Archives of Physical Medicine and Rehabilitation
Volume 90, Issue 1 , Pages 51-57, January 2009

Can the Six-Minute Walk Test Predict Peak Oxygen Uptake in Men With Heart Transplant?

Physiology Institute, Medicine Faculty and Hospital, University Hospital of Strasbourg, Strasbourg, France

Article Outline

Abstract 

Doutreleau S, Di Marco P, Talha S, Charloux A, Piquard F, Geny B. Can the six-minute walk test predict peak oxygen uptake in men with heart transplant?

Objective

To determine whether the six-minute walk test (6MWT) might predict peak oxygen consumption (Vo2peak) after heart transplantation.

Design

Case-control prospective study.

Setting

Public hospital.

Participants

Patients with heart transplant (n=22) and age-matched sedentary male subjects (n=13).

Interventions

Not applicable.

Main Outcome Measures

Exercise performance using a maximal exercise test, distance walked using the 6MWT, heart rate, and Vo2peak.

Results

Compared with controls, exercise performance was decreased in patients with heart transplant with less distance ambulated (516±13m vs 592±13m; P<.001) and a decrease in mean Vo2peak (23.3±1.3 vs 29.6±1mL·min−1·kg−1; P<.001). Patients with heart transplant showed an increased resting heart rate, a response delayed both at the onset of exercise and during recovery. However, the patient's heart rate at the end of the 6MWT was similar to that obtained at the ventilatory threshold. The formula did not predict measured V̇o2, with a weak correlation observed between the six-minute walk distance and both Vo2peak (r=.53; P<.01) and ventilatory threshold (r=.53; P<.01) after heart transplantation. Interestingly, when body weight was considered, correlations coefficient increased to .74 and .77, respectively (P<.001).

Conclusions

In heart transplant recipients, the 6MWT is a safe, practical, and submaximal functional test. The distance-weight product can be used as an alternative method for assessing the functional capacity after heart transplantation but cannot totally replace maximal V̇o2 determination.

Key Words: Exercise, Functional residual capacity, Rehabilitation, Transplantation

List of Abbreviations: 6MWD, six-minute walk distance, 6MWT, six-minute walk test, SEM, standard error of the mean, V̇o2, oxygen uptake, Vo2peak, peak oxygen uptake, VT, ventilatory threshold

 

ALTHOUGH BOTH FUNCTIONAL status and exercise performance progressively increase during the first 2 years after surgery,1, 2 patients with heart transplant still present low activity level and exercise capacity compared with matched sedentary subjects.3, 4, 5, 6, 7 This can be, in part, explained by both central and peripheral limitation, leading to a reduced quality of life in patients with heart transplant.8, 9, 10, 11, 12 Thus, cardiac diastolic dysfunction6 and chronotropic insufficiency10, 13 on the one hand and muscular11, 14 and endothelial dysfunction15 on the other hand appear to be the main limiting factors of exercise capacity after heart transplantation.

Standardized laboratory maximal exercise test with oxygen uptake measurement can, noninvasively and objectively, quantify this limitation by measuring VT and Vo2peak. Thus, a maximal cardiopulmonary exercise test is considered a criterion standard, used in order to prescribe appropriate and individualized physical rehabilitation, allowing a complete assessment of all systems involved in exercise performance.16 However, laboratory tests of maximal exercise performance require sophisticated equipment and specially trained people. They are costly and time-consuming. Furthermore, these tests are not always well accepted, and some participants may have difficulties achieving a maximal exercise test.

Therefore, easier modalities to assess exercise capacities have been developed. Particularly, the 6MWT is the most popular and commonly used submaximal exercise test. Indeed, it is easy to perform, is well tolerated, and better reflects daily life activities of the subjects than other walk tests.17 The 6MWT can predict functional change resulting from disease progression or therapeutic intervention, morbidity and mortality in heart18 or lung disease,19 and capacity in older, healthy, sedentary people.20 The 6MWT can also be used to assess functional exercise performance across various subjects like young21 or older, sedentary people,22 patients with heart failure,18, 23, 24 and patients with a wide variety of pulmonary alterations.25, 26

The 6MWT performance has previously been shown to be reduced in kidney transplant recipients.27 Interestingly, a weak but significant correlation between the 6MWD and the Vo2peak has been reported in patients with heart18, 28, 29 or lung failure.19 But curiously, whereas this test is commonly used in patients with heart failure needing cardiac transplantation,30 data on the 6MWT characteristics and usefulness in heart transplant recipients are lacking.

