Six-Minute Walk Akin to Treadmill METS for Predicting Risk in Stable CAD

June 18, 2012

June 18, 2012 (San Francisco, California) — The six-minute-walk test (6MWT), available on short notice wherever there are hallways, forecast risk of heart failure, MI, or death independently of conventional risk factors in patients with stable heart disease in a cohort study secondary to a prospective trial [1].

Well established as a risk-stratifier in patients with heart failure or chronic pulmonary disease, the 6MWT in this much broader population was prognostically comparable to exercise capacity at treadmill testing.

The analysis of a cohort from the Heart and Soul Study was published online today in the Archives of Internal Medicine, with lead author Dr Alexis L Beatty (University of California, San Francisco).

The analysis suggests that the 6MWT has potential for evaluating patients with stable coronary disease, "and I think it would remain in the outpatient setting," Beatty told heartwire . "The way I see it, the six-minute-walk test would be to estimate prognosis and exercise capacity in a stable outpatient. It would be a potential alternative to a formal treadmill exercise test in that setting."

It's such a simple test that could be used to help motivate people to improve their physical fitness.

But, she added, "One big area where I think this test has potential is if we could do multiple six-minute-walk tests over time to see if they can improve. If a future study were to show that improving on the test is associated with a lower risk of events, I think that would be great."

In an accompanying commentary [2], Dr David T Nash (State University of New York Upstate Medical Center, Syracuse) said, "The 6MWT allows convenient assessment of the patient’s cardiovascular disease risk factors and can help guide personalized risk-factor reduction based on lifestyle measures--diet, physical activity, and control of blood pressure and lipid levels."

Beatty, who when interviewed had not yet seen the editorial, agreed. "It's such a simple test that could be used to help motivate people to improve their physical fitness. It's very accessible to patients, something they can easily understand, and off-the-cuff think on their own about what things they can do to improve their overall health."

The group offered the 6MWT to 769 patients in the trial, and 556 accepted and participated; their performance was compared with other functional tests and other parameters assessed as part of the larger trial. The study was conducted from September 2000 to December 2002.

Functional Capacity by 6MWD and Treadmill Exercise and Clinical Outcomes Across 6MWD Quartiles in the Heart and Soul Study Cohort

End point Q1, n=139 Q2, n=139 Q3, n=139 Q4, n=139
6-min-walk distance (m) 87–419 420–480 481–543 544–837
Treadmill exercise capacity (METS)* 5.1 7.1 8.6 10.6
Death, MI, or HF (%) 61.9 43.2 30.2 21.6

6MWD=six-minute-walk distance; Q=quartile

*p<0.001 for trend

Not surprisingly, those in the lowest quartile for six-minute-walk distance, compared with the highest quartile, had significantly more hypertension, dyslipidemia, diabetes, and peripheral vascular disease and higher body-mass index; greater use of ACE inhibitors or angiotensin receptor blockers and diuretics; and more inducible ischemia on stress echocardiography, lower LVEF, and higher natriuretic-peptide levels. Their treadmill exercise capacity was also significantly lower.

Of the 556 patients, 39.2% died or developed heart failure or MI over a median of 8.0 years; events were adjudicated as part of the larger trial.

Risks of the composite end point and its individual components were significantly increased after adjustment for traditional CAD risk factors and at least a dozen measures of disease severity. The adjusted composite-end-point risk went up 30% per standard-deviation (104 m) drop in six-minute-walk distance.

Hazard Ratio (95% CI) Per Standard Deviation Decrease (104 M) in 6MWD

End point HR (95% CI)* p
Heart failure 1.38 (1.04–1.82) 0.03
MI 1.36 (1.00–1.85) 0.047
Death 1.25 (1.04–1.50) 0.02
Heart failure, MI, or death 1.30 (1.10–1.53) 0.002

*Adjusted for age, sex, smoking, hypertension, dyslipidemia, diabetes, peripheral vascular disease, revascularization, chronic lung disease, body-mass index, systolic blood pressure, estimated glomerular filtration rate, hemoglobin, ACE-inhibitor or angiotensin-receptor-blocker use, diuretic use, inducible ischemia, LVEF, diastolic function, natriuretic-peptide level, and C-reactive-protein level

Combined with traditional risk factors, 6MWT results led to a 39.3% net classification improvement in predicting the composite end point. That compares to 39.6% as the corresponding figure for treadmill exercise capacity added to traditional risk factors.

Among the study's limitations: patients were ineligible for the overall trial if they couldn't walk one block. Also, not all selected patients joined the 6MWT cohort, and not all who joined the cohort could complete the test. But their outcomes were similar to the lowest quartile for those who did.

"Given the greater ease and lesser cost of the 6MWT compared with cardiovascular stress testing," Nash writes, "I would recommend that physicians interested in improving their patients' level of fitness use the 6MWT as a means of getting the patient started on regular exercise. Once the patients become familiar with the ease and safety of the 6MWT, they can be encouraged to repeat the 6MWT more frequently, even on a daily basis. It is then possible to lengthen the walk at appropriate intervals."

Beatty had no disclosures; disclosures for the coauthors are listed in the paper. Nash had no disclosures. The Heart and Soul study was funded by grants from the Department of Veterans Affairs; National Heart, Lung, and Blood Institute; Robert Wood Johnson Foundation; American Federation for Aging Research; Ischemia Research and Education Foundation; and Nancy Kirwan Heart Research.

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