Validating the Six-Minute Walk Test as a Measure of Recovery After Elective Colon Resection Surgery

Article Outline

• Abstract
• Methods
  • Measurement
  • Statistical Analysis
• Results
  • Description of the Study Sample
  • Cross-Sectional Construct Validity
  • Known Groups Construct Validity
  • Convergent Construct Validity
  • Longitudinal Validity
• Discussion
  • Study Limitations
• Conclusions
• References
• Copyright

Abstract 

Moriello C, Mayo NE, Feldman L, Carli F. Validating the six-minute walk test as a measure of recovery after elective colon resection surgery.

Objective

To provide evidence for construct and longitudinal validity of the six-minute walk test (6MWT) as a measure of postsurgical recovery.

Design

Data from a randomized clinical trial.

Setting

A major teaching hospital in a Canadian urban city.

Participants

Patients (N=63) undergoing elective colon resection.

Interventions

Not applicable.

Main Outcome Measures

Functional walking capacity was measured using the 6MWT at before surgery and at 3 and 6 weeks after surgery.

Results

At 3 weeks, 26 (41%) patients recovered to baseline or greater on the 6MWT distance, and 37 (59%) were at baseline or better by 6 weeks postdischarge. At all time points, the 6MWT distance correlated with age, the American Society of Anesthesiologists (ASA) score of surgical risk, albumin, the physical function subscale of the Medical Outcomes Study 36-Item Short-Form Health Survey (SF-36), and the physical component summary score of the SF-36. Baseline 6MWT distance correlated with postoperative 6MWT recovery, and baseline SF-36 and ASA were associated with postoperative recovery. Patients with intraoperative complications had a clinically relevant lower 6MWT than those without complications at all time points.

Conclusions

This study provides evidence for construct validity and sensitivity to change for the 6MWT as a measure of surgical recovery.

Key Words: Recovery of function, Rehabilitation, Reliability and validity, Surgery

SURGICAL RECOVERY IS a nebulous concept because there is no accepted definition of recovery and there are few validated tests or indices for this construct.¹, ², ³, ⁴ The literature is inconsistent with respect to indicators used to reflect surgical recovery or when surgical recovery should be measured. The ones used include length of stay (LOS), mortality, morbidity, complications, quality of life (QOL), depression, and pain medication usage.⁵, ⁶, ⁷, ⁸ A commonly used indicator of recovery is LOS in hospital, but this may not be the optimal indicator because there are many health system factors that influence duration of stay.¹ To date, an optimal test or index for assessing surgical recovery remains elusive.

One potential indicator of the impact of surgery is exercise tolerance; postoperatively, patients experience deterioration as a result of surgical stress, pain, and fatigue.² Although the most accepted measure of exercise tolerance is the amount of oxygen consumed at maximal effort (V̇o₂max), the testing protocol requires specialized equipment and training and may be too demanding to be safely performed postsurgery.

In addition to being feasible and safe, an ideal index of surgical recovery should be consistent with a biologic model, sensitive to the influence of other variables, relate to longer-term outcomes, and have favorable statistical properties. A biologic model for surgical recovery was proposed by Carli and Mayo¹ linking measurable physiologic and systemic changes that occur immediately after surgery to short-term functioning, such as the ability to mobilize (change position and walk) and perform basic activities of daily living (ADLs). The ability to perform basic ADLs (BADLs), therefore, would be a promising indicator of surgical recovery because this capacity is affected by the immediate effects of surgery and by symptoms of pain, fatigue, and muscle weakness. In turn, recovery of the capacity for BADLs is required for longer-term outcomes such as return to work and restoration of QOL. A common feature of the capacity for ADLs, work, and even the broader construct of health-related quality of life (HRQOL) is the capacity for walking. A functional walk test, such as the six-minute walk test (6MWT), may, therefore, satisfy the requirements for an ideal indicator of surgical recovery, because it is a test of mobility that relates to ability to perform ADLs.⁹

Validating a test or measure is an ongoing process of evidence accumulation, because there are many types of validity and many surgical populations for which evidence of validity is required. Because currently there is no criterion standard test or index that measures surgical recovery, concurrent criterion validity cannot be assessed. The primary objective of this article, therefore, was to contribute evidence for construct (cross-sectional, convergent, known groups) and longitudinal validity of the 6MWT for measuring postsurgical recovery.

