Heart Rate Response During Treadmill Exercise Test in Children and Adolescents With Congenital Heart Disease

Chronotropic Response to Exercise is Decreased in Patients With Congenital Heart Disease Compared to Cardiomyopathy Following Pediatric Heart Transplantation

BACKGROUND

Two common indications for pediatric heart transplantation are congenital heart disease and cardiomyopathy. Prior studies suggest differences in chronotropy on cardiopulmonary exercise testing outcomes depending on indication for heart transplantation. We aimed to determine whether the number of pretransplant sternotomies is associated with differences in heart rate response during exercise testing.

METHODS

A retrospective analysis of our institutional pediatric heart transplant data between 2004 and 2022 was performed. Patients were categorized by indication for transplantation into a cardiomyopathy (CM) group if they had a congenital or acquired cardiomyopathy or a congenital heart disease (CHD) group including all other forms of congenital cardiac anatomic abnormalities.

RESULTS

CHD patients (n = 40) differed from CM patients (n = 53) by mean number of sternotomies prior to transplant (2.4 ± 1.8 vs. 0.5 ± 0.9, p < 0.001). There were no significant differences in echocardiographic function or catheterization hemodynamics. In cardiopulmonary exercise testing performance, the congenital heart disease group had a significantly higher resting heart rate (91.8 ± 11.2 vs. 86.4 ± 10.2 bpm, p = 0.019), lower percent predicted age-predicted maximal heart rate achieved (78.3 ± 8.5% vs. 83.2 ± 11.4%, p = 0.032), and lower heart rate reserve (68.6 ± 19.8 vs. 84.4 ± 24.0 bpm, p = 0.001) despite a similar age and average time from transplantation. Regression analysis confirmed number of pretransplant sternotomies as a main predictor of heart rate metrics.

CONCLUSIONS

There is greater chronotropic incompetence in patients who underwent transplantation due to congenital heart disease compared to cardiomyopathy. The groups differ significantly by number of sternotomies, potentially supporting the hypothesis that prior surgical disruption of cardiac innervation may cause decreased chronotropic response to exercise following transplantation.

The effect of chronotropic incompetence on physiologic responses during progressive exercise in people with Parkinson's disease

PURPOSE

Heart rate (HR) response is likely to vary in people with Parkinson's disease (PD), particularly for those with chronotropic incompetence (CI). This study explores the impact of CI on HR and metabolic responses during cardiopulmonary exercise test (CPET) in people with PD, and its implications for exercise intensity prescription.

METHODS

Twenty-eight participants with mild PD and seventeen healthy controls underwent CPET to identify the presence or absence of CI. HR and metabolic responses were measured at submaximal (first (VT1) and second (VT2) ventilatory thresholds), and at peak exercise. Main outcome measures were HR, oxygen consumption (VO2), and changes in HR responses (HR/WR slope) to an increase in exercise demand.

RESULTS

CI was present in 13 (46%) PD participants (PDCI), who during CPET, exhibited blunted HR responses compared to controls and PD non-CI beyond 60% of maximal workload (p ≤ 0.05). PDCI presented a significantly lower HR at VT2, and peak exercise compared to PD non-CI and controls (p ≤ 0.001). VO2 was significantly lower in PDCI than PD non-CI and controls at VT2 (p = 0.003 and p = 0.036, respectively) and at peak exercise (p = 0.001 and p = 0.023, respectively).

CONCLUSION

Although poorly understood, the presence of CI in PD and its effect on HR and metabolic responses during incremental exercise is significant and important to consider when programming aerobic exercises.

Effects of childhood obesity on myocardial performance, autonomic and conduction properties of the heart

BACKGROUND

Childhood obesity tends to persist into adulthood and associated with increase in developing ischemic and non-ischemic cardiovascular diseases. We aimed to evaluate the effect of obesity on cardiac functions, atrial electromechanical coupling, and heart rate response, which are considered to be predictors of atrial fibrillation and sudden cardiac arrest.

METHODS

Study population included 52 obese children and 52 healthy controls. We performed 12-lead electrocardiography, echocardiographic examination, and treadmill exercise testing. Mitral, septal, and tricuspid segments were analysed by tissue Doppler imaging.

