Few data exist on the use of the 6-minute walk test (6MWT) to measure the exercise capacity of patients with severe symptomatic aortic stenosis considered at very high surgical risk. The objectives of the present prospective study were (1) to determine the feasibility and safety of the 6MWT as a measure of exercise capacity before and after transcatheter aortic valve implantation (TAVI), and (2) to determine the clinical and hemodynamic parameters associated with the exercise capacity changes in such patients. A total of 64 patients (age 80 ± 8 years, logistic European System for Cardiac Operative Risk Evaluation score 21 ± 15%, Society of Thoracic Surgeons’ score 7.5 ± 3.9%) who had undergone successful TAVI were included. The 6MWT was performed within the month before TAVI and at the 6-month follow-up visit. The mean distance walked increased from 165.3 ± 79.7 to 231.7 ± 88.9 m (p <0.0001); however, up to 25% of the patients did not improve or even decreased their exercise capacity. After adjustment for the baseline distance walked, multilinear regression analysis showed that a greater degree of renal dysfunction, as evaluated by the serum creatinine levels ( r 2 = 0.05, p = 0.03), lower postprocedural hemoglobin values ( r 2 = 0.13, p = 0.0012), and a longer hospitalization length ( r 2 = 0.08, p = 0.007) were associated with lower improvement in exercise capacity. In conclusion, exercise capacity, as evaluated by the 6MWT, was very poor in patients with severe symptomatic aortic stenosis considered at very high surgical risk. TAVI was associated with a significant increase in exercise capacity, although no improvement was observed in 1/4 of the patients. A greater degree in renal dysfunction, lower postprocedural hemoglobin values, and longer hospitalization stay were predictors of lower improvement in exercise capacity after TAVI. These results suggest that the 6MWT might become an important tool as a part of the evaluation process for TAVI candidates.
During the past 2 decades, the 6-minute walk test (6MWT) has become one of the most popular clinical and research tools used to evaluate the functional exercise capacity, the effects of therapy, and their predictive value for morbidity and mortality in a broad spectrum of patients with cardiopulmonary diseases. The 6MWT is a simple, practical, and inexpensive test that does not require training or complex equipment and reflects the capacity to perform the activities of daily life. The test has also shown its feasibility in elderly patients who are not able to perform standard, maximum, symptom-limited exercise tests, as well as in the evaluation of patients diagnosed with severe symptomatic aortic stenosis (AS). Few data exist on the use of the 6MWT for the evaluation of the exercise capacity of elderly patients with severe symptomatic AS considered at very high surgical risk and the effect of transcatheter aortic valve implantation (TAVI) on the exercise capacity of such patients. Thus, the aims of the present prospective study were: (1) to determine the feasibility and safety of the 6MWT in a high-risk population of elderly patients with symptomatic severe AS as a measure of functional exercise capacity before and after TAVI, and (2) to evaluate the baseline, procedural, and hemodynamic variables determining the exercise capacity changes in this high-risk group of patients.
Methods
Of the 119 patients diagnosed with severe symptomatic AS who had undergone TAVI at our center, 64 patients, who had performed a 6MWT before and 6 months after the procedure, were included in the present study. Of the remaining 55 patients, 20 were unable to perform the 6MWT at baseline because of New York Heart Association (NYHA) class IV in 7, severely limited mobility in 3, oxygen dependency in 3, and logistical reasons in 7. Another 28 patients did not perform the 6MWT at follow-up because of death (n = 10), hospitalization from pneumonia (n = 1), or logistical reasons (n = 17). Finally, 5 patients were excluded from the present study because they had participated in the Placement of AoRTic traNscatheter valve (PARTNER) trial, and 2 patients were excluded because of unsuccessful TAVI. The Edwards-SAPIEN valve (Edwards Lifesciences, Irvine, California) was used in all cases and was implanted using either a transfemoral (n = 14, 22%) or a transapical (n = 50, 78%) approach. TAVI was performed under a compassionate clinical program approved by the Canadian Department of Health and Welfare (Ottawa, Ontario, Canada), and all patients provided informed consent for the procedures. The TAVI procedures have been described in detail in previous reports. All clinical, echocardiographic, procedural, and postprocedural data were prospectively collected.