The main objective of the present investigation was therefore to determine whether the 6MWT could predict Vo2peak in patients with heart transplant. Other objectives were to determine whether the 6MWT is a submaximal or a maximal exercise test, and to compare the Vo2peak predicted by formula with the measured Vo2peak after heart transplantation.

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Methods 

Participants 

The study was carried out between January 2005 and May 2006. During this period, the chirurgical team transplanted 8 patients. Eight heart transplant recipients were excluded: 3 refused to perform a maximal exercise test, and 5 were enrolled in exercise rehabilitation or in sport. Twenty-two male sedentary patients with heart transplant, at least 6 months since transplant, in sinus rhythm, and clinically stable (free of rejection with no sign of heart failure and without cardiac allograft vasculopathy on the coronary arteriography) participated in the study. Thirteen healthy sedentary age-matched people served as controls. Patients with diabetes, pulmonary dysfunction, or conditions that might impair a successful completion of the 6MWT or of the maximal exercise test, such as orthopedic or muscular problems, were excluded. All patients with heart transplant took their usual immunosuppressive therapy composed of cyclosporine (189±12mg/d), prednisolone (6.9±0.8mg/d), and mycophenolate mofetil (909±170mg/d). All of them had a systemic hypertension treated with angiotensin-converting enzyme inhibitor (n=15) and/or calcium antagonist (n=9), but none of them took beta-blockers.

Clinical, hemodynamic, and echocardiographic characteristics of the subjects are presented in table 1. Systolic and diastolic left heart functions were obtained through the left ventricular ejection fraction and the mitral ratio of peak early and late transmitral filling velocities, respectively, following the recommendations of the American Society of Echocardiography.31 All patients gave written informed consent, and the study was approved by the hospital and university review board for human studies.

Table 1. Resting Clinical, Hemodynamic, and Echocardiographic Characteristics of Both the Control Subjects and the Subjects With Heart Transplant
CharacteristicsControl (n=13)Heart Transplant (n=22)
Age (y)57.70±1.5056.00±1.70
Height (cm)179.20±1.10174.50±1.30
BMI, (kg.m−2)25.80±0.5026.00±0.70
Heart rate (b.min−1)73.00±3.2095.10±2.40⁎
Blood pressure (mmHg)
Systolic125.00±2.30143.00±2.80⁎
Diastolic81.00±1.5088.00±1.70†
EF (%)65.00±3.0064.00±2.00
E/A ratio1.55±0.081.49±0.09

Abbreviations: BMI, body mass index; E/A, ratio of peak early (E) and late (A) transmitral filling velocities; EF, ejection fraction.

Differences between groups:

⁎P<.001;

†P=.01.

Interventions 

Both patients with heart transplant and control groups performed, 1 hour after a standardized meal, a 6MWT followed after a 2-hour resting period by a maximal upright bicycle cardiopulmonary exercise test in a quiet air-conditioned room (21°C). To avoid circadian variation, all exercise tests were performed between 2 pm and 4 pm.

Exercise Testing Protocol 

Six-minute walk test 

The 6MWT was coached by the same physicians, according to the American Thoracic Society recommendations.32 Briefly, patients were instructed to walk the most distance possible in 6 minutes in a 13-m straight calibrated track. Standardized encouragements were given every minute. Before the test, patients rested in a chair, located near the starting position, for 10 minutes. After 6 minutes, the distance walked was recorded to the nearest meter. Results are expressed as actual distance walked in meters.

Heart rate (b.min−1) and pulse oxygen saturation (%) were continuously measured before the walk, during the test, and during the first 5 minutes of recovery by using a lightweight pulse oximetera on the finger.

A reference equation for healthy adults33 was used to compute the percent predicted of 6MWD for individual adult patients with the following formula:

Predicted Vo2peak from the 6MWT was calculated by using the following formula from Cahalin et al:18

Because the body weight of the patient directly affects the work/energy required to perform the walk, whereas it is of minor importance during cycling exercise, we used the body weight-walking distance (body weight × walking distance) product to assess the walking capacity of the subject (6-minute walk work = kg.m).34, 35, 36

We also compared the 6MWD × body weight product to the maximal workload (maximal power tolerated), the oxygen uptake on the VT, and the Vo2peak reached during the maximal bicycle exercise test.

