Pediatric Exercise Science, 2001, 13, 392-401 O 2001 Human Kinetics Publishers, Inc.

New Standards for the Bruce Treadmill Protocol in Children and Adolescents

Hans U. Wessel, Janette F. Strasburger, and Brett M. Mitchell

We have developed normal standards for the Bruce exercise (EX) protocol since a review of 875 studies in patients with congenital or acquired heart disease showed that only 5.1 % achieved the predicted 50th percentile for EX time of the standards reported by Cumming, Everatt, and Hastman (Am. J Cardiol41:69, 1978). Our data are based on 160 males and 103 females, age 4-18 years who met the following criteria: trivial or no heart disease, maximal effort, maximal EX heart rate (HR) > 180 beatshin, and normal resting and EX ECG without arrhythmia. The ECG was monitored continuously and HR computed from the ECG and the end of each minute of EX. Comparison with the predicted data of Curnrning et al. for each age group by stage showed essentially identical submaximal EX heart rates but slightly lower maximal HR (-2%), which averaged 197 beats per minute in males and females. EX times were on average 15% lower than the predicted 50th percentile for most age groups in males and females. We developed regression equations, which predict exercise time from age and body size or age, body size and 2nd stage exercise heart rate. They better reflect the capabilities of untrained, asymp- tomatic children and adolescents seen in our laboratory in the 1990s than the Canadian data of 1978.

The Bruce treadmill protocol is almost universally utilized in exercise stud- ies of adult patients (1). It has also been common in pediatric studies. In a recent review of 87 exercise studies of pediatric patients with tetralogy of Fallot, the Bruce protocol was used more often than any other exercise protocol (27.8%) (6). Yet, when we began using this protocol in 1987 the study of Cumming et al. (2) represented the only published data of normal values in children. This study in- volved 327 Canadian children, 167 males, and 160 females with an age range of 4-18 years. The predicted exercise times and heart rates at each stage of the Bruce protocol are given for six age groups from ages 4-5,6-7, 8-9, 10-12, 13-15 and 16-18 years. We soon noted that the majority of our patients could not achieve the 50th percentile predicted for exercise time from Curnming's study. Since the ex- ternal work done on a treadmill at a given speed and grade is also dependent on body weight, we postulated that the lower exercise times in our patients may be in

The authors are with the Division Of Cardiology at The Children's Memorial Hospi- tal, Chicago, IL 60015.

Bruce Treadmill Protocol - 393

part related to different anthropometrics, particularly to greater weight. Unfortu- nately no height or weight information was presented in the study of Cumming et al. (2). We therefore set out to develop standards for the Bruce treadmill protocol in children and adolescents, which are more appropriate for our patient populations.

Patients and Methods

We reviewed 1,186 Bruce treadmill studies of 725 males and 461 females, which had been carried out between 1987 and 1994 in the Division of Cardiology exer- cise laboratory, mostly in patients with congenital heart disease. From these stud- ies we culled a control group of 160 males and 103 females, age 4-18 years, which met the following criteria: trivial or no heart disease, maximal effort during exer- cise, a maximal heart rate of >=I80 beatslmin, and a normal resting and exercise electrocardiogram without arrhythmia. Fifty-one studies were done in normal sub- jects. One hundred-nine patients were evaluated for chest pain, which was not reproduced during the exercise study and considered to be non-cardiac. Sixteen patients had trivial aortic stenosis or bicuspid aortic valve. Fifty-two were evalu- ated for possible exercise induced broncho-constriction, which was excluded by the test and lung function studies. Thirty-five had Kawasaki disease without coro- nary involvement by echocardiogram. All were exercised in an air-conditioned laboratory maintained at 20 OC and 4040% relative humidity. All tests were su- pervised by one of two physicians. Patients and parents gave informed consent, and the exercise studies of the laboratory were approved by the hospital Institu- tional Review Board.

