treadmill testing at any point, they may have exerted themselves beyond the intensity at which they felt comfortable. Prior practice of the 6-minute
Fig. 5. Percentage heart rate recovery pre- and post-training (6-minute walk, boxplots representing median value, interquartile range and absolute range). % = HR change during previous 30s/HR at beginning of the previous 30s × 100.
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walk test could have resulted in greater correlation. Perhaps patients could have achieved an even higher workload had they been asked to exercise to a pre-determined rating of perceived exertion. These findings suggest that the 6-minute walk test may be a valid tool to assess improved exercise capacity and heart rate recovery in a cohort of patients undergoing exercisebased cardiac rehabilitation. It was not possible to quantify the effect of the training programme itself in this study as there was no control group. If these results are confirmed in larger cohorts, the 6-minute walk test could become one of a number of standard tests to evaluate the above parameters.
Limitations A full metabolic analysis of the effects of each exercise test, based on parameters derived from haemodynamic and expired air analysis, would have provided extremely useful information. However, such detailed assessment was beyond the scope of this study. The guidelines referred to for this study state that ‘a practice test is not needed in most clinical settings but should be considered’ [14]. A practice 6-minute walk test is often thought to be necessary prior to measuring parameters for the sake of scientific enquiry. The cohort in this study did not have an opportunity to practice the walk. However, they did not have the opportunity to practice the treadmill test either, where unaccustomed stride length and cadence do mean that a learning effect can occur [16]. Ideally, all patients would have had practice treadmill and 6-minute walk tests. The fact that they did not is a limitation but still allows comparison between the two tests to be made, as the patients were essentially novices at both. Approximately half of the group had suffered myocardial infarction, while the other half had undergone revascularisation prior to being enrolled in the programme. While this reflects clinical practice, it should be noted that results for the group as a whole do not necessarily translate proportionately to the constituent groups.
Key message The six-minute walk test is a valid tool to assess Heart rate recovery in a cardiac rehabilitation setting.
Ethical approval: Queen Elizabeth Hospital Ethics Committee, Hong Kong and the Research Coordination Team, Centre for Exercise and Nutrition Science, University College Chester, UK.
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