Maximal cardiopulmonary bicycle exercise test 

All patients performed a symptom-limited exercise test in the upright position using an electronically braked bicycle ergometer.b The protocol was the same for all patients, and after a warm-up on the bicycle for 3 minutes at 30W, exercise workload was increased by 15W every minute. Exhaustion was defined as the inability to maintain the pedal frequency above 50rev/min because of leg fatigue and/or dyspnea.

Breath-by-breath V̇o2 (mL.min−1), carbon dioxide output (mL.min−1), and minute ventilation (L.min−1) were measured throughout using a Vmax 229.c The system was calibrated before each test using known gas concentrations, and a 3-L calibrated syringe was used to ensure the accuracy of the pneumotach.

Peak values of Vo2 were averaged on the last 30 seconds of the exercise test (Vo2peak). The VT was manually determined by 2 blinded examiners, using the V-slope and ventilatory equivalents methods.37 The interobserver variability for VT determination was 4.8±1.64%. Vo2peak was compared with age/sex-adjusted peak oxygen uptake (peak % Vo2).38

Electrocardiographic activity was monitored continuously using a standard 12-lead configuration,d and systemic arterial blood pressure was registered every 2 minutes using a sphygmomanometer. Maximal exercise heart rate was defined as the maximal value reach at the end of the test.

Statistical Analysis 

Sigma-Stat Softwaree was used for statistical analysis. All data are presented as mean ± standard error of the mean. Differences between predicted V̇o2, 6MWD, and measured values were evaluated using paired t tests. For comparison of chronotropic response between groups, a 2-way analysis of variance with repeated measures was performed. When the F value indicated significant differences between means at different time, a Student-Neman-Keuls test for multiple comparisons was performed. A Pearson correlation analysis was used to determine whether there was a significant association between the 6MWD and the 6-minute walk work with measured Vo2peak, maximal power tolerated, and VT. Correlation was also determined between V̇o2 from predicted equations versus the measured Vo2peak. A P value less than .05 was considered significant.

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Results 

All 35 men completed the 6MWT and the cycle ergometry with gas exchange. No complication occurred during both tests.

Resting Characteristics of the 2 Groups 

The reason for cardiac transplantation was a dilated cardiomyopathy for 14 and an ischemic cardiomyopathy for the other 8. The subjects were similar concerning their age and body mass index. The delay between transplantation and the study was 60.2±6.9 months, and the mean number of rejection episodes was 1.3±0.2.

As expected, patients with heart transplant presented an increased heart rate and blood pressure compared with control values. Both systolic and diastolic cardiac functions were in the normative range as inferred from the left ventricular ejection fraction and the mitral ratio of peak early and late transmitral filling velocities, respectively (see table 1).

Distance Walked, Oxygen Saturation, and Heart Rate Responses to the Six-Minute Walk Test 

The mean distance covered during the 6MWT was 516.5±12.8m and 596.2±13.0m in patients with heart transplant and control group, respectively (P<.001). When compared as a percentage of the predicted normative age-specific and sex-specific values, patients with heart transplant presented with a significant (–14.7±2.5%; P<.001) reduction in the total walk distance compared with the control group, which was in normative range (–0.2±2.2%) (fig 1).

Work for the 6MWT, calculated as the product of the walk distance (m) × body weight (kg), was significantly reduced in patients with heart transplant compared with the control group (40,861.2±1883.1kg.m and 49,329.8±1447.8kg.m in patients with heart transplant and control group, respectively; P=.004).

Oxygen saturation remained unaltered throughout the walk and mean resting period and end-walking values were respectively 97.3±0.2% and 97.2±0.1% in the 2 groups.

The chronotropic response is shown in figure 2. The heart rate significantly increased during the walk test in the 2 groups (P<.001). The mean resting and end-walking heart rate values were 95.1±2.4b.min−1 and 123.5±2.8b.min−1 and 73.0±3.2b.min−1 and 123.4±3.0b.min−1 in the heart transplant and control groups, respectively. Interestingly, despite reaching similar absolute values during exercise, the chronotropic response, calculated as peak heart rate minus resting heart rate, was significantly decreased in transplanted patients compared with the control group (28.4±2.9 and 50.4±2.8, respectively; P<.001).

  • View full-size image.
  • Fig 2. 

    Chronotropic response at rest, exercise, and during the recovery period (R1 to R5). *P<.001 between heart transplant (black point) and control groups (white point).

The peak heart rate reached during the 6MWT was similar to the heart rate reached at the VT during the maximal exercise test, suggesting that the 6MWT is a submaximal test. It was significantly (P<.001) lower than the maximal heart rate (fig 3) in both groups.

