Introduction

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Exercise testing is useful in determining prognosis, exercise prescription, and in the evaluation of new and existing treatments in patients with cystic fibrosis. The most precise method of assessing exercise capacity is by formal laboratory tests with online analysis of expired air. However, formal tests are not widely available to clinicians working in cystic fibrosis centres, and there is debate as to the most appropriate protocol for testing. Furthermore, many patients with cystic fibrosis find these tests excessively stressful and are reluctant to perform such tests on a routine basis. Some centres have attempted to use informal tests to assess and monitor exercise capacity in patients with cystic fibrosis, but some studies have highlighted the controversy surrounding the reliability, validity, and sensitivity of many of these tests.^1 2

The shuttle walking test is an incremental externally paced informal exercise test which overcomes many of the problems associated with existing informal exercise tests. The original authors of the shuttle walking test have shown this test to be a reliable (after just one practice test), valid, and sensitive measure of exercise capacity in patients with chronic obstructive pulmonary disease.^3-5 We have carried out preliminary work with the shuttle walking test in adult patients with cystic fibrosis, and have shown that the walking speeds in the original test (up to a maximum of 2.37 m/s) do not elicit a maximal response in adult patients with cystic fibrosis and minimal disability as well as in patients with more severe disability. On the basis of these preliminary findings the original test was modified by the addition of three levels and, further, by permitting the patients to run. The additional stages to the original 12 stage test were: level 13, 5.63 mph, 15 shuttles; level 14, 6.00 mph, 16 shuttles; and level 15, 6.38 mph, 17 shuttles. It was hypothesised that this modified shuttle test (MST) could be used to measure peak exercise capacity objectively in adult patients with cystic fibrosis. The aim of this study was therefore to compare patients' performance on the MST with peak oxygen consumption (O₂peak) measured directly during a treadmill test.

	Methods

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Twenty patients (14 men) of mean (SD) age 25 (7) years, weight 58 (8) kg, height 1.68 (0.08) metres volunteered for the study. All patients had been familiarised with the MST and the treadmill test prior to entry into the study. Patients undertook the treadmill test on one visit to the hospital and the MST on a separate visit. The order of the tests was randomised in a counterbalanced design. The mean (SD) duration between visits was 7 (4) days. The tests were performed at approximately the same time each day. Baseline spirometric measurements (Vitalograph Alpha), resting oxygen saturations and resting heart rate (Ohmeda SaO₂ monitor with ear probe), and rating of perceived breathlessness (Borg scale⁶) were recorded before the exercise test on each study day. The study was approved by the hospital ethical committee and informed consent was obtained from all patients.

TREADMILL TEST
The treadmill test was a symptom limited maximal exercise test performed according to the standardised treadmill exponential exercise protocol (STEEP).⁷ During the treadmill test measures of O₂ (ml/min), CO₂ (ml/min), and minute ventilation (VE, 1/min) were recorded at 15 s intervals (PK Morgan). The equation forced expiratory volume in one second (FEV₁) × 35 was used to predict maximum voluntary ventilation (MVV).⁸ The heart rate was measured at one minute intervals using a 12 lead electrocardiogram (Model Marguette Case 15) and SaO₂ was continuously monitored using an Ohmeda SaO₂ monitor. At the end of the test the peak heart rate, SaO₂, and peak rate of perceived breathlessness were recorded. Reasons for stopping or failing to maintain the correct pace were also recorded.

SHUTTLE TEST
Using the 15 level MST, patients were required to walk/run at increasing speeds back and forth on a 10 metre course.³ They were accompanied by an operator during the first minute of the test to help them pace themselves with the audiosignal. At the end of each level the patients were also told to go a little faster and were reminded that they were permitted to run at any time during the test. Patients continued with the test until they were unable to do so or failed to maintain the set pace.³ Heart rate was measured at 15 s intervals using a short range telemetry device (Polar Sports Tester) and SaO₂ was continuously monitored using an Ohmeda SaO₂ monitor. At the end of the test the peak heart rate, SaO₂, and peak rating of perceived breathlessness were recorded. Reasons for stopping or failing to maintain the correct pace were also recorded.