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Methods 

The data for the current study were obtained from a randomized trial evaluating the impact of epidural anesthesia and analgesia on functional exercise capacity and HRQOL for patients undergoing elective colonic resection. Full details of this trial have been presented elsewhere.¹⁰ Briefly, 63 participants were recruited from 2 hospitals within the McGill University Health Centre between April 1998 and April 2000 and randomized, on the day of surgery, to either thoracic epidural analgesia with bupivacaine (n=32) or patient-controlled analgesia with morphine (n=31). Participants were evaluated preoperatively and at 3 and 6 weeks postoperatively on measures of HRQOL, functional walking capacity, and nutritional status. Perioperatively, data were collected on pain, mobility, gastrointestinal function, adverse events, and hospital LOS.

Measurement 

The primary variable of interest in the study is functional walking capacity, evaluated using the 6MWT.¹¹, ¹², ¹³, ¹⁴ This test was initially developed for patients with chronic obstructive pulmonary disease.¹¹ It evaluates the ability of a person to maintain a moderate level of walking for a period of time, reflective of ADLs. It is a submaximal test that provides a global evaluation of the integrated responses required during exercise.⁹ It has been applied widely in other patient populations, where walking capacity is an important limitation. It does not require specialized equipment or training. It is a reliable and valid measure of function. The test-retest reliability has been reported to range from .73 to .99 among a variety of populations, including the elderly¹², ¹⁵, ¹⁶ and people with chronic heart and lung disease, heart failure, chronic obstructive pulmonary disease, fibromyalgia, peripheral arterial disease, and end-stage lung disease.¹⁷ In community-dwelling elderly, measurement error was estimated at 20m,¹², ¹³ and this was used as the threshold value for classifying recovery. Furthermore, among persons without health conditions associated with musculoskeletal impairments, the 6MWT shows moderate to strong correlation (.64–.90) with V̇o₂max,¹⁸, ¹⁹, ²⁰, ²¹, ²² the most accepted test of exercise tolerance. There are few data supporting a value for clinically meaningful change. In a population of stroke survivors, the value is close to 60m.²³

Patients were instructed to walk back and forth for 6 minutes at a pace that would make them tired by the end of the walk, in a 20-m stretch of hallway, according to published guidelines.²⁴ They were allowed to rest during the test if needed, and a rest period was provided to them before the walk tests were administered. The total distance traveled was then recorded. The percentage of the distance predicted based on a reference equation including age and sex was calculated¹⁴:

(1)

HRQOL was assessed using the Medical Outcomes Study 36-Item Short-Form Health Survey (SF-36).²⁵ The SF-36 is a reliable and valid generic index of perceived health status.²⁵, ²⁶, ²⁷ It incorporates behavioral functioning, subjective well-being, and perceptions of health by assessing, on a 0-to-100 scale, 8 health concepts: (1) physical function (limitations in physical activities because of health problems), (2) role–physical (limitations in role activities because of physical health problems), (3) role–emotional (limitations in usual role activities because of emotional problems), (4) social functioning (limitations in social activities because of health problems), (5) bodily pain (pain), (6) general health (general health perceptions), (7) vitality (energy and fatigue), and (8) mental health (general mental health). Two summary scores have been developed—the physical component summary (PCS) and mental component summary (MCS)—and have been standardized to have a mean of 50 and a standard deviation (SD) of 10. A higher score on the SF-36 subscales or component summary measures indicates a better QOL. A change of as little as 2 units on the PCS has been shown to be the minimum clinically meaningful change; 5 points is often targeted by medical intervention studies, and surgical interventions can have an impact as large as 10 points.²⁵

Serum albumin level is an indicator of nutritional status. Its distribution supported its treatment as a continuous variable in the analysis.

LOS was the number of days from admission to discharge; time-to-readiness for discharge was the number of days until the patient was ready for discharge, based on clinical criteria related to infection control and bowel function. Both of these were assessed prospectively.

Surgical risk was assessed preoperatively by the anesthesiologist assigned to the case using the American Society of Anesthesiologists (ASA) grading system with the following grades: 1, a normative healthy person; 2, mild systemic disease that does not limit activity; 3, severe systemic disease that limits activity but is not incapacitating; 4, incapacitating systemic disease that is constantly life threatening; and 5, being moribund, not expected to survive 24 hours with or without surgery.²⁸

Intraoperative complications, such as cardiac arrest, bladder injury, splenic injury, excessive blood loss, and silent myocardial infarction, were also documented and coded as present or absent.