RESULTS

Myocardial performance index (p = 0.011, p < 0.001, and p = 0.001, respectively) was higher and E'/A' ratio (p = 0.011, p < 0.001, and p < 0.001, respectively) was lower in obese group than controls. Atrial electromechanical coupling was longer in the obese group at all three segments (p < 0.001, p = 0.009, and p = 0.04, respectively). They had significantly longer interatrial (p < 0.001) and intra-atrial (p = 0.003) electromechanical conduction delay. While chronotropic index was similar between two groups, heart rate reserve was lower in obese children than controls (p = 0.043). The 1st- and 2nd-minute heart rate recovery indices of the obese group were lower compared to controls (p < 0.001 and p = 0.03, respectively). Body mass index was positively correlated with intra- and inter-atrial conduction times, whereas it was negatively correlated with heart rate recovery indices.

CONCLUSION

We showed a deterioration in the diastolic function, atrial conduction, and heart rate response properties in children with obesity. Given the prognostic importance of these parameters, obese patients are might be at risk for atrial fibrillation and severe dysrhythmias from a young age.

Long-Term Results of Serial Exercise Testing and Echocardiography Examinations in Patients with Pulmonary Stenosis

Pulmonary stenosis (PS) affects cardiopulmonary function and exercise performance. Cardiopulmonary exercise testing (CPET) together with transthoracic echocardiography (TTE) can measure exercise performance, PS progression, and treatment effects. We assessed exercise capacity in PS patients using these methods. We enrolled 28 PS patients aged 6-35 years who received surgery, balloon pulmonary valvuloplasty, and follow-up care. The control population was selected by a 1:1 matching on age, sex, and body mass index. Baseline and follow-up peak pulmonary artery pulse wave velocity (PAV) were compared using TTE. Initial CPET revealed no significant differences in anaerobic metabolic equivalent (MET), peak oxygen consumption (VO₂), and heart rate recovery between the two groups, nor were significant differences in pulmonary function identified. Within the PS group, there were no significant differences in MET, peak VO₂, and heart rate recovery between the baseline and final CPET. Similarly, no significant differences were observed between the baseline and final PAV. The exercise capacity of patients with properly managed PS was comparable to that of healthy individuals. However, during the follow-up, declining trends in pulmonary function, aerobic metabolism, and peak exercise load capacity were observed among adolescents with PS. This study provides long-term data suggesting that PS patients should be encouraged to perform physical activity. Regular reevaluation should also be encouraged to limit performance deterioration.

Fitter Fontans for future—Impact of physical exercise on cardiopulmonary function in Fontan patients

Background

In Fontans, exercise tolerance is poorer compared to their healthy peers. Higher V˙O2peak represents a strong predictor for mortality and morbidity in these patients. Cardiac rehabilitation programs have been shown to improve cardiopulmonary function in Fontans. More habitual physical activity should therefore lead to a better exercise tolerance.

Methods

We performed cardiopulmonary exercise testing in 24 Fontan patients who had engaged in physical activity for a minimum of 3 h per week over their lifetime. As a control we performed cardiopulmonary exercise testing in 20 Fontan patients who had undertaken no physical activity or &lt;3 h per week in the past.

Results

A total of 44 Fontan patients was included (mean age 18.1 years). The mean parameters measured at peak exercise differed significantly between the active and inactive group (peak oxygen uptake [V˙O2peak] of 34.0 vs. 25.0 ml/min/kg, peak heart rate (HR) of 169.8/min vs. 139.8/min). Even though the O2pulse and the EF did not differ significantly between both groups, N-Terminal-Pro-B-Type Natriuretic Peptide (NT-pro BNP) was significantly higher in the inactive group. The two groups did not differ with respect to their cardiac function determined by magnetic resonance imaging (MRI). V˙O2peak was positively correlated with hours of sports performed by Fontans.

Conclusions

V˙O2peak and maximum HR were significantly higher in Fontans who had been physically active compared to those who had been inactive. The values reported in this study were higher than in other studies and reached normal values for V˙O2peak for most Fontans in the physically active group. The positive correlation between V˙O2peak and physical activity is an indicator of the importance of incorporating physical exercise programs into the treatment of Fontan patients.

Comprehensive cardiac evaluation to maximal exercise in a contemporary population of prepubertal children

BACKGROUND

Heart rate (HR) is a biomarker used to measure physiological function, health status and cardiovascular autonomic function. The purpose of this study was to determine sex- and age-specific reference values for cardiac autonomic function at rest, during maximal exercise and the recovery phase in prepubertal children.