After a physical examination, and height and weight measurement, the pa- tients exercised on a motor driven programmable treadmill (Quinton). A twelve- lead electrocardiogram (ECG) was monitored continuously and recorded at 1-min intervals. Systemic cuff blood pressure was measured at rest, at the end of each stage, and during each of 5 min of recovery. Oxygen hemoglobin saturation by pulsed oximetry andlor transcutaneous PO2 and PC02 were measured in the ma- jority of the studies. However, the patients wore no mask or mouthpiece and nose clip, duplicating the conditions of the study of Cumming et al. (2).

The resting ECG, heart rate, and systemic blood pressure were obtained dur- ing 3 min just prior to exercise, while the patient was standing on the treadmill. Similarly, the patient remained standing on the treadmill during the 5 min of re- covery. Once exercise began, the patients were required to exercise without hold- ing on to the handrail. Exercise was terminated when the patient was unable to continue running without holding on. The physician supervising the test judged maximal effort. The judgment is based on the heart rate achieved and comrnunica- tion with and observation of the patient. The latter includes sweating, and esti- mates of the respiratory effort, which includes an estimate of the respiratory fre- -ffe-et'.Pw@agl@uthe respiratory variations of the ECG base line. The heart rates were measured at the end of each minute from the ECG. The data were entered by hand into a computer and stored on disk. Statistical analysis was done with a UNIX based STATA statis- tical package. In each patient, observed heart rates at each stage were compared to the mean value predicted from the Cumming et al. data. Exercise times were com- pared to the predicted 50th percentile of exercise time for age group.

394 - Wessel, Strasburger, and Mitchell

For both genders and each age group we computed the mean +1 SD of the heart rate at rest, at the end of each stage and for the 2nd and 5th min of recovery. Exercise times were expressed as mean +1 SD and in percentiles for each age group. ANOVA and linear regression analysis were utilized to examine the rela- tionship of exercise time to age and anthropometric data: height, cm (ht), weight, Kg (wt.), body surface area, M2 (BSA), and body mass index, Kg 1 (~rn/100)~ (bmi). Similarly, we examined the effect of the referring diagnosis on exercise time and heart rate response to exercise.

Results

In the different age groups height and weight was similar in males and females, except for the 16-18 year old males, who were taller and heavier than the girls (Table 1). Except for the youngest males, the weights were all above the 50th percentile and close to or above the 75th percentile in most age groups. Compari- son of the observed exercise times and heart rate data with the predicted values from Cumming et al. (2) are summarized in Table 2 for the pooled data of all males and all females separately. The complete breakdown of the data by gender and age group are shown in Tables 3 and 4. The exercise times were significantly lower in our male and female control groups. Resting heart rates were significantly higher and maximal heart rates slightly but significantly lower than those of the Canadian children were. By comparison, the submaximal heart rates at each stage were es- sentially identical from the 1st through the 4th stage in males and for all five stages in females. Heart rate recovery as measured during the 2nd and 5th min of recov- ery was more rapid in the younger patients.

Correlation of exercise time with age, height, weight, body surface area (BSA), and body mass index (bmi) yielded significant correlations only between exercise time and age (r .348,p < .WOO) and height (r 0.259, p < .002). The other correlations in males and all correlations in females were not statistically signifi- cant. Regression of exercise time on age and bmi yielded the best predictions for both genders. In males: Ex time (min) = 11.12 + 0.343 x Age - 0.202 x bmi, r 0.511, F 2 7 . 1 , ~ > F 0.0000. In females: Ex time (min) = 11.1 + 0.178 x Age - 0.189 x bmi, r 0.323, F 5.8, p > F 0.0041. The difference between the observed exercise times and the times predicted from these equations is: -.004 k1.6 min, range -3.4 to +6.4 min in males, and 0.003 k1.7, range -3.2 to +5.0 min in fe- males. Inclusion of the heart rate at the end of the second stage of exercise signifi- cantly improved the prediction of the observed exercise time, r 0.715 and 0.650 in males and females, respectively, and decreased the range of the differences be- tween observed and predicted time. Males: EX time (min) = 17.55 + (0.292 x Age) - (0.033 x wt) - (0.958 x HR-2nd stage)

(r = 0.714, F = 53.81, p < .0000) Females:

EX time (min) = 16.07 + (0.82 x Age) - ( 0.138 x BMI) - (0.029 x Age+) - (0.059 x HR-2nd stage) (r = 0.675, F = 19.40, p = 0.000)

The differences between the observed exercise times and those predicted from these regressions were -.016 k1.32 min in males (range = -2.53 to +4.85 min), and 0.009 21.32 min in females (range = -2.97 to +2.32 min).

