Download Distance Walked in the 6-Minute Test Soon After Cardiac Surgery*

Distance Walked in the 6-Minute Test
Soon After Cardiac Surgery*
Toward an Efficient Use in the Individual Patient
Cristina Opasich, MD; Stefania De Feo, MD; Gian Domenico Pinna, MS;
Giuseppe Furgi, MD; Roberto Pedretti, MD; Domenico Scrutinio, MD; and
Roberto Tramarin, MD
Study objectives: To describe the results of the 6-min walking test performed on admission to an
intensive rehabilitation program after cardiac surgery and to develop, through an algorithm
based on a few clinical indicators, reference tables in order to apply distance walked values more
efficiently in the individual patient at his/her entry into a cardiac rehabilitation program.
Setting: Intensive cardiac rehabilitation units.
Patients and intervention: A total of 2,555 consecutive patients admitted between January 2001
and December 2002 to the Cardiac Rehabilitation Department of the S. Maugeri Foundation
early after cardiac surgery performed a 6-min walking test within the fourth day of hospital
admission.
Results: The mean walked distance was 296 ⴞ 111 m (ⴞ SD). At multiple regression analysis, age,
sex, and comorbidity were independent predictors of walking test performance. The left
ventricular ejection fraction only influenced the walked distance in men. Starting from these
variables, we propose an algorithm and specific reference tables.
Conclusions: Reference values for gender-, age-, comorbidity-, and systolic function-related test
performance in patients after cardiac surgery at the beginning of the rehabilitative phase are
provided. Once a new patient has been categorized through simple parameters, the actual
distance walked could be compared with the matched reference value, thus making the
interpretation of the result more efficient. The walked distance might be used to define different
levels of disability and to personalize therapeutic exercise prescriptions.
(CHEST 2004; 126:1796 –1801)
Key words: cardiac surgery; rehabilitation; 6-min walking test
Abbreviations: LVEF ⫽ left ventricular ejection fraction; 6MWT ⫽ 6-min walking test
oon after a cardiac surgery procedure, a patient is
S prescribed
an individual rehabilitation program
based on his or her demographic, clinical, psychological, and functional variables.1 In this perspective,
early execution of the 6-min walking test (6MWT)
can be useful. The 6MWT is a practical, simple test,
which requires only the ability to walk; its use can be
*From the Istituto di Ricovero e Cura a Carattere Scientifico
Salvatore Maugeri Foundation, Cardiology Department (Drs.
Opasich, De Feo, Furgi, Pedretti, Scrutinio, and Tramarin),
Scientific Institute of Pavia, Pavia; and Department of Biomedical Engineering (Dr. Pinna), Scientific Institute of Montescano,
Montescano, Italy.
Manuscript received February 4, 2004; revision accepted July 13,
2004.
Reproduction of this article is prohibited without written permission from the American College of Chest Physicians (e-mail:
[email protected]).
Correspondence to: Cristina Opasich, MD, IRCCS Salvatore
Maugeri Foundation, Cardiology Department, Via Ferrata 8,
I-27100 Pavia; e-mail: [email protected]
extended to frail and limited patients.2 The distance
that a patient can walk on a flat surface in a period of
6 min may be used either as a generic one-time
measure of functional status or as an outcome measure for the rehabilitation program. The results of
the 6MWT are generally interpreted as a percentage
of predicted values in healthy subjects.3 However, a
correct interpretation of the 6MWT in a given
patient can only be achieved by comparing that
patient’s 6MWT performance with appropriate reference values for the specific population from which
the patient comes: the higher the affinity of the
patient with the reference population, the lower the
risk of approximation in the interpretation.
Thus, the aims of the present study were as
follows: (1) to assess the descriptive parameters of
6MWT performance in a large sample of patients
early after hospital admission to an intensive inhospital rehabilitation unit after cardiac surgery, (2)
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Clinical Investigations
to investigate the influence of some relevant clinical
and demographic variables on walking performance,
and (3) to develop efficient reference tables for this
population of patients based on a few easily measurable demographic and clinical indicators. These tables will be used for each patient at his/her entry to
the cardiac rehabilitation program.
Methods and Materials
Subjects
Consecutive patients admitted between January 2001 and
December 2002 to the Cardiac Rehabilitation Department of the
Salvatore Maugeri Foundation (which includes seven in-hospital
rehabilitation units) early after cardiac surgery were considered.
