The main purpose of this study was to evaluate heart rate recovery (HRR) in patients with the Down syndrome (DS) after peak dynamic exercise and compare their responses to those of nondisabled subjects of similar age, gender, and body mass index. Eighteen participants with the DS (14 men, 4 women; mean age 33.6 ± 7.6 years) and 18 nondisabled controls (14 men, 4 women; mean age 33.8 ± 8.5 years) performed peak treadmill tests with metabolic and heart rate measurements. Compared to controls, subjects with the DS presented lower peak values of heart rate, oxygen uptake, and minute ventilation (p <0.05). In contrast, the 2 groups attained similar respiratory exchange ratio values at peak exercise. Even after controlling for the effects of reduced peak heart rate and body mass index, participants with the DS showed slower HRR than controls at 1 minute (DS: 25.3 ± 7.2 beats/min; controls: 34.1 ± 12.1 beats/min) and 2 minutes (DS: 36.3 ± 5.8 beats/min; controls: 53.6 ± 14.1 beats/min) of recovery (p <0.05). In conclusion, adults with the DS had reduced HRR (at 1 and 2 minutes of recovery) compared to nondisabled controls, and this was independent of their lower chronotropic response to peak exercise. Additionally, despite showing attenuated HRR from peak exercise, adults with the DS did not present increased cardiovascular risk by general diagnostic criteria (HRR >12 and 22 beats/min, respectively).
Patients with the Down syndrome (DS) have reduced aerobic capacity (V o 2peak ) and chronotropic incompetence. Reduced heart rate response to exercise has been identified as the primary contributor to the low physical work capacity and cardiorespiratory fitness in this population. There is compelling evidence that the chronotropic incompetence experienced by patients with the DS in response to exercise may be associated with alterations in cardiac autonomic control. Although attention has been given to the clinical implications of changes in heart rate during exercise, the prognostic value of the rate of decrease in heart rate after the cessation of exercise is of considerable relevance. Delayed heart rate recovery (HRR) has been found to be associated with several negative health outcomes and is an independent predictor of subsequent mortality in adults who undergo exercise testing for screening purposes. Because previous studies have demonstrated delayed HRR in adults with the DS performing isometric exercise, we hypothesized that their HRR after peak dynamic exercise would also be reduced. Consequently, the purpose of this study was to evaluate HRR in subjects with the DS after peak dynamic exercise and compare their responses to those of nondisabled subjects of similar age, gender, and body mass index (BMI).
Methods
A total of 36 healthy participants, 18 with the DS (14 men, 4 women) and 18 controls without disabilities (14 men, 4 women), aged 18 to 50 years were included in the present study. Descriptive statistics are listed in Table 1 . A health screening questionnaire was completed by each participant and/or his or her parent or legal guardian. Exclusionary criteria included any contraindications to exercise, severe or profound mental retardation, active smoking status, congenital or atherosclerotic heart disease, metabolic disease, respiratory disorders including asthma, atlantoaxial instability, orthopedic issues that would limit treadmill performance, and heart rate–altering medications. Also, all participants had normal thyroid function per family member or physician report. Subjects in the 2 groups were either sedentary or moderately active, but none was involved in any formal exercise endurance training for ≥6 months. All subjects and the parents and/or guardians of the participants with the DS provided informed consent to participate in the study. Participants with the DS were recruited from a vocational center for individuals with intellectual disabilities. They all lived at home and were bused to the center daily. Most of the vocational activities at this center site involved light physical work for 5 to 6 hours 5 days a week. Control participants were recruited from the local and university communities. This study was approved by the university’s internal review board. Before data collection, each participant was familiarized with the laboratory setting, treadmill protocols, and use of the headgear and face mask. Familiarization sessions were continued until the subject could comfortably walk on the treadmill with the headgear and mouthpiece. All subjects were adequately familiarized within 1 or 2 sessions.