Bruce Treadmill Protocol - 395

Table 1 Data Collected in Each Patient for Analysis

Sex Age, yrs Height, cm Weight, Kg BMI N

Age 4-5 M 5.6 f 0.3 F 5.3 f 0.1

Age 6-7 M 7.0 + 0.7 F 7.0 f 0.6

Age 8-9 M 9.0 + 0.6 F 8.8 f 0.6

Age 10-12 M 11.4f1.0 F 11.6 f 1.1

Age 13-15 M 14.2 f 0.7 F 14.6 f 0.9

Age 16-18 M 17.2 f 0.8 F 17.0 f 0.4

Table 2 Exercise Time, Heart Rate (HR) at Rest and at the End of the 1st Through 5th Stage, and Maximal Heart Rate (HRmax)

Males Females

Variable N Meanf 1 SD p N Mean+ 1 SD p

EX time min. HR Rest

HR 1st Stage HR 2nd Stage HR 3rd Stage HR 4th Stage

- - - H K r Stage HR max

Note: Data represent average values for all patients. W: current study; C: predicted values from Cumrning et al. study; P: difference between W and C data, obtained by paired comparison; N: number of patients.

396 - Wessel, Strasburger, and Mitchell

Table 3 Exercise Times in Min + 1 SD and Percentiles by Age Groups

Exercise time, min Percentiles

Males N 10th 25th 50th 75th 90th M e a n f l S D

Age 4-5 W C

Age 6-7 W C

Age 8-9 W C

Age 10-12 W C

Age 13-15 W C

Age16-18 W C

Females

Age4-5 W 5 C 36

Age 6-7 W 16 C 34

Age 8-9 W 19 C 26

Age10-12 W 22 C 28

Age13-15 W 29 C 24

Age 16-18 W 11 C 12

-

W: current study; C: predicted from Cumming et al. data; N: number of patients. Top, males. Bottom, females. *Indicates significant difference, p < .05.

There was no effect of the refemng diagnosis on resting heart rate or submaximal and maximal exercise heart rate. The normal subjects and patients with Kawasaki disease exercised significantly longer than the other three diagnos- tic groups. Exercise times did not differ significantly between Normal and Kawasaki patients. However, in both males and females, exercise times of all diagnostic groups were significantly shorter than the predicted values of the data of Cumming et al. (2). At the same time there was no significant difference between observed and predicted submaximal exercise heart rates.

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Bruce Treadmill Protocol - 399

Discussion

This study was undertaken to examine whether normal standards for the Bruce treadmill protocol, based on a cohort of Canadian children studied in the 1970s (2), are valid for children seen in our laboratory in the 1990s. The Canadian con- trols were volunteers recruited from patients referred for evaluation of an innocent murmur. Our controls were selected from patients referred to the exercise labora- tory for evaluation. In both studies the controls reflect the demographics of the hospital population. However, our controls may have been biased in favor of pa- tients with overprotective parents, as the majority was referred for trivial com- plaints. In a previous study we recruited controls for a single suburban school district, which was not representative of the demographics of our hospital popula- tion (7). By necessity these controls were volunteers, and the cohort may have been biased in favor of athletic children, which may also have been the case in the Cumming et al. study (2).