METHODS

Five hundred and twelve healthy children 7-11 years of age performed a Léger test. A heart RR-interval monitor recorded the heart data and a specific software analysed the cardiac autonomic response through HR and HR variability (HRV). It analysed HR before the test (resting HR, RHR), during the test (HR_peak) and HR recovery (HRR) in the first minute (HRR1) and the fifth minute (HRR5). The values are mean ± SD.

RESULTS

Collectively, 91.2% of girls and 92.3% of boys were within the recommended ranges regarding RHR. The average HR_peak was 199 ± 10.83 b.p.m. and 96.8% of girls and 95.3% of boys were within the minimum threshold value recommended (180 b.p.m.). Boys showed lower values of RHR than girls (p < 0.001) and larger values of HRR 1 and HRR5 (p < 0.001).

CONCLUSIONS

This study comprehensively provides a reference set of data for the most important HR variables that can be obtained during exercise testing in prepubertal children regarding age and sex and in a field setting.

IMPACT

This is the first study to provide reference values of autonomic cardiac function at rest, during maximal exercise and during the recovery period in prepubertal children aged 7-11 years. Despite the early age of participants, cardiorespiratory fitness, RHR and HRR are different according to sex. Aerobic performance and HR_peak have a negative correlation with body mass index and cardiometabolic risk.

Cardiac Evaluation of Exercise Testing in a Contemporary Population of Preschool Children: A New Approach Providing Reference Values

The objective of this study was to evaluate cardiac autonomic function at rest, during maximal exercise, and in post-exercise recovery, to determine sex-specific and age-specific differences in resting heart rate (RHR), linear and spectral parameters of Heart Rate Variability (HRV), HR_peak, and heart rate recovery (HRR) after one and five minutes, in preschool children. This study involved a cohort of 167 healthy children (79 girls) aged 3 to 6 years that were selected from several schools in southern Spain. A 10 × 20 m test was conducted, and the cardiovascular response was recorded. No significant differences were found in all variables between the sexes. However, a significant reduction in RHR and an increase in HRR were found from age 4 to age 6. HRV parameters at rest were higher in older children. No associations between 10 × 20 m performance, weight status, and cardiac parameters were found. Simple linear regression analysis revealed that heart rate reserve (HRr), HRR_5min, RMSSD, and HF were the variables that showed association with all HR parameters. There was also a significant correlation between HRr and HRR_5min. In conclusion, cardiovascular autonomic function during rest, exercise, and recovery in Spanish preschool children was not influenced by sex, although older children showed greater cardiovascular modulation. Cardiorespiratory fitness status was not associated with HR response.

Cardiopulmonary Exercise Test Outcomes in Fontan Patients With Right Versus Left Single Ventricle Morphology

Background: Congenital heart defects (CHDs) palliated with Fontan surgery often result in a functional single ventricle that is either a morphologically right or left ventricle, and much less commonly undefined. Given this departure from normal physiology, especially for systemic right ventricle Fontan patients, our study sought to compare cardiopulmonary exercise test (CPET) results of adult patients with single right ventricle (SRV) and single left ventricle (SLV) morphology. Methods: Of 237 Fontan patients from the Ahmanson/UCLA Adult Congenital Heart Disease Center database, 135 patients met the inclusion criteria and were split into 2 groups: SRV (n = 44) and SLV (n = 91). Data were collected on baseline demographics, cardiac history, and CPET results. The 2 groups were compared using unpaired t-test, Mann-Whitney, or Chi-square test. Results: Regarding baseline demographics, SRV patients underwent CPET at a slightly younger age than the SLV group (26.5 ± 6.2 vs 29.6 ± 8.5 years, P = .03). There were no significant differences in CPET parameters (including peak heart rate, oxygen saturation, and maximum VO₂/kg) between the SRV and SLV groups. When evaluated subsequent CPET at 3 to 4 years, there was no difference in CPET peak heart rate, peak oxygen saturation, and maximum VO₂/kg between the 2 groups. Conclusions: This single-center retrospective analysis suggests that dominant single ventricle morphology may not be associated with an appreciable difference in exercise performance in adult survivors with a Fontan palliation.