Among these patients, those who performed a 6MWT within the
fourth day of hospital admission were included in this study, the
others being physically severely frail or presenting actual clinical
contraindication to the exercise. The demographic and clinical
variables of these patients were extracted from the database of
the Cardiac Rehabilitation Department. The following data were
considered in this study: age; sex; presence of comorbid conditions, such as chronic cerebrovascular disease, diabetes mellitus,
renal failure (defined as a creatinine value ⬎ 1.5 mg/dL), and
COPD; left ventricular ejection fraction (LVEF), measured by
echocardiography on admission to the rehabilitation units; number of atherosclerotic risk factors; and time between the index
event and admission to the rehabilitation unit.
In order to test the usefulness of reference tables in clinical
practice, we also considered a new group of 567 consecutive
patients admitted to our department after cardiac surgery. As for
the main group, the 6MWT was performed in these patients
within 4 days of hospital admission. At discharge from the
hospital, the test was repeated and the time spent in the
rehabilitation unit was recorded.
6MWT
The 6MWT was performed according to the standardized
procedure.4 The test was supervised by a physical therapist and
was performed, if indicated by the physician, using telemetry
monitoring. Subjects were asked to walk at their own maximal
pace along a 35-m long, flat, and straight hospital corridor. No
encouragement was offered. The test was symptom limited, so
patients were allowed to stop if signs or symptoms of significant
distress occurred (severe dyspnea, dizziness, angina, skeletal
muscle pain), though they were instructed to resume walking as
soon as possible. The distance covered during the test was
recorded in meters.
Statistical Analysis
The associations between demographic and clinical variables
and 6MWT performance were assessed by a Pearson coefficient
of correlation or by analysis of variance with post hoc multiple
comparisons (Scheffé test), depending on whether the variables
were continuous or categorical, respectively. Variables found to
be significant in univariate analysis were the further analyzed by
a multivariate linear regression model in order to identify a set of
variables independently associated with 6MWT performance.
Besides main effects, we also tested for possible interaction
effects between pairs of variables (eg, between LVEF and
gender).
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Reference cross-tabulation tables for 6MWT performance
were obtained by grouping the patients of the study according to
the value of the variables selected by the multivariate model, and
computing relevant 6MWT statistics (mean, SD, median, lower
and upper quartiles) for each group. As some classification
variables were continuous, they were first categorized into two or
three levels.
To test the practical usefulness of reference tables, we reasoned that patients with a poor 6MWT performance on admission to a postoperative in-hospital rehabilitation program would
be less likely to recover. Accordingly, each patient enrolled in the
new test group was classified as having poor or preserved 6MWT
performance if the walked distance on program admission was,
respectively, within or above the lowest quartile statistic reported
in the reference tables for his/her specific combination of
demographic and clinical variables. An analysis of variance was
then carried out to determine the following: (1) whether patients
with poor walking test performance on program admission stayed
longer in the rehabilitation unit than did those with preserved
performance, and (2) whether they had a persistently reduced
performance at discharge.
Data are reported as mean ⫾ 1 SD, unless otherwise specified.
All tests of hypothesis were carried out using 0.05 as the level of
statistical significance.
Results
Patients
During the 2 years considered, 5,482 patients
(mean age, 65.6 ⫾ 9 years; 72% male gender) were
admitted for intensive in-hospital cardiac rehabilitation after a cardiac surgery procedure (bypass surgery, 73%; valvular surgery, 25%; other, 2%). Among
them, 2,555 patients performed the 6MWT within 4
days of hospital admission and were included in this
study. The clinical characteristics of the studied
population are summarized in Table 1. Almost all
patients were ⬎ 70 years old and had ischemic heart
disease. Moreover, 46% of patients had at least one
comorbid condition. About one fourth of cases had
systolic left ventricular dysfunction (LVEF ⬍ 50%).
Women had a significantly higher LVEF than did
men (51.5 ⫾ 10% vs 49.8 ⫾ 12%; p ⫽ 0.0005).
6MWT Performance
During the test, the patients walked an average of
296 ⫾ 111 m (range, 15 to 630 m), which was
59.5 ⫾ 22% of their predicted values according to
the regression equation proposed by Enright and
Sherrill3 in healthy subjects. Figure 1 shows the
distribution of the distance walked in the overall
population.