Statistical Analysis 

Descriptive statistics were calculated for sociodemographic variables and outcomes. The Pearson correlation, a statistic that explains the extent to which the relationship between variables can be described by a straight line, was used as an indicator of cross-sectional construct validity. However, when the variables deviated from linearity based on visual inspection of the residuals, the Spearman correlation was used. Preoperatively and 3 and 6 weeks postdischarge, the Pearson correlation coefficient was estimated for the 6MWT and the variables age, serum albumin, and LOS. The Spearman correlation was also calculated for the 6MWT and the 8 subscales of the SF-36 and its 2 summary components (PCS, MCS). An analysis of variance (ANOVA) was used to assess whether the 6MWT at 3 weeks postsurgery differed between the groups of patients defined according to the presence and absence of complications or levels of the ASA (known groups construct validity). The impact of baseline 6MWT on level of recovery was assessed using ordinal regression²⁹ after categorizing recovery into 3 levels: excellent (greater than baseline value by 20m or more), acceptable (recovered to within 20m of their baseline value), and poor (not recovered to within 20m of baseline). Adjustment was made for age, ASA, and albumin (convergent validity). ANOVA was used to assess whether scores on the SF-36 differed according to the degree of recovery as measured by return to baseline values on the 6MWT (convergent construct validity).

Standardized response mean (SRM), which is the mean change in score over the SD of the change in score, and a paired t test were used to determine whether the value on the 6MWT differed over the course of recovery from surgery (longitudinal validity). SRM values are interpreted similarly to effect sizes: trivial, 0.0 to 0.2; small, 0.2 to 0.5; moderate, 0.5 to 0.8; and strong, greater than or equal to 0.8.³⁰ For overall deterioration, SRM is negative on all measures; for overall improvement, SRM is positive on all measures.

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Results 

Description of the Study Sample 

The clinical characteristics of the 63 patients who participated in the study are presented in table 1. There were a total of 7 various types of surgery performed on participants. The frequency and type of surgery varied and included anterior or low anterior resection (35%), sigmoid resection or left hemicolectomy (27%); right hemicolectomy or ileocolic resection (19%), abdominal perineal resection (8%), total abdominal colectomy (6%), left and right hemicolectomy (2%), and transverse colectomy (2%).

Table 1. Selected Participant Characteristics (N=63)

Characteristics	Value
Age (y)	60.4±12
Sex (male/female)	33/30(52/48)
Albumin (g/L)	41.3±3.9
ASA (1/2/3)	14/45/4(22/72/6)
Intraoperative complications	8(13)
Postoperative complications	31(49)
LOS (d)	8.6±3.9

NOTE. Values are mean ± SD or n (%).

The distributions of the distance walked at each of the 3 time points are presented in table 2. Preoperatively, the distance covered during the 6MWT was, on average, 73% of that predicted for this group, which dropped to 65% and 70%, respectively, at 3 and 6 weeks postdischarge.

Table 2. Distribution of Distances Walked on the 6MWT

NOTE. Predicted distance is calculated using predicted distance (m) = 868 – (age × 2.9) – (female × 74.7), where age is recorded in years and if female then female is 1 and if male then female is 0.

Abbreviation: NA, not applicable.

Cross-Sectional Construct Validity 

Table 3 presents the Pearson or Spearman correlation coefficients between the 6MWT at the 3 points in time (preoperative, 3 and 6 weeks postdischarge) and variables hypothesized to be associated with the 6MWT. Age and ASA negatively correlated with the 6MWT at all 3 time points, indicating that older people had lower values on the 6MWT, as did people with higher ASA ratings. Serum albumin measured preoperatively correlated moderately with the 6MWT at baseline (Pearson r=.36) and at 6 weeks posthospital discharge (r=.35). The 6MWT at 3 weeks correlated negatively (r=−.39, P<.05) with LOS, indicating that people with longer hospital LOSs had lower values. The 6MWT at the different time points correlated strongly with subsequent measurements on this test, with correlations ranging from .75 to .87.

