The six-minute walk test (6MWT) is a simple, low-cost, valid and reliable method for evaluating functional capacity.1,2 This test is widely used in the clinical management of patients suffering from chronic lung3,4 and heart5,6 diseases, but its use in acute myocardial infarction (AMI) prior to hospital discharge is still recent and has been little investigated.7 A possible reason would be that the main guidelines for the 6MWT2 do not recommend its performance within 30 days of an AMI event. There is no evidence to support restricting the use of the 6MWT after a recent AMI.8

Early exercise testing after AMI can be used to assess functional capacity and the ability to perform tasks at home and at work, to evaluate the efficacy of medical therapy, and to assess the risk of a subsequent cardiac event.8 It is also important to initiate cardiac rehabilitation programs early, which is associated with decreased mortality9,10 and improved quality of life.9 The 6MWT may be a safe and viable option for the early evaluation of functional capacity following AMI,11 as it is a self-regulated effort test that can reflect the level of activities of daily living.1

The improvement in treatment strategies for AMI is reflected by hospitalization time and patients without complications are often discharged in less than 5 days,12 a fact that may hamper exercise testing before discharge. However, according to the guidelines of the American College of Cardiology/American Heart Association (ACC/AHA),8,13 submaximal exercise tests can be performed 3–5 days after AMI in patients without complications. Despite this, limited data is available on the safety of early exercise testing after AMI8 to support the performance of exercise testing before discharge.

Therefore, the objective of this study was to evaluate the safety of early performance of the 6MWT in patients following uncomplicated AMI from the moment of discharge from the Coronary Care Unit and to compare cardiac behavior between patients who performed the test up to 4 days or more than 4 days after the event. We hypothesized that the percentage of severe complications (i.e., angina, drop in blood pressure, and arrhythmias14 as identified by Holter) during the test would be less than 2% and that they would not be influenced by the time since AMI.

The study was approved by the Research Ethics Committee of Universidade Federal de Minas Gerais (UFMG), Belo Horizonte, MG, Brazil (approval no. ETIC 0515.0.203.000-10) and developed at the University Hospital of UFMG from March 2011 to April 2013.

Possible candidates were identified among patients admitted to the Coronary Care Unit with a diagnosis of uncomplicated AMI (Killip class I or II),15 with or without ST-segment elevation (STE AMI or non-STE AMI) regardless of the reperfusion therapy – conservative, thrombolysis, percutaneous coronary intervention, or waiting for coronary artery bypass grafting. Different physicians using the same Coronary Care Unit protocol carried out the diagnosis. Furthermore, patients were required to meet the following inclusion criteria: age equal to or greater than 18 years, clinical and hemodynamic condition enabling discharge from the Coronary Care Unit and approval of the medical staff. The study did not include patients whose AMI event had taken place one week prior to the study or more, candidates with persistent arrhythmias (identified daily by electrocardiography), previous conditions associated with exercise intolerance (like pulmonary or peripheral arterial disease), signs and/or symptoms of angina at rest, unstable blood pressure levels, signs of active infection, or individuals/patients who presented changes that affected ambulatory ability. Killip class II patients with third heart sound and with extensive pulmonary crackles were also excluded.

Patients who met the research criteria were identified within 24h of discharge from the Coronary Care Unit (even if they stayed in Coronary Care Unit waiting for a room) by means of medical record analysis, clinical evaluation, and discussion with the medical staff. When all of the criteria were met, the selected patient received clarification regarding the study and its objectives, risks, and benefits. Patients were then provided with an informed consent form and invited to participate in the study. At this time, depending on the number of days since AMI, patients were allocated into two groups: Up to 4 Days Group or After 4 Days Group. This cut-off point (4 days) was based on the time proposed by ACC/AHA Guidelines8,13 for conducting submaximal exercise tests after AMI.

Fig. 1 shows the flow chart with the number of participants.

Figure 1.

Flow chart of the participants.

(0.41MB).

The sample (n=152) was comprised of mainly middle-aged, hypertensive, sedentary, and overweight men (Table 1). Most patients had experienced their first AMI (87.5%), predominantly an STE AMI (80.3%), and had up to 2 injured coronary arteries (73.7%). Only 1.3% of the patients did not undergo catheterization. Two patients had less than 50% left main coronary artery lesions. Most patients were classified as Killip class I upon admission and did not display signs of ventricular dysfunction throughout hospitalization, in accordance with the median ejection fraction of 54% (45–63%) measured by echocardiography. Diastolic function, however, was predominantly abnormal; 55.9% of the patients had abnormal relaxation patterns, 17.1% had pseudonormal relaxation patterns, and 0.7% displayed restrictive filling patterns.

All patients completed the 6MWT. Patients walked a mean distance of 442.4m during the 6MWT, which corresponds to 85% of the expected distance (515.7m)17 (Table 2). During the test, BP and HR displayed physiological behavior, elevating during exertion and gradually returning to baseline values during rest. The elevation was mild, considering the entire patient sample. There was no statistical difference between groups for the variables assessed.

One hundred and five 6MWTs were performed under Holter monitoring. Among these, 7 Holter analyses were excluded due to the percentage of artifacts being higher than 5%. Therefore, 98 6MWTs were considered for analysis (Table 3).