| Variable | Patients With DS (n = 18) | Controls (n = 18) |
|---|---|---|
| Age (years) | 34 ± 8 | 34 ± 8 |
| Height (cm) | 153.9 ± 8.7 † | 174.3 ± 6.7 |
| Body mass (kg) | 67.2 ± 9.1 ⁎ | 78.6 ± 16.1 |
| Body mass index (kg/m 2 ) | 28.5 ± 4.3 | 25.7 ± 4.5 |
| Heart rate at rest (beats/min) | 68 ± 11 | 69 ± 11 |
| Oxygen uptake at rest (ml/kg/min) | 4.1 ± 0.7 | 3.8 ± 0.9 |
| Minute ventilation at rest (L/min) | 7.8 ± 1.3 ‡ | 9.2 ± 1.1 |
| Respiratory exchange ratio at rest | 0.86 ± 0.07 | 0.85 ± 0.06 |
All subjects were tested in a postprandial state, approximately 2 to 4 hours after their last meal. Participants refrained from vigorous exercise 24 hours before testing. Subjects were also asked to refrain from caffeine ingestion on the testing day. Testing consisted of (1) a standardized anthropometric assessment and (2) a peak graded exercise protocol. Testing was carried out in the laboratory with an environmental temperature of 21°C to 24°C and a relative humidity of 44% to 56%. In an attempt to control for possible circadian variations, the measurements were performed from 7 to 11 am at approximately the same time of day for all subjects. Body mass was measured using a calibrated digital scale, and height was measured using a stadiometer (Seca 770 standing digital scale with height rod attached; Seca, Hamburg, Germany). BMI was calculated by dividing mass in kilograms by the square of height in meters. Expired gas measurements were made using a respiratory gas analysis system (Quark b ; Cosmed Srl, Rome, Italy), which was calibrated before each test with a known volume and with known gas concentrations. Heart rate data were obtained using a Polar RS 800 G3 heart rate monitor (Polar R-R Recorder; Polar Electro, Kempele, Finland).
The participants’ cardiopulmonary data at rest were obtained during a 5-minute seating period, following a quiet rest period of 10 minutes in the same position. Subsequently, participants’ cardiorespiratory data were collected while exercising on a motorized treadmill (H/P/COSMOS Mercury Med 4.0; Sports & Medical Gmbh, Nussdorf – Traunstein, Germany). Testing began with a submaximal horizontal walk on a motorized treadmill at a constant speed of 4 km/hour. Grade was increased by 2.5% every 5 minutes until a 7.5% grade was reached. Grade was then increased every 2 minutes by 2.5% until a 12.5% grade was reached. From this point, grade was held constant, whereas speed was increased by 1.6 km/hour every minute until exhaustion. This protocol has been shown to be a valid and reliable measure of maximal cardiorespiratory fitness in patients with the DS and in control participants without disabilities. V o 2 data were displayed in 20-second averages. A valid V o 2peak was defined as the highest value obtained during the last stage of exercise with a respiratory exchange ratio >1.0. Recovery from peak exercise consisted of a 3-minute treadmill walk at a speed of 2.4 km/hour and a grade of 2.5%. HRR was defined as the reduction in heart rate from the rate at peak exercise to the rate at 1 and 2 minutes after the cessation of exercise.
Descriptive statistics were calculated for all variables. Before comparing the 2 groups, data were tested for normality and homoscedasticity using the Kolmogorov-Smirnov and Levene tests, respectively. Potential group differences were evaluated using analyses of variance. Analyses of covariance were conducted to compare HRR between groups, controlling for peak heart rate and BMI. All data are reported as mean ± SD. Statistical significance was set at p <0.05. All data analysis was carried out using SPSS version 16.0 (SPSS, Inc., Chicago, Illinois).
Results
Descriptive and rest data are listed in Table 1 . No differences were observed between groups for age. Participants with the DS were shorter and had lower body mass than controls. Additionally, they had lower minute ventilation at rest. The comparisons between patients with the DS and control subjects at peak exercise intensities are listed in Table 2 . Subjects with the DS achieved lower peak values for heart rate, V o 2 , and minute ventilation. In opposition, the 2 groups attained similar respiratory exchange ratio values at peak exercise. Postexercise recovery resulted in significantly different HRR between subjects with the DS and controls, even after controlling for the effects of peak heart rate and BMI. Participants with the DS had lower HRR than controls at 1 and 2 minutes of recovery ( Table 2 ).
| Variable | Patients With DS (n = 18) | Controls (n = 18) |
|---|---|---|
| Peak heart rate (beats/min) | 162 ± 14 † | 188 ± 10 |
| Peak oxygen uptake (ml/kg/min) | 29.1 ± 6.3 † | 45.1 ± 9.5 |
| Peak minute ventilation (L/min) | 61.0 ± 17.6 † | 122.9 ± 33.9 |
| Peak respiratory exchange ratio | 1.19 ± 0.17 | 1.25 ± 0.12 |
| Heart rate recovery at 1 minute after exercise | 25 ± 7 ⁎ | 34 ± 12 |
| Heart rate recovery at 2 minutes after exercise | 36 ± 6 ‡ | 54 ± 14 |
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