Table 3. Significant Pearson and Spearman Correlations of 6MWT With Commonly Used Measures of Surgical Risk and Outcome

Variables	6MWT Measured
	Preoperative	3 Week Postoperative	6 Week Postoperative
Characteristics
Age	−.60⁎	−.54⁎	−.56⁎
ASA	−.35†	−.29†	−.43†
Albumin	.36†	NS	.35†
LOS	NS	−.39†	NS
6MWT
Preoperative	NA	.75⁎	.87⁎
3wk	NA	NA	.82⁎
SF-36: physical function
Preoperative	(.39†)	NA	NA
3wk	NA	(.47⁎)	NA
6wk	NA	NA	(.50⁎)
SF-36: PCS
Preoperative	(.38†)	NA	NA
3wk	NA	(.35†)	NA
6wk	NA	NA	(.28†)

NOTE. Spearman correlations (in parentheses) were calculated when linearity assumption of Pearson correlation was violated; reported only when different from Pearson.

Abbreviations: NS, not significant. NA, not assessed.

The 6MWT correlated cross-sectionally with physical health—that is, HRQOL, as assessed by the physical function subscale of the SF-36 and the PCS of the SF-36—with correlations ranging from low (Spearman ρ=.28) to moderate (ρ=.50). Not presented is a positive correlation between the 6MWT at 6 weeks with the general health subscale of the SF-36 (ρ=.43, P<.05). The 6MWT did not correlate with the vitality subscale.

Known Groups Construct Validity 

Only 8 patients experienced one or more intraoperative complications, resulting in low power to detect differences. Compared with persons without complications, patients with intraoperative complications including excessive blood loss, trauma to spleen, respiratory failure requiring tracheostomy, myocardial ischemia during surgery, and rectovaginal fistula had clinically meaningful lower distances on the 6MWT at 3 and 6 weeks postsurgery (66m and 57m, respectively) and close to this value at baseline (46m).

A total of 31 patients experienced postoperative complications, such as a wound infection, ileus, pulmonary edema, bowel obstruction, hyporeflexic bladder, perforated gastric ulcer, anastomotic leak, and abscess. The difference in the distance walked between those with or without postoperative complications was not statistically significant. However, at 3 weeks postoperatively, those patients without a postoperative complication walked on average 41m more than those without a complication, a value approaching clinical importance.

Figure 1 shows the average distance walked during the 6MWT for ASA levels 1 to 3. ANOVA showed that the ASA was significantly associated with preoperative 6MWT and with the 6-week 6MWT.

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• Fig 1. 

  ASA score as a predictor of preoperative 6MWT and 6-week postoperative 6MWT (average distance). NOTE. Values in italics show the percentage of people recovering to baseline values. Legend: ☐, ASA level 1 (n=14); , ASA level 2 (n=45); , ASA level 3 (n=4). *Statistically significant.

Convergent Construct Validity 

The distance walked during the preoperative 6MWT was divided into quartiles (1st: <417m; 2nd: 417–495m; 3rd: 495–545m; 4th: >545m). Recovery of walking was assessed as the difference in distance from the baseline and 6-week postoperative 6MWT and categorized into excellent (>20m over baseline), acceptable (within 20m of baseline), and poor (<20m of baseline). Excellent recovery at 6 weeks postoperatively was achieved for 21%, 38%, 25%, and 24% in the first, second, third, and fourth quartiles, respectively. An acceptable recovery was obtained for 29% in the first quartile, 19% in second quartile, 38% in third quartile, and 41% in fourth quartile. A poor recovery was obtained for 50% in the first quartile, 44% in second quartile, 38% in third quartile, and 35% in fourth quartile (r=−.98). Using logistic regression for ordinal categories, there was a trend for an increasing proportion of people recovered with higher baseline 6MWT (odds ratio, .76; 95% confidence interval, .45–1.3), although these differences in proportions did not reach statistical significance in this sample of 63 people.

Recovery status (excellent, acceptable, poor) was significantly associated with HRQOL as measured by the SF-36, as determined by ANOVA. Table 4 shows the mean and SD of the SF-36 subscale scores according to category of recovery. Compared with persons with poor recovery, people with excellent recovery had substantially higher scores on most of the SF-36 subscales and on the PCS scores.