  • View full-size image.
  • Fig 3. 

    Comparison of heart rate measured at the end of the 6MWT and during the incremental exercise test (IET) at the VT and at the peak exercise both in the heart transplant (grey) and control groups (white). Abbreviation: NS, not significant. *P<.001.

Cardiorespiratory Responses to the Maximal Bicycle Exercise Test 

All patients also performed a maximal exercise test with an end-exercise respiratory exchange ratio of 1.16±0.03 and 1.14±0.02 in the heart transplant and control groups, respectively. Patients stopped exercise because of leg fatigue (n=28) or dyspnea (n=7). None of the exercise tests had to be terminated prematurely because of an abnormal hemodynamic response. No subject had a ventilatory limitation. Ventilatory reserves were 27.5±5.3% and 32.4±6.4% in heart transplant and control subjects, respectively.

The heart transplant population presented with a moderate exercise capacity limitation compared with the age-matched control values (table 2). Overall, the mean Vo2peak during cycle ergometry was 23.3±1mL.min−1.kg−1 at a mean workload of 135.0±9.1W. This represented 82.5±3.5% of the predicted maximal V̇o2. A similar reduction in other gas exchange indexes was noted (see table 2). The VT was determined in all patients and tended to occur earlier in heart transplant patients than in control groups. VT correlated with the measured Vo2peak (r=.64; P<.001) in both groups.

Table 2. Cycle Ergometry Gas Exchange Data on VT and at Peak Exercise
VariablesControls (n=13)Heart Transplant (n=22)
VT
Work (W)116.3±8.592.7±3.9§
V̇o2
mL.min−1.kg−117.9±0.715.7±0.6‡
% predicted61.0±2.855.3±1.9
Vco2 (mL.min−1)1520.6±78.41179.3±63.9⁎
Spo2 (%)97.1±0.396.7±0.3
HR (beats.min−1)126.1±4.3128.8±2.3
Peak exercise
Work (W)194.2±7.2135.0±9.1†
Exercise time (min)14.5±0.410.1±0.6†
V̇o2
mL.min−1.kg−129.6±1.023.3±1.3⁎
% predicted100.2±3.582.5±3.5⁎
Vco2 (mL.min−1)2749.8±185.22130.1±144.6‡
V̇E (L.min−1)94.2±15.579.3±4.4
Spo2 (%)96.7±0.396.1±0.5
SBP (mmHg)171.5±5.8177.3±4.2
DBP (mmHg)85.1±3.693.7±2.1‡
HR (beats.min−1)168.4±2.7148.7±3.4†

Abbreviations: DBP, diastolic blood pressure; HR, heart rate; SBP, systolic blood pressure; Spo2, pulse oxygen saturation; V̇co2, carbon dioxide output; V̇E, minute ventilation.

Differences between groups:

⁎P=.002;

†P<.001;

‡P<.05.

Relationship Between Six-Minute Walk Test and Measured Oxygen Uptake 

There were relatively weak but significant correlations (fig 4A) between 6MWD and measured Vo2peak, VT, and maximal power in the 2 groups.

  • View full-size image.
  • Fig 4. 

    Relationship between both distance walked during 6MWT (6MWD) (A, left) or distance walked during the 6MWD (m) multiplied by the body weight (kg) (B, right), with measured Vo2peak, the VT, and peak exercise workload (Wpeak) in subjects with heart transplant.

When body weight was considered, correlations between 6MWD × body weight product (6-minute walk work) and the measured maximal power tolerated, VT, and Vo2peak became stronger (fig 4B). Thus, for example, 28% of the variance in V̇o2 on VT was accounted for by the distance walked in 6 minutes in the heart transplant group, while 59% was accounted for when the 6-minute walk work calculation was applied, representing an improvement of 31% in the variance for the same patients. Similar improvements were noted for the Vo2peak and maximal power tolerated.

The maximal power tolerated estimated from performance on the 6MWT (see equation in fig 3) was 133.8±6.8W and did not differ significantly from the measured maximal power tolerated (135±9W; P=.89).

The V̇o2 predicted by the equation by Cahalin et al18 was significantly lower than the measured Vo2peak (19.6±0.4 vs 23.3±1mL.min−1.kg−1; P<.001), with an SEM of 3.4mL.min−1.kg−1. However, there was a moderate but significant correlation between predicted and measured Vo2peak (r=.50; P<.02).