Heart Rate Recovery Following Exercise Testing in Pediatric Patients with Acyanotic Repaired Congenital Heart Disease

Attenuated heart rate recovery (HRR) following peak exercise has been shown to be a predictor of mortality in populations of adults with Fontan palliation, coronary artery disease, heart failure, and heart transplantation. However, few have studied HRR in children and adolescents with congenital heart disease (CHD). This case-control study compared HRR patterns from exercise stress testing in children and adolescents with and without repaired acyanotic CHD (raCHD). Retrospective analysis included patients aged 10-18 years who had exercise testing between 2007 and 2017. The raCHD cohort included patients with Tetralogy of Fallot, transposition of the great arteries, coarctation, truncus arteriosus, atrioventricular septal defect, pulmonary outflow obstruction, aortic stenosis and/or insufficiency, or septal defects. Those in the control cohort were matched for age, sex, BMI, peak METs achieved, and peak heart rate (HR). HR at 1-min intervals throughout the 10-min recovery period and HRR patterns were analyzed. The study included n = 584 individuals (raCHD: n = 146), median age 14 years old, 67.1% male. The cohorts had similar resting and peak HRs. Linear mixed-effects models (LMM) suggested statistically significant cohort-by-time interaction for HR in exercise recovery, with the largest mean difference at minute-6 (2.9 bpm, p = 0.008). When comparing lesion types, LMM found no cohort or cohort-by-time interaction. While minute-6 of exercise recovery was statistically significant, the difference was 2.9 bpm and may not have clinical significance. These results suggest that HRR in pediatric raCHD patients should not vary from their healthy peers, and an attenuated HRR may not be directly attributed to underlying raCHD.

A New Approach for Evaluation of Cardiovascular Fitness and Cardiac Responses to Maximal Exercise Test in Master Runners: A Cross-Sectional Study

The aim of this study was to analyze the cardiac autonomic function at rest, at maximum exercise, and in recovery after exercise and to determine sex-specific and age-specific values for resting heart rate (RHR), hear rate (HR)-peak, HR recovery (HRR), and HR variability at rest in master runners. Fifty endurance runners (21 women) participated in this study (43.28 ± 5.25 years). The subjects came from different athletic clubs in Andalusia (Spain), and the testing protocol was performed in-season. A 3-km running test was performed and the cardiovascular response was monitored. Regarding sex, no significant differences were found regarding cardiovascular autonomic function at rest, during exercise, and following maximal exercise, only at rest, the standard deviation of all R-R intervals and low frequency values displayed significantly (p < 0.05) lower scores in women. 46% of athletes showed an RHR < 60 bpm. Additionally, HR-peak showed a significant correlation with age (r = −0.369; p = 0.009) and HRR5min (r = 0.476, p = 0.001). Also, endurance performance was inversely associated with obesity traits and cardiometabolic risk factors. In summary, age, sex, fitness, or anthropometrics characteristics did not show a relevant influence on cardiovascular autonomic modulation in master runners. However, the 3-km performance displayed a significant negative association with several factors of cardiometabolic risk.

Effects of exercise training on heart rate variability in children and adolescents with pulmonary arterial hypertension: a pilot study

Background

Pulmonary arterial hypertension (PAH) is often associated with cardiac autonomic dysfunction, and heart rate variability (HRV) as marker of cardiac autonomic function is even related to disease severity. Knowledge about the effects of physical activity on HRV is limited in these patients. We aimed to assess whether HRV parameters can be influenced by a supervised exercise training program and whether respective changes are related to levels of activity.

Methods

Six children and adolescents with moderate PAH (3 female, mean age 15.0±4.4 years; mean pulmonary to systemic arterial pressure ratio 0.5±0.2) performed supervised endurance and resistance training for 16 weeks. PAH-specific targeted medication remained unchanged during the study period. HRV was assessed before training and after 16 weeks of training by the use of ECG Move accelerometers. HRV indices included: standard deviation of normal-to-normal (NN) intervals (SDNN), square root of the mean of the sum of the squares of differences between successive NN-intervals (RMSSD), proportion of the number of pairs of successive normal-to-normal intervals that differ by more than 50 ms divided by total number of normal-to-normal interval (pNN50), and the Baevsky stress index (BSI) calculated by the histogram method.

