The 6-minute walk distance (6MWD) test is a useful prognostic tool in chronic heart failure. Its usefulness after percutaneous coronary intervention is unknown.
In a prospective observational study, patients underwent a 6MWD test within 2 weeks after percutaneous coronary intervention. The primary endpoint was major adverse cardiovascular events (MACE) (death, acute coronary syndrome, and heart failure admission) at one year. Receiver operating characteristic curves and area under the curve were used to determine the 6MWD test's predictive power, and the Youden index was used to measure its effectiveness.
A total of 212 patients were enrolled (98% men; mean age, 65 ± 9 yr). Major comorbidities were hypertension in 187 patients (88%), dyslipidemia in 186 (88%), and diabetes mellitus in 95 (45%). Among the 176 patients (83%) who completed the 6MWD test, the incidence of MACE at one year was 22% (acute coronary syndrome in 17%; heart failure admission in 4%; and death in 3%). The area under the curve for MACE was 0.59, and 6MWD was shorter for patients with MACE than for those without (290 vs 326 m; P=0.03). For 39 patients with previous heart failure who completed the 6MWD test, the area under the curve was 0.64 for MACE and 0.78 for heart failure admission.
The 6MWD test predicted reasonably well the incidence of MACE one year after percutaneous coronary intervention. In a subgroup of patients with previous heart failure, it fared even better in predicting heart failure admission. Larger studies are needed to confirm these findings.
The 6-minute walk distance (6MWD) test is a well-validated prognostic tool in patients with chronic heart failure (HF). This simple evaluation helps to predict peak exercise oxygen consumption (Vo2) and is a good predictor of cardiovascular morbidity and death and hospital admission in patients with HF.1–6 Six-minute walk distance also has prognostic value in patients undergoing coronary artery bypass grafting (CABG). A 6MWD >300 m has been associated with lower mortality rates in elderly patients undergoing CABG.7 Other data suggest that 6MWD also helps predict outcomes in patients with aortic stenosis undergoing surgical or transcatheter aortic valve procedures.8,9 Before percutaneous coronary intervention (PCI) was introduced into clinical practice, 6MWD was also a useful predictor of major adverse cardiovascular events (MACE) in patients with ST-segment-elevation myocardial infarction undergoing fibrinolysis.10 The 6MWD test is easily reproducible and sensitive to changes in quality of life. It has also been suggested that the 6MWD test is prognostically comparable to the exercise stress test.11 However, the usefulness of 6MWD in predicting outcomes in patients undergoing PCI is unknown. We therefore conducted this prospective observational study to evaluate the ability of 6MWD to predict MACE and HF admission one year after PCI.
Patients and Methods
Screened for participation in this study were all patients who underwent PCI either for stable angina—with or without an abnormal noninvasive functional study result—or for acute coronary syndrome (ACS) at the Oklahoma City Veterans Affairs Medical Center from June 2010 through December 2011. Excluded were patients who could not walk effectively because of chronic severe musculoskeletal issues, severe symptomatic peripheral artery disease, neurologic conditions such as disabling stroke or severe myoneuropathies, or severe obstructive pulmonary disease or advanced HF with New York Heart Association functional class IV symptoms that limited exercise capacity. Informed consent for this study was obtained from each patient after PCI. The study was approved by the local institutional review board. Demographics, clinical characteristics, and procedural details about coronary interventions were documented at baseline.
Patients underwent a standard 6MWD test in our physical therapy department within 2 weeks after PCI. The 6MWD test was administered by a dedicated physical therapist and involved the patient walking at his or her own pace with opportunities to rest during the walk if needed. Total distance walked (in meters) over 6 minutes and symptoms during the test were documented. Follow-up in the clinic or by telephone occurred at one year (12 ± 2 mo) after PCI. Clinical outcome and medication use were documented at the time of follow-up. The primary endpoint was MACE, which included death, ACS, and admission for HF (left ventricular ejection fraction, <0.40).
Continuous data are presented as mean ± SD. Continuous variables were compared by using the Student t test or analysis of variance as appropriate. Categorical variables were compared by using the χ2 test. A 2-tailed P value of ≤0.05 was considered statistically significant. Receiver operating characteristic (ROC) curves and areas under the curve (AUCs) were calculated and used to determine the ability of 6MWD to predict MACE. The Youden index, which maximizes a function of sensitivity and specificity, was calculated and used to determine the optimal cutoff point of 6MWD for classifying MACE outcome. The optimal cutoff point was defined as the maximum vertical distance between the ROC curve and the diagonal. A P value of ≤0.05 was considered statistically significant. Data were analyzed with SAS version 9.3 (SAS Institute Inc.).
A total of 274 patients underwent PCI and were screened for participation in this study. Of those, 62 patients were excluded because they were unable to walk effectively. The remaining 212 patients were enrolled in this study (Fig. 1). Tables I and II summarize their baseline characteristics, indications for PCI, and procedural details of coronary intervention. The mean age of the patients was 65 ± 9 years, and 208 (98%) were men. Major comorbidities were hypertension in 187 patients (88%), dyslipidemia in 186 (88%), and diabetes mellitus in 95 (45%). A total of 118 patients (56%) with ACS underwent revascularization. Drug-eluting stents were used in 171 patients (81%).
Of the 212 patients who underwent PCI and were enrolled in this study, 176 (83%) completed the 6MWD test after PCI and had returned for a follow-up visit or had been contacted by telephone at one year. The remaining 36 patients (17%) did not return to complete the 6MWD test after PCI, for reasons such as ride availability, lack of interest, inability to be contacted during the required pretest time frame, illness, or forgetting to keep the test appointment.
