Reproducibility and validity of the incremental shuttle walking test in patients following coronary artery bypass surgery
Introduction
The importance of cardiac rehabilitation after coronary artery bypass grafting has been widely acknowledged. Improved physical fitness is an important outcome for cardiac rehabilitation. Therefore, accurate measurement of the exercise tolerance of cardiac patients prior to, and upon completion of, a cardiac rehabilitation programme using an objective measure is important. Patients’ cardiorespiratory fitness can be defined, appropriate exercise training intensities can be prescribed and the response to intervention can be measured objectively. The gold standard measure is maximum oxygen uptake (O2 peak) calculated by expired gas analysis. However, the equipment, training and time necessary to undertake this form of testing limits its widespread clinical use [1]. A field test which correlates with O2 peak that can be carried out with minimal equipment or training is desirable.
Six and 12-minute walking tests have been used and these have the advantage of needing very little equipment [2], [3]. However, self-paced tests are not standardised and performance can be influenced by encouragement and the patient's motivation or mood [4]. These tests also require up to three practice walks before the results are reproducible, and this makes them time consuming [5], [6], [7]. Various forms of step tests have been used in cardiac populations but these are not validated. Most step test protocols maintain a constant work rate throughout, and it has been suggested that this form of exercise testing may produce less useful information [8].
The incremental shuttle walking test (ISWT) is an incremental field-based exercise test that stresses the individual to a symptom-limited maximum. The patient walks around a 10-m course at the speed dictated by pre-recorded audio signals played on a cassette. The ISWT was developed from a 20-m running test to determine the functional capacity of patients with chronic obstructive pulmonary disease [9], [10]. It has been shown to correlate well with O2 peak and to be reproducible after one practice walk [10], [11]. More recently, it has been used as an outcome measure for pulmonary rehabilitation [12].
The aim of this study was to identify the reproducibility and validity of the ISWT for patients following cardiac surgery. We also explored its sensitivity to change in the context of a cardiac rehabilitation programme.
Section snippets
Methods
Thirty-nine patients [34 males, mean age 61.2 (S.D. 8.5) years] were recruited from a regional cardiothoracic unit 6–8 weeks after coronary artery bypass graft surgery. All patients fulfilled the inclusion criteria for the hospital-based cardiac rehabilitation programme which excluded patients with neurological and locomotor disorders. Informed consent was obtained from all patients. None of the patients were on beta-blocker therapy, and all patients were recruited on a convenience basis from
Statistical analysis
Statistical analysis was carried out using SPSS Version 10 (SPSS Inc, Chicago, IL, USA). Data are presented as means and standard deviations unless otherwise stated. Differences between performances on Tests 1–3 were examined using analysis of variance along with resting and peak heart rate measurements. The relationship between performance in the ISWTs and treadmill tests was evaluated using the Pearson product moment coefficient. Post-rehabilitation walking distances and differences in
Reproducibility study
Two sets of data were not completed due to equipment failure; therefore, results are missing for one subject for Test 2 and another subject for Test 3. In addition to this, one patient (Subject 2) is missing data for Test 3. This patient had an elevated resting heart rate prior to commencing the test due to a viral illness, for which he was taking medication. Due to his condition, the ISWT was not performed. No patient completed the 12-level protocol (1020 m), and distances walked during the
Discussion
Measurement of the exercise tolerance of cardiac patients is necessary in order to assess baseline fitness, to allow appropriate prescription of post-operative exercise and to evaluate the effectiveness of any intervention. The gold standard measurement of O2 peak secured by complex measurements on a treadmill is not always practical in a clinical setting due to the equipment and expertise required. Although other forms of field test have been used for this patient group, they have inherent
Acknowledgements
Ethical approval: Leicestershire Local Research Ethics Committee.
Funding: Research grant awarded by Glenfield Hospital.
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