Results

Before and after the training program, SDNN, RMSSD and pNN50 correlated with the level of physical activity. Mean values of SDNN, RMSSD and pNN50 did not change significantly due to the training program. Notably, activity level depending SDNN increased markedly after the exercise program (during activity +12.4%, at rest +6.6% and reclining +8.1%, Hedge's g of 0.28, 0.14 and 0.27, respectively). BSI decreased during activity due to the training program reflecting a reduction of stress level (Hedge's g -0.87 indicating an effect of clinical relevance).

Conclusions

According to the experience of this pilot study a workout program of 16 weeks revealed an activity level dependent effect on parameters of autonomic cardiac function in children and adolescents with PAH. This mechanism might contribute to the positive effects of exercise training in patients with PAH.

Cardiopulmonary function findings of pediatric patients with patent ductus arteriosus

Transcatheter occlusion and surgical ligation are the treatments of choice for most patent ductus arteriosus (PDA) in children. Fifty-five children who had PDA completed a pulmonary function test and a symptom-limited treadmill exercise test from 2016 to 2018 at 1 medical center in southern Taiwan. The study group was divided into surgical ligation and catheterization groups, which were compared to a healthy control group matched for age, sex, and body mass index. Data about the performance on the exercise test, including metabolic equivalent at anaerobic threshold and peak, were analyzed. No differences in the pulmonary function and ventilatory parameters were observed between the surgery, catheterization, and control groups. Heart rate at peak and at anaerobic threshold significantly differed in the investigated groups. The post hoc analysis showed that the surgery group had a lower heart rate at peak and threshold compared to the catheterization and control groups (P = .02, P < .001, respectively). No significant difference was found between the catheterization group and the control group. A larger and younger group of patients were recruited, allowing for newer data about the cardiopulmonary function to be obtained. The findings suggest that patients with PDA could undergo physical training after intervention. The imposition of restrictions to limit sports activities should be avoided.

Exercise Performance in Adolescents With Fontan Physiology (from the Pediatric Heart Network Fontan Public Data Set)

In the pediatric population, exercise capacity differs between females and males and the gap widens through adolescence. However, specific age- and sex-based changes in adolescents with congenital heart disease and Fontan palliation have not been reported. The purpose of the current study is to identify age- and sex-specific changes in exercise performance at peak and ventilatory anaerobic threshold (AT) for adolescents with Fontan physiology. Retrospective review of the Pediatric Heart Network Fontan cross sectional study (Fontan 1) public use dataset. Comparisons were made for peak and AT exercise parameters for females and males at 2-year age intervals. In addition, normative values were generated by sex and age at 2-year intervals. χ² test was used for comparison for categorical variables. Changes in exercise parameters between age groups by sex were compared by ANOVA with post-hoc analysis. Exercise testing was performed in 411 patients. AT was reached in 317 subjects (40% female), of whom, 166 (43% female) reached peak exercise. Peak oxygen consumption decreased 32% through adolescence in females and did not have the typical increase through adolescence for males. Oxygen consumption at AT also decreased with age in both sexes. In conclusion, age- and sex-based exercise performance for adolescents with Fontan physiology are predictably low, but there are additional significant decreases through adolescence for this population, especially in females. We have established normative exercise values for several parameters for this population which will better identify at risk patients and allow for earlier intervention.

Resting electrocardiographic differences in ventricular repolarization between children and young adults with congenital heart disease and those with a structurally normal heart are diminished by exercise

OBJECTIVE

Exercise-induced repolarization changes have not been systematically evaluated in children and young adults with congenital heart disease (CHD). We carried out this study to assess the QTc responses during exercise in children and young adults (≤ 21 years) with CHD with comparison to those with structurally normal hearts.

METHODS

Baseline QRS duration, calculated baseline QTc, QTc at 4 min of recovery and delta QTc was measured in 360 exercise stress tests which were performed in 360 subjects (137 stress tests in patients with CHD [CHD group] and 223 stress tests in patients with structurally normal hearts). The effects of presence of CHD and potential confounders on primary outcome measure, change in QTc (delta QTc), and secondary outcome measures (QTc at baseline and QTc at 4 min of recovery) were determined using multiple linear regression analyses.

RESULTS

The baseline QTc and the QTc at 4 min of recovery in the CHD group was longer than patients with structurally normal hearts (respective p values = .00 and .001). No significant difference was noted in delta QTc between the CHD and structurally normal heart groups.

CONCLUSIONS

While patients with CHD had a longer QRS duration and QTc interval at baseline than those with structurally normal hearts, these differences did not persist or augment with exercise.