Exercise times and heart rates in the Cumming et al. study were presented by age groups (2). However, the external work done on a treadmill is significantly influenced by body size. Therefore, predicted exercise times and heart rates will provide a valid comparison only if a patient's height and weight at a given age falls within the distribution of height and weight of the age groups reported in the Cana- dian study. Since Cumming et al. did not provide height and weight data of their cohort, one can only speculate if the current population is significantly different in this respect. However, it seems likely. In the U.S. height and weight data are cur- rently referenced to standards published in 1979 (3). By these standards the weights of our current study cohort are almost all at or above the 75th percentile for age of these standards. By contrast, we found the weights to be generally at the 50th percentile in a cohort of 223 male and 160 female normal children, which we studied in 1970 (7), suggesting that the average weight of the patients seen in our laboratory significantly increased over the past 30 years. Thus it may be preferable to predict the exercise times for treadmill studies by standards which, apart from age, take body size into account. However, recalculation of predicted exercise times with the equations cited above (ex time = a x Age - b x bmi), assuming that all our patients were at the 50th percentile for height and weight, did not significantly narrow the difference in exercise times between our study and the data of Cumming et al. (2). Thus other factors than body weight must account for the differences. Significant differences in physical fitness between the Chicago and Canadian chil- dren seem unlikely in view of essentially identical submaximal heart rates. More likely, lack of motivation of our children andfor our inability to push our subjects to work to exhaustion account for the lower exercise times. The finding that of 84 subjects who terminated exercise during the third stage of the protocol, only 49% completed the stage supports this view. Similarly, of those who terminated during the 4th stage (n = 140), only 22.9% completed the stage. Moreover only 4/36

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The resting heart rate of the pooled patient data (Table 3) was significantly faster in both males and females as compared to the Cumming et al. study (2). Break down by age groups (Table 4) indicates that this was true for 8 -15-year-old males and 8-12-year-old females. The differences may be related to the fact that our resting heart rate was done standing. However, the resting heart rates reported by Lenk et al. (4), which were also obtained in the standing position, were not

400 - Wessel, Strasburger, and Mitchell

significantly different when we compared the data by age groups in males and females.

Review of a recent study of treadmill exercise by the Bruce protocol in 280 healthy children from Naples, Italy, by Mafulli et al. (5) showed marked differ- ences height and weightas compared to the present study. Using the U.S. Growth reference (4), height was at the 25th percentile or less for age in both males and females. The weights were at the 50th percentile up to age 11 and dropped down to the 25th percentile or lower. In boys the weights above age 14 years are between the 5th and 10th percentile, resulting in an almost 18 Kg difference between the average weight of the older boys between the two studies. The exercise times and maximal heart rates are not comparable to this or the Cumming et al. study be- cause the patients were allowed to complete the final stage of their exercise at 0 grade and half speed, once they had completed at least 1 min of the stage. As a consequence the reported exercise times are significantly greater than the stan- dards of Cumming et al. (2), while average maximal heart rates were approxi- mately 20 beats per minute lower. No submaximal heart rate data were provided in this study. In another recent study of the Bruce protocol in 80 healthy Turkish Children (40 males and 40 females), age 4 to 15 years (4), the body size is defined only by the range of body surface area (BSA) in each of four age groups of males and females combined. Comparison indicated that the BSA exceeded the upper limits of the Turkish children in 44% of our patients. The Turkish children had significantly lower maximal exercise heart rates with the exception of the 4-6- year-old males and the 7-9-year-old females. At the same time exercise times were significantly longer than in our patients, with the exception of the 4-6-year-old males. In the 13-15-year-old males the average exercise time was 17.86 min, ex- ceeding the exercise time of the Cumming et al. data by more than 3 min, or a full stage. There was a similar large difference in the 13-15-year-old females. The reasons for these large differences are not apparent.

We conclude that the normal standards for the Bruce protocol reported by Cumming et al. ( 2 ) in 1978 are not appropriate for children currently seen in our laboratory with respect to exercise times. It would be preferable to predict exercise times from both age and a measure of body size. In our study the body mass index gave the best predictions. Since virtually all patients will complete the second stage of the Bruce protocol, it may be reasonable to include the heart rate at the end of the second stage in the prediction, which significantly improves the corre- lation. Published data, which are used as laboratory standards, should as a mini- mum define the study population in terms of age, height, and weight, in order to facilitate comparison with data from other laboratories.

References

1. Bruce, R.A. Exercise testing of patients with coronary heart disease. Principles and normal standards for evaluation. Ann. Clin. Res. 3:323-332, 1971.

a clinic population. Am. J. Cardiol. 41:70-75, 1978. A. Drizd, C.L. Johnson, R.B. Reed, A.F. Roche, and W.M. Moore.

: National Center for Health Statistics percentiles. Am. J. Clin. Nuk

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