	Results

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Lung function (FEV₁) ranged from 17% to 96% predicted normal, indicating that the patients exhibited a wide variety of disease impairment. Table 1 shows that there were no significant differences between study days in baseline test parameters (FEV₁, resting heart rate, resting rating of perceived breathlessness, resting SaO₂). Furthermore, there were no significant differences between tests in comparable physiological responses to exercise.

A significant and moderately strong relationship was found between the distance achieved on the MST and lung function (MST vs FEV₁% predicted: r = 0.70, p = 0.001) and between O₂peak and lung function (O₂peak vs FEV₁% predicted: r = 0.78, p<0.001). The relationship between the distance achieved on the MST and directly measured O₂peak was strong (r = 0.95, p<0.00) with 90% of the variance in O₂peak explained by the variance in the MST distance. Using methods adopted from the authors of the original test,⁴ regression analysis was used to further describe the nature of the relationship between O₂peak and MST performance. The relationship was represented by the regression equation and 95% confidence intervals:

O₂peak = 6.83 (2.85 to 10.80) + 0.028 (0.019 to 0.024) × MST distance (fig 1).

View larger version (14K): [in this window] [in a new window]   Figure 1   Regression line (with 95% confidence intervals) for the relationship between performance on the modified shuttle test (MST) and peak oxygen consumption (O2 peak) measured during treadmill testing.

	Discussion

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The purpose of this study was to determine the validity of the MST as a measure of exercise capacity in adults with cystic fibrosis. The results show that there was a strong relationship between O₂peak and MST performance in patients with cystic fibrosis and varying degrees of lung function impairment. Ninety percent of the variation of directly measured O₂peak is explained by the variation in MST performance. This compares very favourably with the original test in which the shuttle walking test performance explained 77.4% of the variance in directly measured O₂. Many cystic fibrosis clinicians have no access to formal exercise testing equipment and the regression analysis used in this study provides additional information to such individuals on the nature of the relationship between O₂peak and MST performance. There was no significant difference between peak heart rate and peak rating of perceived breathlessness recorded during both exercise tests, which indicates the effectiveness of the MST to evoke a symptom limited exercise response in both mildly and more severely compromised adults with cystic fibrosis.

Most of the patients in the present study encroached on their pulmonary reserve during exercise testing (mean peak VE >70% MVV). None of the patients reached their maximum predicted heart rate, as determined by the age related equation (220-age), and "shortness of breath" and "fatigue" were the most common reasons reported for stopping the exercise test. These findings support the assertion that ventilatory factors rather than cardiovascular factors limit exercise tolerance in cystic fibrosis.⁹

The relationship between lung function and O₂peak was moderate. This finding supports previous work which indicated that impaired pulmonary function limits exercise capacity.¹⁰ Lung function is not a good predictor of exercise capacity because of wide intersubject variability of exercise capacity in patients with comparable lung function. In the present study oxygen desaturation (more than 5% fall in SaO₂)¹¹ occurred in all patients with FEV₁ less than 35% predicted, and in two of the patients with FEV₁ of 43% and 50%. Exercises tests to establish if exercise induced desaturation occurs are therefore a necessary prerequisite to exercise prescription in cystic fibrosis. Oxygen saturation should also be intermittently monitored during exercise programmes and, if necessary, supplemental oxygen should be used to avoid oxygen desaturation.

This study has shown that there is a strong relationship between MST performance and O₂peak in adults with cystic fibrosis and thus provides evidence of the validity of this test as a measure of peak exercise capacity in adult cystic fibrosis. Further work is required to establish the intertest reliability, test-retest reliability, and the sensitivity to change of the MST in adult patients with cystic fibrosis.