The distance walked in 6 min was significantly
associated with gender (Table 2), age (r ⫽ ⫺ 0.35,
p ⬍ 0.001), and the presence of at least one comorbid condition (Table 2). LVEF was weakly but
significantly correlated with the 6MWT result only in
men (r ⫽ 0.11, p ⬍ 0.001). This was confirmed by a
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Table 1—Clinical Characteristics of the Patients*
Characteristics
Patients, No.
Mean age, yr
Women
Ischemic heart disease
History of myocardial infarction
Valvular disease
Chronic heart failure
Comorbid conditions
Chronic cerebrovascular disease
Renal failure (serum creatinine ⬎ 1.5 mg/dL)
COPD
Diabetes mellitus
Risk factors, No.
ⱖ 2 risk factors
Smoking
Current
Past
Never
LVEF, %
Missing
ⱖ 50
⬍ 50
Mean
Time from index event, d
Mean length of rehabilitation stay, d
Data
2,555
64.6 ⫾ 10
674 (26)
1,746 (68)
938 (37)
192 (7.5)
218 (8.5)
495 (19)
159 (8)
98 (4)
613 (24)
1.8 ⫾ 1.3
1,357 (53)
676 (26)
1,352 (53)
527 (21)
592 (23)
1,282 (50)
681 (27)
51 ⫾ 12
10 ⫾ 8
18 ⫾ 16
*Data are presented as mean ⫾ SD or No. (%) unless otherwise
indicated.
significant interaction between gender and LVEF in
the multiple linear regression model (p ⫽ 0.006).
Age and comorbidity were independently associated
with 6-min walking performance (p ⬍ 0.001 for
both). LVEF was entered in the multivariate model
as a further independent predictor in men
(p ⬍ 0.001). The 6MWT performance did not correlate with the timing distance between surgery and
the day of the test (which could be influenced by the
surgical complications).
Table 2—Six-Minute Walking Distance Covered by
Patients Grouped According to Sex, Age, Comorbidity,
and LVEF*
Gender
Male
Female
Age, yr
⬍ 61
61–70
⬎ 70
Comorbidity
No
Yes
LVEF
ⱖ 50% and
⬍ 50% and
LVEF
ⱖ 50% and
⬍ 50% and
Distance Walked, m
p Value
319.9 ⫾ 105.5
229.3 ⫾ 99.2
⬍ 0.0001
324.6 ⫾ 99.9
293.95 ⫾ 103.6
251.9 ⫾ 112.6
⬍ 0.0001
310.7 ⫾ 108.3
278.9 ⫾ 112.1
⬍ 0.0001
male gender
male gender
331.6 ⫾ 107.9
316.9 ⫾ 94.9
0.016
female gender
female gender
232.9 ⫾ 102.9
231.2 ⫾ 86.4
0.85
Derivation of Reference Tables
In order to build reference tables for the considered population of patients, we categorized age into
three classes (⬍ 61 years, 61 to 70 years, and ⬎ 70
years). LVEF was initially categorized into three
classes (⬍ 35%, 35 to 49%, and ⱖ 50%), but there
was no difference in the distance walked between
the first two. Thus, LVEF was categorized into two
classes (ⱖ 50% and ⬍ 50%). The 6MWT performance according to these categories as well as to sex
and comorbidity are reported in Table 2.
The reference values for 6MWT performance in
male subjects according to age, presence/absence of
comorbidity, and LVEF are given in Table 3. The
same descriptive statistics are given in Table 4 for
female subjects according to age and presence/
absence of comorbidity.
Testing the Usefulness of Reference Tables
Figure 1. Frequency (N) distribution of the 6MWT results in
the overall population (n ⫽ 2,555).
Of the 567 new patients enrolled for the validation
study, 365 were men 202 were women. The mean
age of these patients was 65.2 ⫾ 10 years (range, 23
to 87 years), and their mean LVEF was 54 ⫾ 12%.
Patients with a poor 6MWT performance on program admission showed a significantly longer length
of rehabilitation stay than did patients with preserved performance, and they also had significant
persistent functional impairment at program discharge (Table 5). Of note, the mean distance walked
in 6 min at program discharge by patients with poor
performance at the beginning of the rehabilitation
program was very close to the mean distance covered
at program admission by patients with preserved
performance.