The 6MWT was performed within the month before TAVI and at 6 months of follow-up according to the American Thoracic Society standardized protocol. Briefly, using an internal flat corridor of a 30-m distance, marked by 2 orange traffic cones, the participants received the following instructions: “walk from end to end of the corridor, around the cones, at your own pace, in order to cover as much ground as possible for 6 minutes, but don’t run or jog.” Each minute, a nurse encouraged the participants only with the standardized statements “you’re doing well” or “keep up the good work”; no other phrases were used. The participants were allowed to stop and rest during the test but were instructed to resume walking as soon as they were able to do so. The patient used a mechanical lap counter, and the distance covered during the test was recorded in meters. The distance that each patient was expected to walk in the 6MWT on the basis of age, gender, height, and weight was calculated using Enright’s formula. Just before starting and at the end of the test, the patients were asked to qualify their feelings of breathlessness as a feeling of an uncomfortable need to breath rather than any other sensation associated with exercise, such as fatigue or pain. They were asked to estimate the level of breathlessness using the visual analog scale (VAS) and the modified Borg scale. The VAS is a 100-mm horizontal line with the boundaries clearly defined as the extremes of breathlessness, with the left side corresponding to “not at all breathless” and the right side to “maximum imaginable breathless.” The resolution of the measurement of the 100-mm line to the nearest millimeter is both convenient and appropriate because 1 part of 100 is sufficiently sensitive and can be transformed easily from a multinomial to near-normal distribution. The Borg scale was the modified version for measuring dyspnea. This consists of a vertical scale labeled from 0 to 10, representing the progressive dyspnea-trigger sensation. It has been shown that both scales have the potential to provide sensitive, reliable, and reproducible estimations of breathlessness during exercise. The NYHA class of all patients was assessed by 1 of the cardiologists of the TAVI team, who was unaware of the 6MWT results, at baseline and at 6 months of follow-up.
Continuous variables are expressed as the mean ± SD or the median (25th to 75th interquartile range), depending on the variable distribution. The univariate normality assumptions were verified with the Shapiro-Wilk tests. A comparison of the numerical variables was performed using the Student t test (unpaired and paired) or the Wilcoxon rank test. The relations between the clinical characteristics and the delta (Δ) distance walked are expressed with point biserial or Pearson’s correlations coefficients. Parameters with p <0.05 were candidates for a stepwise linear regression analysis to find the predictors of exercise capacity improvement at the follow-up visit. Statistical interactions between the Δ-distance walked and Δ-NYHA class were assessed using a multivariate regression model, including a multiplicative interaction term. The results were considered significant at p <0.05. All analyses were conducted using SAS, version 9.1.3 (SAS Institute, Cary, North Carolina).