Table 4. Scores on the SF-36 According to Degree of Recovery Defined by the 6MWT

SF-36	Excellent Recovery (>20m above baseline)	Acceptable Recovery (within 20m of baseline)	Poor Recovery (>20m below baseline)
Physical function
3wk	77.5±16.0⁎	51.1±22.9	50.4±19.1
6wk	82.9±12.6⁎	70.6±18.7	65.9±23.1
Role–physical
3wk	78.1±36.4⁎	8.3±21.0	14.4±20.6
6wk	55.9±47.2⁎	44.5±44.1	28.1±36.9
Role–emotional
3wk	87.5±24.8	62.9±42.6	61.1±38.1
6wk	82.3±35.6	76.9±35.9	60.7±42.3
Social functioning
3wk	79.7±25.8	57.6±25.0	63.3±23.6
6wk	91.2±14.5⁎	80.3±25.7	72.0±22.9
Bodily pain
3wk	87.5±20.9⁎	56.4±23.1	53.1±24.9
6wk	81.8±19.1	77.9±19.2	73.6±21.1
General health
3wk	91.0±8.3⁎	64.1±17.7	67.0±19.2
6wk	84.6±13.0⁎	78.8±16.1⁎	67.3±18.7
Vitality
3wk	73.8±18.1⁎	55.5±19.1	53.2±16.0
6wk	73.8±14.2⁎	70.2±15.6	62.9±18.3
Mental health
3wk	88.0±14.6⁎	67.8±16.0	69.8±17.8
6wk	78.4±16.4	84.3±15.4⁎	73.4±17.7
PCS
3wk	50.5±6.8⁎	34.6±7.3	34.8±5.2
6wk	49.1±9.0⁎	44.0±8.6	41.6±8.4
MCS
3wk	55.5±7.3	48.8±9.8	49.5±9.8
6wk	54.2±10.5	55.2±8.4†	49.6±9.6

NOTE. Values are mean ± SD. Sample size for each recovery group: 3 weeks: excellent, n=8; acceptable, n=18; poor, n=37; 6 weeks: excellent, n=17; acceptable, n=20; poor, n=26.

Longitudinal Validity 

Change over time was assessed using SRM (mean change/SD of change) and t tests. The SRM for deterioration of the 6MWT for the time period preoperatively to 3 weeks postdischarge was −.70 (moderate change); the SRM for recovery for the time period from 3 weeks to 6 weeks was .54 (moderate change). The corresponding values of the paired t test were −5.59 (P<.001) for deterioration and 4.3 (P<.001) for recovery.

An important feature of a test or index is the absence of a floor or ceiling effect; that is, few or none should have the lowest or highest value. For the 6MWT, the lowest value is 0, unable to walk, and the highest value is undetermined but some conceivably could cover a distance of 1km, a distance much greater than achieved by anyone in this sample. The range of the 6MWT preoperatively was 240 to 640m (see table 2). Another way of looking at floor and ceiling effects is to assess the percentage of people who returned to baseline values postoperatively (± measurement error of 20m). If few or all return to baseline then the measures would not be ideal as a measure of recovery. There were 59% and 41% of persons with a poor recovery from baseline to 3 weeks and 6 weeks postoperatively, respectively. By 3 weeks postoperatively, 28% of the patients had recovered to within 20m of their baseline values; by 6 weeks, this proportion was 32%. Thirteen percent had excellent recovery by 3 weeks and 27% by 6 weeks postoperatively.

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Discussion 

Identifying an easy-to-administer indicator of functional recovery from surgery would be of great practical benefit. Patients could be more accurately informed as to the expected recovery and plan accordingly. Surgeons would easily be able to monitor recovery in their own practices, and such a measure would provide quantitative information to evaluate new and modified surgical procedures. The 6MWT is easy to administer, is inexpensive, and requires a minimal amount of training and space. It is directly observable and does not rely on self-report. Evidence of the validity of the 6MWT was presented supporting the use of this measure as an indicator of surgical recovery (table 5).