Practical Judgment of Workload Based on Physical Activity, Work Conditions, and Worker's Age in Construction Site

It is important for construction companies to sustain a productive workforce without sacrificing its health and safety. This study aims to develop a practical judgement method to estimate the workload risk of individual construction workers. Based on studies, we developed a workload model comprising a hygrothermal environment, behavioral information, and the physical characteristics of workers). The construction workers’ heart rate and physical activity were measured using the data collected from a wearable device equipped with a biosensor and an acceleration sensor. This study is the first report to use worker physical activity, age, and the wet bulb globe temperature (WBGT) to determine a worker’s physical workload. The accuracy of this health risk judgment result was 89.2%, indicating that it is possible to easily judge the health risk of workers even in an environment where it is difficult to measure the subject in advance. The proposed model and its findings can aid in monitoring the health impacts of working conditions during construction activities, and thereby contribute toward determining workers’ health damage. However, the sampled construction workers are 12 workers, further studies in other working conditions are required to accumulate more evidence and assure the accuracy of the models.

Evaluation of Physical Fitness in Children With Congenital Heart Diseases Versus Healthy Population

We aimed to evaluate and compare the physical fitness of children with different severity of the cardiac defect with their healthy peers. We included 182 children after cooperation for congenital heart disease (CHD) and 129 healthy children as controls, 6-18 years old, who performed a complete cardiopulmonary exercise test (CPET) in a cross-sectional observational study. The CHD patients were further subdivided into three subgroups according to diagnostic characteristics and surgical methods: simple CHD group (SCHD), complex CHD group (CCHD)and total cavopulmonary connection group (TCPC). Physical fitness was compared between groups using generalized linear model and multiple linear regression analysis. In comparison with age and gender adjusted healthy controls, children in SCHD, CCHD, and TCPC subgroups had accordingly graded down values of peak oxygen consumption (VO_2max: 44.86 ± 5.41 vs. 39.91 ± 5.59 vs. 36.95 ± 5.92 vs. 32.04 ± 5.38 mL/kg/min, P < 0.05) and %predicted VO_2max (1.01 ± 0.14 vs. 0.91 ± 0.13 vs. 0.83 ± 0.15 vs. 0.72 ± 0.13, P < 0.05) for each group. The proportion of %predicted VO_2max above 80% in the CHD and the control group were 60.4% and 96.1% (P < 0.001). The VO_2max decreased by 0.98 mL/kg/min per year in CHD children, and the corresponding %predicted reference diminished by 2.0%. Children with TCPC had the highest mean decrease of VO_2max and %predicted VO_2max per year of age (1.23 mL/kg/min, corresponding 3.0%). Although the mean overall physical fitness of children with simple and complex defect after biventricular correction were significantly different from healthy population, they were close to normal (>80%predicted VO_2max). However, TCPC group had much lower exercise capacity.

Appropriate heart rate during exercise in Fontan patients

OBJECTIVE

To evaluate heart rate against workload and oxygen consumption during exercise in Fontan patients.

METHOD

Fontan patients (n = 27) and healthy controls (n = 25) underwent cardiopulmonary exercise testing with linear increase of load. Heart rate and oxygen uptake were measured during tests. Heart rate recovery was recorded for 10 minutes.

RESULTS

Heart rate at midpoint (140 ± 14 versus 153 ± 11, p < 0.001) and at maximal effort (171 ± 14 versus 191 ± 10 beats per minute, p < 0.001) of test was lower for patients than controls. Heart rate recovery was similar between groups. Heart rate in relation to workload was higher for patients than controls both at midpoint and maximal effort. Heart rate in relation to oxygen uptake was similar between groups throughout test. Oxygen pulse, an indirect surrogate measure of stroke volume, was reduced at maximal effort in patients compared to controls (6.6 ± 1.1 versus 7.5 ± 1.4 ml·beat-1·m-2, p < 0.05) and increased significantly less from midpoint to maximal effort for patients than controls (p < 0.05).

CONCLUSIONS

Heart rate is increased in relation to workload in Fontan patients compared with controls. At higher loads, Fontan patients seem to have reduced heart rate and smaller increase in oxygen pulse, which may be explained by inability to further increase stroke volume and cardiac output. Reduced ability to increase or maintain stroke volume at higher heart rates may be an important limiting factor for maximal cardiac output, oxygen uptake, and physical performance.