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Discussion 

The main results of this study performed in heart transplant recipients are the following: (1) the 6MWT is a submaximal test, safe, and easy to perform; (2) the Vo2peak predicted by equations does not correspond well enough with the measured Vo2peak after heart transplantation; and (3) unlike the simple distance walked, the weight-distance product (6-minute walk work) might be a useful approach of patients' Vo2peak after heart transplantation.

Exercise Intolerance After Heart Transplantation 

As expected, patients with heart transplant showed a tight but significant reduction of their exercise capacity in both maximal bicycle exercise test (82.5±3.5% of the predicted values) and 6MWT (88.7±2.5% of the predicted values) compared with control values. After cardiac transplantation, central factors (cardiac diastolic dysfunction and cardiac denervation resulting in chronotropic incompetence) and peripheral factors such as endothelial and muscular dysfunctions (progressive reduction in muscular fibers, greater contribution of glycolytic metabolism despite the oxidative one) likely contribute to the reduced exercise capacity.4, 10, 39, 40, 41, 42 The values of the Vo2peak were greater than values found in the literature.1, 2, 3, 6, 7 Such a difference could be explained, in part, by the inclusion of patients without cardiac allograft vasculopathy6 and only the bicaval surgical technique.43

In our well patients, it is unlikely that central factors have played a significant limiting role. Indeed, both cardiac systolic and diastolic functions were similar to those of control subjects. The normative diastolic function in our population late after heart transplantation could probably be explained by the inclusion of patients without cardiac allograft vasculopathy.6

Furthermore, although a reduced heart rate reserve was shown—mainly because of the increased resting heart rate secondary to the surgical cardiac denervation—no relationship was observed in our patients with heart transplant between heart rate reserve and exercise capacity. This is not unexpected, because such a relationship has been previously observed mainly in highly trained patients with heart transplant, supporting that greater exercise intensity might be necessary to reveal the central limitation of exercise capacity after heart transplantation.3, 10

Concerning peripheral factors, both vascular and muscular alterations are important to consider. Indeed, besides a possible influence of the amino acid taurine,41 muscular alterations might be a result of a direct deleterious effect of deconditioning and/or the immunosuppressive therapy. Thus, both glucocorticoids and cyclosporine have been shown to damage the muscles of patients with heart transplant. Skeletal muscle dysfunction might also result from relative hypoxia secondary to inadequate perfusion; thus, vascular dysfunction is related to exercise capacity reduction after heart transplantation.41, 42, 44

Exercise Capacity Determination After Heart Transplantation 

To quantify the aerobic performance of patients with heart transplant, we used both a maximal exercise test and a walk test. A maximal bicycle exercise test with oxygen uptake measurement is the best test used to identify the limiting factor of aerobic performance. This test measured the global and integrated response involved in exercise performance, including the potential limitations observed in heart transplant recipients: cardiovascular system and muscle metabolism.

Walk tests are typically administered as a means of evaluating functional status, monitoring treatment effectiveness, and establishing prognosis.17, 45 Indeed, walking is the most common, practical, and convenient form of exercise. Functional walk tests measure particularly the ability to undertake physically demanding activities of daily living.17, 46 Because most activities of daily living are performed at submaximal levels of exertion, the distance walked may better reflect the functional exercise level for daily physical activities. Thus, the 6MWT has been shown to correlate well with formal measures of quality of life.47 Among walk tests, we chose the 6MWT because it is easier to administer, is better tolerated, and better reflects daily life activity than other walk tests.17

Is the Six-Minute Walk Test a Submaximal or Maximal Test After Heart Transplantation? 

This issue appeared controversial in previous studies because authors proposed that the 6MWT corresponded either to a submaximal21, 48 or a maximal exercise test,22 mainly depending on the type of patients investigated. This is not surprising, because in people with an important exercise limitation factor, the walking metabolic demand can approach their maximal capacity and walk speed. Conversely, when people do not have any important limiting factor, such as in sedentary people, the 6MWD mainly depends on the walk speed (they cannot run) with its sex, age, and weight dependence. In our patients with heart transplant, several arguments suggest that the 6MWT was a submaximal test.