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Table 3—References Values in Men Stratified by Age, LVEF, and Comorbidity
Age, yr
ⱕ 60
ⱖ 71
61–70
Comorbidities
LVEF ⱖ 50%
LVEF ⬍ 50%
LVEF ⱖ 50%
LVEF ⬍ 50%
LVEF ⱖ 50%
LVEF ⬍ 50%
Absent
Mean ⫾ SD
Median
Lower quartile
Upper quartile
Present
Mean ⫾ SD
Median
Lower quartile
Upper quartile
n ⫽ 205
369 ⫾ 92
370
310
427
n ⫽ 109
346 ⫾ 102
350
292
416
n ⫽ 119
360 ⫾ 90
360
310
420
n ⫽ 63
341 ⫾ 89
344
282
400
n ⫽ 191
330 ⫾ 98
340
260
400
n ⫽ 156
326 ⫾ 109
334
250
400
n ⫽ 108
302 ⫾ 101
309
241
377
n ⫽ 105
282 ⫾ 100
286
220
360
n ⫽ 113
310 ⫾ 113
300
220
390
n ⫽ 124
287 ⫾ 122
284
200
371
n ⫽ 79
268 ⫾ 102
270
180
340
n ⫽ 85
254 ⫾ 119
248
175
325
Discussion
In the last years, the 6MWT has become one of
the most popular clinical exercise test for evaluating
functional capacity. It is a practical, simple, and
inexpensive test, and does not require any exercise
equipment or advanced training for technicians.4
The test has close similarities to activities of daily
living and can be performed by many elderly, frail,
and severely limited patients who could not be
evaluated by standard maximal symptom-limited exercise tests, as are cardiac patients after recent major
surgery. The test is widely employed in cardiac
rehabilitation in various categories of patients (after
cardiac surgery, after myocardial infarction, chronic
heart failure), both as a functional status indicator
and as an outcome measure.5–10
In our previous experience with elderly patients
after cardiac surgery, the 6MWT performed within
the first week of hospital admission for rehabilitation
was feasible and safe.2 The timing of the test and the
walking performance were strongly influenced by
the patient’s disability and dependence level, assessed by nursing needs. Moreover, the walking
Table 4 —References Values in Women Stratified by
Age and Comorbidity
Age, yr
Comorbidity
ⱕ 60
Absent
Mean ⫾ SD
Median
Lower quartile
Upper quartile
Present
Mean ⫾ SD
Median
Lower quartile
Upper quartile
(n ⫽ 75)
283 ⫾ 96
295
210
350
(n ⫽ 83)
267 ⫾ 100
275
200
340
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61–70
(n ⫽ 101)
255 ⫾ 93
249
200
318
(n ⫽ 151)
220 ⫾ 86
220
160
280
ⱖ 71
(n ⫽ 115)
184 ⫾ 83
178
125
240
(n ⫽ 149)
207 ⫾ 105
200
132
280
capacity affected the patient’s self-perceived heath
status and identified more severely compromised
patients with lower susceptibility to recovery.1,2 In
the database, the reasons why the walking test was
not performed within the forth day were not categorized; consequently, we could not speculate on the
possible clinical differences between patients who
did and those who did not perform the test.
The present study assessed the descriptive parameters of a 6MWT carried out by a large sample of
patients early after admission to an intensive inhospital rehabilitation unit after cardiac surgery, and
investigated the influence of some clinical and demographic variables on the tested walking performance. Moreover, the huge number of patients in
this study offered us the chance to construct reference tables developed in this specific population. A
correct interpretation of the 6MWT in any given
patient can only be achieved using reference values
with the highest affinity in terms of selected population. The reference tables have been built up
taking into account the variables that independently
affected the distance walked. Thus, in order to
interpret the distance walked by a new similar
patient correctly, we propose that this value is
compared with a specific reference value for that
type of patient obtained from an easy and quick-touse algorithm based on few common clinical data.
First, we suggest that the patient is characterized
according to his or her demographic variables. In
fact, the present study confirmed the influence of sex
and age on the walking performance of patients after
cardiac surgery: the distance walked was greater in
men than women and was inversely related to age.
Several studies3,11–18 have already reported that
functional capacity is affected by such demographic
variables, with performance decreasing in older patients and in women. The gradual reduction of
skeletal muscle mass and strength that generally
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Table 5—Duration of Rehabilitation Stay and 6-min Walked Distance on Hospital Admission and Discharge for the
Patients in the Test Group (n ⴝ 567)*
Poor 6MWT Performance (n ⫽ 260)
Variables
Walked distance, m
Duration of stay, d
Admission
168.05 ⫾ 85
17.7 ⫾ 6.7†
Preserved 6MWT Performance (n ⫽ 307)
Discharge
Admission
322.7 ⫾ 126‡
329.6 ⫾ 83
Discharge
14.2 ⫾ 5.3
416 ⫾ 98
*Data are presented as mean ⫾ SD. Patients were classified as having poor or preserved 6-min walking performance if the walked distance on
admission was, respectively, within or above the lower quartile statistic reported in reference Tables 3 and 4.