Supraventricular arrhythmias, particularly extra systoles, were the prevalent finding in Holter monitor analyses, having been observed in 49 patients. Supraventricular tachycardia was rare and there were only 4 non-persistent cases, which occurred in 2 patients from each group (HR 138bpm: 3 beats, HR 179bpm: 3 beats, HR 148bpm: 3 beats, HR 125bpm: 4 beats). Ventricular arrhythmias were also prevalent, particularly extra systoles. Only one patient suffered from non-sustained ventricular tachycardia, belonging to the Up to 4 Days Group (HR 109bpm: 5 beats, the greatest; and HR 114bpm: 3 beats, the fastest). Transient ST-segment changes were observed in 9 patients. These arrhythmias were observed only during the 6MWT.

The frequency of clinical and hemodynamic complications was analyzed considering 152 patients, since the whole sample performed the 6MWT. The frequency of electrocardiographic complications, on the other hand, was analyzed considering 98 patients.

Considering the whole sample (152 patients), 37 (24.34%) had some type of complication during the test and six (3.9%) patients displayed a complication classified as severe (see Table 4 complications marked with #), four (2.6%) from Up to 4 Days Group and two (1.3%) from the After 4 Days Group (p=0.17). Further complications were observed in 31 (20.4%) patients, 14 (9.2%) from the Up to 4 Days Group and 17 (11.2%) from the After 4 Days Group (Table 4). None of the complications regarded as severe led to truly significant complications or death. No other outcomes, such as signs of diminished tissue perfusion, nausea, profuse sweating, or absence of a minimal increase in HR, were identified.

This study suggests that the 6MWT is safe for evaluating patient HR following AMI, even when administered early. During the test, hemodynamic variables displayed physiological behavior of the heart rate and blood pressure. Some types of complications were observed in around 24% of patients. However, severe complications were observed only in 3.9% and disappeared when exercise ceased without any kind of intervention. There were no relevant statistical differences between groups. Therefore, we found no literature-based7,18,19 or experimental reason to justify the absolute contraindication of early performance of the 6MWT in patients following AMI.

In this sense, Sancho et al.7 evaluated the functional capacity of 23 patients from the fourth to the sixth day following AMI in the Coronary Care Unit and concluded that the 6MWT is a safe, simple, well-tolerated, and low-cost test for evaluating functional capacity. Moreover, Nogueira et al.18 carried out the 6MWT after exercise testing (the Bruce protocol) in 25 patients and concluded that the test provides a safe, reliable, and reproducible measure of the functional capacity of stable patients after uncomplicated AMI, even when performed within one week of the event.

Recently, a low frequency of serious adverse events during the 6MWT was found in hospitalized cardiac patients when administered two days after admission. Of the 30 patients, 13.3% had adverse effects considered as severe. However, the classification of severity was different and the sample included patients with higher risk, such as heart failure patients.19 Despite the higher identified percentage of severe complications during the test than the hypothesized percentage (3.9 and 2%, respectively), the frequency was still low and more than that, it did not lead to clinical complication or death.

Patients walked a reasonable distance during the 6MWT (85% of the expected distance), considering the proximity to the event and factors such as inappropriate floor and footwear and the circulation of people in the hallway. This condition reflects the real life condition that will be found in the majority of the hospitals that administer the test. Furthermore, although the main instruction for the test was to “walk as fast as possible, without running”,2 some patients clearly feared exertion and many did not walk at their maximum tolerated speed. Another factor mentioned by patients was their insecurity in walking after having undergone catheterization through the femoral artery. Many of them performed the test within 24h of the procedure. In these cases, the medical staff was asked to assess thoroughly the puncture sites. There were no related complications. In addition, in patients with STE AMI treated with fibrinolysis, the addition of 6MWT assessment pre-discharge to the traditional risk score improved the prediction of cardiovascular events at the 3-month follow-up.11

In association with the physiological behavior of the hemodynamic variables during the 6MWT, median peak HR was low – 104bpm, which corresponds to 63% of the maximum age-estimated HR and reflects the submaximal nature of the test.2 It must be considered that the level of the test, even with submaximal effort, can be higher than daily living tasks for some patients. However, in this study, the level of effort required for the test is consistent with that of basic activities of daily living (3.14 METS),1,2 such as showering or changing clothes.

Median resting HR was 76bpm, despite the fact that most patients were taking beta-blockers. A possible reason is the coincidence between the recently started medication and the early performance of the test. Nonetheless, the beta-blockers may have influenced the mild rise in HR and BP during exertion.14 Consequently, the median value of the double product was low,20 representing myocardial oxygen consumption, an extremely important variable during this stage of recovery following AMI.12

Although electrocardiographic complications were the most prevalent overall, 85.7% of the patients did not show electrocardiographic changes stemming from exertion during Holter monitoring. Nine patients displayed ST-segment depression during the 6MWT, a sign of low-load ischemia given that median energy expenditure for the test was 3.14 METs. Among all complications, this complication is noteworthy because it is a strong indicator of a poor prognosis.14

The 6MWT is a safe instrument for evaluating patient function capacity following uncomplicated AMI. During the test, hemodynamic variables displayed a physiological behavior and 3.9% of the patients displayed some type of important complication, none of which led to an unfavorable outcome. Moreover, the performance and safety of the test by the Up to 4 Days Group resembled those of the After 4 Days Group. Therefore, there is no experimental or literature-based evidence to justify the absolute contraindication of the early performance of the 6MWT in this population. The availability of a safe and viable instrument for the early evaluation of these patients’ functional capacity may facilitate their inclusion in cardiac rehabilitation programs.

The authors declare no conflicts of interest.