Table 5. Summary of the Evidence Provided to the Validation of the 6MWT

The data presented in this study provide evidence for cross-sectional convergent, known groups construct, and sensitivity to change validity of the 6MWT. The face validity of the 6MWT as a measure of surgical recovery may not be obvious to all stakeholders. To put its relevance into context, it is a measure of the amount of exercise capacity required for ADLs.³¹ The age of a person must also be taken into consideration when assessing surgical recovery. The stress of surgery including psychologic distress, tissue trauma, lack of activity, and quasi-starvation produces immediate systemic changes³² that may be more profound in the elderly, who have lower functional and physiologic reserves.³³ The impact of surgical stress may be a trigger accelerating the frailty process in the elderly; bedrest-induced muscle weakness can have negative impact on balance and muscle strength, hindering walking capacity. The 6MWT would seem to be an ideal way of capturing the effect of surgery on the person, because it integrates all the components of functional walking (balance, speed, endurance) in 1 measure.

Recovery after surgery is multifaceted. The fact that the 6MWT is a single indicator of surgical recovery may suggest that it cannot cover the complete construct. Myles et al³, ⁴, ³⁴ identified 40 items indicative of patient's and health professionals' perceptions of postsurgical recovery. Apart from items referring to emotional recovery and psychosocial support, almost all other items reflect some aspect of physical recovery, which would be manifested in exercise capacity. Thus, 1 simple test would contribute substantially to the quantification of recovery and would not be dependent on perceptions of physical performance but on measured capacity. Although self-report indices capture recovery from the very important perspective of the patient, these are known to be affected by a phenomenon called response shift.³⁵, ³⁶, ³⁷, ³⁸ This refers to recalibration or re-conceptualization of the underlying construct owing to individual differences in the experience of surgery and recovery. Response shift invalidates mathematical calculations performed on the scores to estimate changes over time, because the postmeasure may differ conceptually from the premeasure.

The SF-36 as a measure of health status could also be considered a useful index for surgical recovery and in fact has been used extensively in studies of surgical outcomes.¹⁰, ³⁹, ⁴⁰, ⁴¹, ⁴², ⁴³ This measure, however, is also at risk for response shift, making it difficult to assess change over time. We have shown here that the 6MWT correlates moderately with the SF-36. Because these 2 measures do not capture identical constructs this correlation is expected. However, the relationship between degree of recovery measured by the 6MWT and the SF-36 indicates that the 6MWT is capturing an aspect of postoperative recovery without the disadvantages associated with self-report.

Tests of functional exercise capacity have been used only sparingly to assess surgical recovery. We previously studied surgical recovery in a group of 35 patients undergoing laparoscopic donor nephrectomy. One month postoperatively, only 37% of patients had returned to baseline functional walking capacity. The change in 6MWT distance correlated moderately well with changes in the physical function, role–physical, bodily pain, role–emotional, and PCS domains of the SF-36 (0.4–0.5). Brooks et al⁴⁴ provided evidence for the validity of the two-minute walk test (2MWT) in cardiac surgery patients. The correlation of the 2MWT with the physical function subscale of the SF-36 was moderate (preoperatively, .44; postoperatively, .48). There was a difference of approximately 10m between persons with New York Heart Association class I and II compared with class III and IV but no difference between those who developed complications and those who did not. This test was, however, highly sensitive to change in health status because the average values (n=102) preoperatively, postoperatively, and at follow-up at 6 months were 138, 84, and 151m, respectively.⁴⁴

Study Limitations 

This study has several limitations. This validation was performed using an existing dataset rather than designing a specific study to address this objective a priori. Because of this, not all the relevant measures were included. For example, the criterion standard for exercise tolerance is the V̇o₂max test, but this was not available on the existing dataset. Also, the sample size to detect differences between groups and targeted correlations would have been estimated in advance and the study powered to detect differences. The sample size included here was insufficient for determining statistical significance in rare outcomes such as complications. Finally, although evidence has been provided regarding the validation of the 6MWT as a measure of surgical recovery, the population in this study comprised only 1 surgical group, people undergoing elective colon resection. To generalize these results, additional studies with different surgical populations need to be conducted.

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Conclusions 

The evidence presented here supports the validity of the 6MWT as a measure of surgical recovery. This information should encourage clinicians and researchers to consider adopting the 6MWT as a way of appraising surgical recovery. The consistent use of a standard test across patients and studies will permit knowledge to accumulate as to the risks and benefits of medical and surgical interventions and provide information for prognostication and for evaluation of new and existing procedures. Detailed guidelines for administering the 6MWT have been prepared by the American Thoracic Society,⁴⁵ and these should be consulted before implementing this test either for clinical practice or for research.

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