Results
The clinical, echocardiographic, and procedural characteristics of the study population are listed in Table 1 . All patients performed the 6MWT with no complications. The results of the 6MWT are summarized in Table 2 . The mean distance walked improved from 165.3 ± 79.7 m at baseline to 231.7 ± 88.9 m at 6 months after TAVI, for a mean increase of 66.4 ± 81.7 m (Δ83%, 95% confidence interval 39 to 127) versus baseline (p <0.0001). However, this mean distance walked remained much shorter than the expected distance in a healthy population of the same age, gender, weight, and height (363.7 ± 49.8 m, p <0.0001). Of the 64 patients, 48 (75%) showed some degree of improvement in the distance walked compared to baseline, and 16 patients (25%) had no increase or even a decrease in the distance walked compared to baseline ( Figure 1 ). Among those 28 patients unable to perform the 6MWT at 6 months of follow-up, the distance walked at the baseline 6MWT was 137.1 ± 77.4 m (p = 0.12 compared to the study population). The relation between the baseline clinical, echocardiographic, and procedural characteristic of the patients according to the changes (Δ) in the distance walked before and 6 months after TAVI are listed in Table 3 . After adjustment for the distance walked at baseline, the variables independently associated with a lower improvement in exercise capacity at 6 months of follow-up were a greater degree of renal dysfunction as evaluated by the creatinine values (estimated −18.5 ± 9.8, r = 0.05, p = 0.03), lower postprocedural hemoglobin values (estimated −18.6 ± 6.6, r = 0.13, p = 0.001), and longer hospitalization (estimated −3.1 ± 1.1, r = 0.08, p = 0.007). At 6 months of follow-up, the mean aortic gradient, aortic valve area, and left ventricular ejection fraction, were 10.5 ± 4.9 mm Hg, 1.53 ± 0.3 cm 2 , and 57 ± 13%, respectively. The changes in the left ventricular ejection fraction, pulmonary pressure, mean gradient, and aortic valve area did not correlate with the changes in the distance walked between the baseline and follow-up 6MWT.
| Characteristic | Value |
|---|---|
| Clinical | |
| Age (years) | 80 ± 8 |
| Men | 21 (33%) |
| Weight (kg) | 66.9 ± 14.3 |
| Body mass index (kg/m 2 ) | 26.2 ± 3.9 |
| Diabetes mellitus | 17 (27%) |
| Dyslipidemia ⁎ | 54 (84%) |
| Hypertension † | 59 (92%) |
| Current smokers | 4 (6%) |
| History of congestive heart failure | 38 (59%) |
| New York Heart Association class III | 64 (100%) |
| Coronary artery disease ‡ | 40 (63%) |
| Cerebrovascular disease | 14 (22%) |
| Peripheral vascular disease | 23 (36%) |
| Chronic obstructive pulmonary disease | 14 (22%) |
| Porcelain aorta | 18 (28%) |
| Frailty | 10 (16%) |
| Serum creatinine (mg/dl) | 1.34 ± 0.84 |
| Estimated glomerular filtration rate (ml/min/1.73 m 2 ) | 53 ± 21 |
| Hemoglobin (g/dl) | 11.9 ± 1.5 |
| Logistic European System for Cardiac Operative Risk Evaluation (%) | 21 ± 15 |
| Society of Thoracic Surgeons score (%) | 7.5 ± 3.9 |
| Echocardiographic | |
| Left ventricular ejection fraction (%) | 56 ± 13 |
| Left ventricular ejection fraction <55% | 16 (25%) |
| Peak systolic transvalvular gradient (mm Hg) | 71 ± 4 |
| Mean transvalvular gradient (mm Hg) | 43 ± 17 |
| Aortic valve area (cm 2 ) | 0.60 ± 0.15 |
| Pulmonary artery systolic pressure (mm Hg) | 43 ± 14 |
| Moderate/severe mitral regurgitation | 18 (28%) |
| Periprocedural | |
| Approach | |
| Transapical | 50 (78%) |
| Transfemoral | 14 (22%) |
| Successful procedure | 64 (100%) |
| Major vascular complications | 3 (5%) |
| Minor vascular complications | 6 (9%) |
| Life-threatening or disabling bleeding § | 9 (14%) |
| Major bleeding § | 30 (47%) |
| Minor bleeding § | 10 (16%) |
| Decrease in hemoglobin level (g/dl) | 3.2 ± 1.5 |
| Lowest postprocedural hemoglobin (g/dl) | 8.7 ± 1.3 |
| Need for transfusions | 34 (53%) |
| Median number of units | 1 (0–1) |
| Myocardial infarction | 1 (2%) |
| Stroke | 2 (3%) |
| Sepsis | 3 (5%) |
| Need for hemodialysis | 0 |
| Median hospitalization days | 7 (6–11) |
⁎ Dyslipidemia defined as low-density cholesterol levels >3.50 mmol/L or treatment with lipid-lowering medication.