†p ⬍ 0.001 compared to patients with preserved 6MWT.
‡p ⬍ 0.001 compared to patients with preserved 6MWT.
occurs with aging and the increasing prevalence of
debilitating diseases are probably responsible for the
shorter distance walked in elderly patients. Moreover, women show lower functional capacity with
respect to men independently of differences in age,
depression score, or frequency of comorbidity.9,14
Subsequently, the presence of comorbidity should
be assessed. The prevalence of comorbid conditions
is high among patients admitted for in-hospital cardiac rehabilitation intervention after surgery, and
indeed cardiac surgery is being ever increasingly
offered as a therapeutic option for elderly, frail
patients.
Performance of the 6MWT is poor in patients with
many chronic diseases, such as osteoarthritis, pulmonary, vascular, neurologic, and muscular diseases. In
these settings, the test is widely used as a measure of
exercise capacity.19 –25 In this study, we considered
comorbid conditions that are known or believed to
affect the results of the test: diabetes mellitus,
present in almost one quarter of the entire population; renal failure; chronic cerebrovascular diseases;
and COPD. The symptomatic peripheral occlusive
arterial disease was considered as exclusion criteria
for the 6MWT. In our population, the presence of
one or more comorbid conditions negatively affected
the walking performance, independently of sex and
age.
As the final step of the proposed algorithm, left
ventricular systolic function should be assessed, but
considered only for men. In fact, in our study, this
parameter was independently associated with the
distance walked in men (possibly explained by physical deconditioning in patients with left ventricular
dysfunction), but there was a significant interference
between female sex and left ventricular fraction. It is
possible that the usually higher values of LVEF in
women, confirmed in our population, negatively
affected the correlation.
Thus, appropriate reference values for interpreting the walking performance of a new patient should
be selected in function of the patient’s gender, age,
comorbidity, and left ventricular function (Fig 2).
Once the patient has been categorized, the distance
walked can be compared with his/her matched reference value, and the interpretation of the result
becomes more efficient. For instance, we tested the
contribution of the proposed algorithm in a sample
of ⬎ 500 consecutive patients. Half of them walked
a shorter distance than the distances in the lowest
quartile of their respective reference tables. In these
patients, the functional impairment on admission
into the intensive rehabilitation programs reflected a
greater nursing need, correlated with a longer time
spent in the rehabilitation unit, and negatively influenced the functional recovery achieved by the end of
the rehabilitation phase. These patients might benefit from more specific and longer rehabilitation
programs.
Conclusions
In order to interpret the results of the 6MWT
more accurately in any single patient after cardiac
surgery, we propose the use of an easy algorithm and
comparison with an appropriate reference table de-
Figure 2. Suggested algorithm for an efficient use of the walking
test in the individual patient early after cardiac surgery.
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rived from a similar population. The degree of
physical impairment, assessed early after admission
to an in-hospital cardiac rehabilitation unit, can be
used both to assess the care and nursing needs of the
patient and in the perspective of prescribing a
personalized intensive rehabilitation program.
ACKNOWLEDGMENT: We thank the therapists and technicians of the Cardiologic Department of the Salvatore Maugeri
Foundation for making this study possible: Cassano: Amelia
Mancini, Walter Monaco, Florinda Minerva; Gussago: Daniela
Voltolini, Monica Benzoni, Ciro Rangioletti; Pavia: Antonio
Mazza, Federica Camera, Antonella Maestri; Montescano: Agostina Civardi, Angela Lupo, Giulia Salvaneschi, Anna Maria
Ventura, Milena Scabini, Patrizia Leonelli, Alessandra Pitocchi;
Telese: Maria Giovanna Beatrice, Ivana De Pierro, Silvana
Ievolella, Maria Gabriella Porcaro; Tradate: Mario Pribetich,
Eleonora Milani, Laura Gracilla; Veruno: Alfio Agazzone, Elena
Bonanomi. Moreover, we acknowledge D. Brovelli and P. Vaghi
for support with informatics.
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