First, considering the values obtained in healthy subjects, the distance walked by our healthy patients with heart transplant remained in the sedentary predicted range. Indeed, healthy subjects have been shown to walk about 400 to 700m during the 6MWT, with the main predictors of the distance walked being sex, age, and height.21

Second, we determined the heart rate response to the 6MWT in our patients with heart transplant. This parameter deserves discussion after heart transplantation because patients presented with a cardiac denervation secondary to the surgical procedure.9 Accordingly, their resting heart rate was increased and, during exercise, cardiac denervation resulted primarily in a chronotropic incompetence characterized mainly by delays in the heart rate response both at the beginning of exercise and during recovery. However, even if denervated, the transplanted heart remains under hormonal control and thus, during exercise, its increase is related to exercise intensity.4, 8, 48 Thus, the end exercise heart rate is similar in patients with heart transplant and controls at the end of the 6MWT and, importantly, the heart rate of the patients with heart transplant at the end of the 6MWT was comparable to that observed at the VT during the maximal exercise test (124±3 and 129±2b.min−1, respectively in the heart transplant group) (see fig 4).

Thus, it appeared likely that the 6MWT was a submaximal test in these patients with heart transplant.

Can the Six-Minute Walk Test Predict Peak Oxygen Uptake After Heart Transplantation? 

In heart failure with systolic dysfunction18 or in patients with end-stage lung diseases,19 the distance ambulated during the 6MWT predicts the maximal oxygen consumption. Using the heart failure prediction equation,18 we found higher values than predicted with an SEM of 3.4mL.min−1.kg−1. Our results are similar to those reported recently in older patients with systolic and diastolic heart failure, supporting the idea that such equations might not always be clinically useful.23

What about the direct determination of V̇o2 at the VT and at the end of exercise (Vo2peak)? Although positively correlated, the relationship between the 6MWT distance and V̇o2 both at the VT and at peak were relatively weak. Thus, the distance walked during the 6MWT did not predict V̇o2 after heart transplantation accurately.

However, the energy cost of walking depends on both weight and walking speed,49 and the preferred minimum walking speed (to minimize the energy cost for walking) of healthy-weight adults is higher than that of obese adults who prefer to walk more slowly.50 In fact, overweight persons expend much more metabolic energy during walking than healthy-weight persons,50, 51 and the distance walked during the 6MWT is weight-dependent.

Accordingly, the correlation between performance on timed walking tests and Vo2peak from a cycle test becomes stronger if distance walked is multiplied by body weight (distance × weight = work of walking at horizontal level).34, 36 To take into account this effect of weight on the distance walked by the patients, we compared the distance-weight product during the 6MWT and the patients' V̇o2. Very interestingly, the significant relationship between this parameter and V̇o2 at the VT and at peak exercise was much stronger than that obtained with the 6MWD. Such data increase the clinical relevance of the 6MWT in patients with heart transplant and suggest that the distance walked multiplied by the body weight might be used as a reasonable approach to the patients' Vo2peak after heart transplantation.

Study Limitations 

This study has several limitations. We included only a low number of men without cardiac allograft vasculopathy, late after heart transplantation. A greater number of all kinds of patients are required to analyze the relation between the 6MWT and the maximal V̇o2.

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Conclusions 

The 6MWT is a safe and practical test, easy to use in patients with heart transplant. The heart rate response analysis support that this walk test is a submaximal test after heart transplantation. Like the equation, the use of the walking distance alone does not allow accurate prediction of the patients' V̇o2. However, the distance-weight product obtained during the 6MWT might be used as a clinically relevant approach to the patients' Vo2peak after heart transplantation. This will allow a broader use of exercise testing after heart transplantation and might help convince more patients to perform exercise training, the beneficial effects of which have been largely demonstrated.

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  • a Oxypleth, Novametrix; Soma Technology Inc, 166 Highland Park Dr, Wallingford, CT 06002.
  • b Medifit 1000 S; Planetenbaan 48, 3951 EK Maaren, The Netherlands.
  • c Sensormedics, 22705 Savi Ranch Pkwy, Yorba Linda, CA 92887.
  • d Cardiovit CS-200; Schiller, Altgasse 68, PO Box 1052, CH-6341 Barr, Switzerland.
  • e Version 3.1; Systat Software Inc, 501 Canal Blvd, Ste C, Point Richmond, CA 94804.

 No commercial party having a direct financial interest in the results of the research supporting this article has or will confer a benefit on the authors or on any organization with which the authors are associated.

PII: S0003-9993(08)01537-2

doi:10.1016/j.apmr.2008.07.010

Archives of Physical Medicine and Rehabilitation
Volume 90, Issue 1 , Pages 51-57, January 2009

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