† Hypertension defined as blood pressure ≥140/90 mm Hg or treatment with antihypertensive medication.
‡ Coronary artery disease defined as presence of untreated coronary stenosis ≥50% or previous coronary revascularization (percutaneous coronary intervention, coronary artery bypass grafting).
§ According to Valve Academic Research Consortium definitions.
| Variable | Before TAVI | After TAVI | Δ | p Value |
|---|---|---|---|---|
| Distance walked (m) | 165.3 ± 79.7 | 231.7 ± 88.9 | 66.4 ± 81.7 | <0.0001 |
| Oxygen saturation at rest (%) | 96 (95–97) | 96 (95–98) | 0 (−1–1) | 0.25 |
| Oxygen saturation after exercise (%) | 95 (94–97) | 95 (94–97) | 0 (−1–1) | 0.88 |
| Pre-test Borg scale | 0 (0–3) | 0 (0–0) | 0 (−3–0) | 0.06 |
| Post-test Borg Scale, median (min-max) | 4 (0–9) | 3 (0–7) | −1 (−8–5) | 0.003 |
| Visual Analogue Scale | 7 (4–8) | 4 (2–5) | −2 (−4–1) | <0.0001 |
| Characteristic | Distance Walked (Δ Meters) r , p Value |
|---|---|
| Clinical | |
| Age | −0.23, 0.07 |
| Men | 0.10, 0.45 |
| Height | −0.04, 0.74 |
| Weight | −0.14, 0.28 |
| Body mass index | −0.13, 0.32 |
| Diabetes mellitus | 0.20, 0.12 |
| Dyslipidemia | 0.14, 0.28 |
| Hypertension | −0.07, 0.56 |
| Current smokers | −0.02, 0.87 |
| Coronary artery disease | 0.17, 0.18 |
| Peripheral vascular disease | 0.14, 0.31 |
| Chronic obstructive pulmonary disease | −0.11, 0.41 |
| Porcelain aorta | 0.17, 0.19 |
| Frailty | 0.17, 0.18 |
| Serum creatinine | −0.29, 0.02 |
| Estimated glomerular filtration rate | 0.27, 0.03 |
| Hemoglobin | 0.10, 0.43 |
| Logistic European System for Cardiac Operative Risk Evaluation score | −0.04, 0.78 |
| Society of Thoracic Surgeons score | −0.23, 0.07 |
| Echocardiographic data | |
| Left ventricular ejection fraction | 0.13, 0.33 |
| Left ventricular ejection fraction <55% | −0.12, 0.34 |
| Peak systolic transvalvular gradient (mm Hg) | 0.13, 0.28 |
| Mean transvalvular gradient (mm Hg) | 0.16, 0.21 |
| Aortic valve area | −0.11, 0.38 |
| Pulmonary artery systolic pressure | −0.02, 0.91 |
| Moderate/severe mitral regurgitation | 0.14, 0.28 |
| Periprocedural variables | |
| Approach (transfemoral/transapical) | −0.21, 0.09 |
| Any vascular complications | 0.0, 1.0 |
| Life-threatening or disabling bleeding | −0.07, 0.57 |
| Major bleeding | −0.35, 0.005 |
| Minor bleeding | 0.07, 0.57 |
| Decrease in hemoglobin level | −0.30, 0.01 |
| Lowest postprocedural hemoglobin | 0.40, 0.001 |
| Need for transfusions | −0.32, 0.01 |
| Number of red blood cell units | −0.24, 0.06 |
| Myocardial infarction | −0.11, 0.38 |
| Stroke | −0.01, 0.93 |
| Sepsis | −0.13, 0.30 |
| Hospitalization days | −0.40, 0.0013 |
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