Of the 176 patients who completed the 6MWD test, 39 (22%) experienced MACE within one year after PCI (Table III). Overall, the AUC was 0.59 for MACE and 0.69 for HF admission (Fig. 2A–B). Optimal cutoff points of 225 m for MACE and 286 m for HF admission were calculated by using the Youden index. The odds ratio (OR) for MACE was 0.26 (95% CI, 0.11–0.62; P =0.002) among patients who walked >225 m as compared with those who walked ≤225 m. The mean 6MWD was shorter for patients who had MACE than for those who did not (290 vs 326 m; P=0.03).
A subset of 50 enrolled patients (24%) had a baseline diagnosis of HF, 39 of whom completed the 6MWD evaluation. Of those 39 patients, 8 (21%) experienced MACE within one year after PCI (Table IV). For this subset of patients, the AUC was 0.64 for MACE and 0.78 for HF admission (Fig. 2C–D).12 Optimal cutoff points of 195 m for MACE and 335 m for HF admission were determined by using the Youden index. The OR for MACE was 0.25 (95% CI, 0.04–1.46; P=0.12) among patients who walked >195 m as compared with those who walked ≤195 m. The OR for HF admission was 0.81 (95% CI, 0.037–5.38, P=0.52) among patients who walked >335 m as compared with those who walked ≤335 m.
Our findings suggest that 6MWD is a reasonably good predictor of MACE and HF admission in patients undergoing PCI and an even better predictor of HF admission in a subset of patients with previous HF. The 6MWD test is an inexpensive, easily available noninvasive test that has been well validated in patients with both cardiac and noncardiac disease. In patients with chronic HF, 6MWD predicts death and recurrent hospitalization.1,6 Therefore, this test is quite often used in routine HF follow-up. Unlike other functional tests such as the exercise stress test, the 6MWD test can be administered to a wider population of patients who have cardiovascular disease and very limited functional capacity but who can walk short distances. Although 6MWD has been proved useful in the HF population, evidence of its usefulness in patients after PCI is lacking. Our study evaluated the prognostic value of 6MWD in a sample of such patients and showed that 6MWD may help to identify high-risk patients after PCI, especially those with a previous history of HF.
The predictive value of 6MWD in our study was much more robust in terms of predicting HF admission after index PCI, as evidenced by a C statistic (AUC) of 0.69 overall (Fig. 2B) and 0.78 in patients with a previous history of HF with reduced left ventricular ejection fraction (Fig. 2D). Although 6MWD was relatively weaker at predicting MACE than it was at predicting HF admission, this difference needs validation in a larger population. Our study sample included not only patients with ACS, but also patients who underwent elective revascularization because of an abnormal functional or stress test result. Moreover, our study was not large enough to compare the predictive capability of 6MWD in each subset of the sample. Thus, it would be interesting to determine if the ability of 6MWD to predict MACE and HF admission in patients with ACS is superior to its ability to do so in patients with stable angina or abnormal functional or stress test results who then undergo elective revascularization.
In our study, we determined different cutoff points for the 6MWD in subsets of patients. As noted in our results, the optimal 6MWD cutoff point for predicting HF admission after PCI was 286 m overall and 335 m in the HF subset. We also calculated cutoff points for predicting MACE after PCI. The resulting ORs were probably clinically insignificant, except when predicting MACE in the overall sample at a 6MWD cutoff of 225 m, because of the small sample size. These findings warrant further study in a larger group of patients undergoing PCI to define more precisely the optimal cutoffs for different subsets of these patients.
The 6MWD test can be used to estimate peak O2 consumption indirectly without the need for an exercise stress test.13 Unlike the exercise stress test, the 6MWD test is inexpensive, easily administered, highly reproducible, and available at all centers. The only conditions are a level, premeasured hallway; a stopwatch; and a set of instructions. If optimal cutoffs are established for patients undergoing PCI (whether for ACS or for elective revascularization), then using a tool as simple as the 6MWD test will enhance the ability to predict MACE and HF admission during the first year after PCI. This may in turn enable earlier identification and triage and closer follow-up of patients who perform poorly on the 6MWD test, most of whom are likely to have a history of HF. The 6MWD may also be a useful quality-of-life variable and a good surrogate endpoint for PCI procedures. A 6MWD obtained before PCI, and another 6MWD obtained 6 months to one year after PCI, may then become one of several quality-of-life variables that reflect a patient's functional status.
Our study had several limitations. First, it excluded patients who had severely limited functional capacity and could not walk. This in turn may have undermined the ability of 6MWD to specifically predict MACE, because many noncardiac conditions (such as severe pulmonary, neurologic, and musculoskeletal diseases) can also limit functional capacity and 6MWD. Second, the study was conducted in the Veterans Affairs system, where most patients are men. Thus, our findings may not be readily and generally applicable to women. Third, our study's relatively small sample size may have adversely affected the AUC-based predictive value of 6MWD. Larger, more adequately powered studies are needed to confirm our current findings.
Six-minute walk distance was reasonably good at predicting MACE and even better at predicting HF admission within one year after PCI. These findings warrant larger confirmatory studies, especially in patients with HF.
From: Cardiovascular Section, Department of Internal Medicine (Drs. Dasari, Latif, Lozano, Patel, Thadani, and Wayangankar, and Mr. Alexander); Department of Biostatistics and Epidemiology; College of Public Health (Dr. Zhao and Mr. Leung); and Physical Therapy Section (Ms Schlegel); University of Oklahoma Health Sciences Center and Veterans Affairs Medical Center, Oklahoma City, Oklahoma 73104
Presented as an abstract at the 19th Heart Failure Society of America Annual Scientific Meeting, Washington, DC, 27 September 2015.