Cardiac output during exercise in pubertal boys

Sex differences in fitness and cardiac function during exercise in adolescents with chronic fatigue

Females demonstrate less robust Frank-Starling mechanism with respect to cardiac preload than males at rest. We asked whether this phenomenon would also affect cardiac performance during exercise. We hypothesized that stroke volume (SV) response to exercise would be more limited in deconditioned females such that cardiac output would be mainly rate dependent, compared with males. We conducted a chart audit of clinical exercise tests performed by adolescents with chronic fatigue. Oxygen uptake (V˙O2) was measured breath-by-breath at rest and during cycle ergometry, while cardiac output was measured by acetylene rebreathing at rest plus 2-3 subthreshold workloads. SV response was analyzed in two ways: after normalization for body surface area (SV index, SVI) and as percentage change from resting values. Among 304 adolescents (78% females) with chronic fatigue, 189 (80%) of 236 females and 52 (76%) of 68 males were deconditioned (peakV˙O₂ <90% predicted). Heart rate trajectory during exercise was steeper for unfit than fit females, 70 vs 61 beat·min^-1 per L·min^-1 V˙O2, (P=.003); but not for males, 47 vs 42 beat·min^-1 per L·min^-1 V˙O2 (P=.23). The highest measured SVI did not differ between unfit vs fit females (42.8 vs 41.5 mL·m^-2 , P=.39) while fit males showed larger SV during exercise than their unfit peers (highest SVI 55.9 vs 48.0 mL·m^-2 , P=.014). Both qualitative and quantitative sex differences exist in SV responses to exercise among chronically fatigued adolescents, suggesting volume loading may be more efficacious in girls.

Resting and exercise cardiorespiratory function in survivors of congenital diaphragmatic hernia

Our objective was to study exercise capacity and cardiorespiratory response to exertion in survivors of congenital diaphragmatic hernia (CDH). This was a cross-sectional cohort study of 23 CDH survivors, aged 10-16 years, and 23 gender- and age-matched controls. Exercise testing was performed on a cycle ergometer, with cardiac output measurements made using exponential CO2 rebreathing. Pretest cardiorespiratory assessment was done by echocardiography and pulmonary function testing. Statistical analysis was performed using Student's t-test, regression analysis, and longitudinal model computing with spatial covariance structure. No echocardiographic evidence for pulmonary hypertension was found at rest (right ventricular systolic pressures, 27 +/- 6 mmHg). Mean pulmonary artery diameter on the side of the CDH was significantly smaller than contralaterally, but was within normal range (z-score, 0 +/- 1.1 vs. 1.2 +/- 1.6, P < 0.01). Exercise capacity was mildly reduced in CDH compared to controls and predictive data (maximum workload, 77% +/- 12% vs. 91% +/- 16% of predicted, P < 0.01). Cardiorespiratory response to exertion was not significantly different between groups. In conclusion, most adolescent CDH survivors have nearly normal exercise capacity and cardiorespiratory response to exertion. This study may prove useful in comparisons with future cohorts comprising more severely affected individuals now surviving due to improved neonatal care.

End-tidal estimates of arterial PCO2 for cardiac output measurement by CO2 rebreathing: a study in patients with cystic fibrosis and healthy controls

We set out to determine the effects of various estimates of arterial PCO2 (PaCO2) on calculation of cardiac output (Q) by the indirect Fick (CO2) method in healthy children and children with cystic fibrosis (CF), and to develop a prediction equation for children for PaCO2, based on end-tidal PCO2 (PetCO2). The study had 3 parts: 1) Twenty-three healthy children exercised lightly and moderately while arterialized capillary blood gases and PetCO2 were measured simultaneously so that a prediction equation for PaCO2 could be derived from PetCO2. Cardiac output was measured by CO2 rebreathing at each workload; different values for PaCO2 (measured in arterialized capillary blood, end-tidal, and PaCO2 derived from the Bohr equation assuming normal dead space) were used to calculate Q; 2) our equation PaCO2 = 0.647 PetCO2 + 12.4 was tested prospectively to measure Q in 9 healthy children; and 3) cardiac output based on arterialized capillary PaCO2 was compared with that based on Jones-corrected PetCO2 during light and moderate exercise in 16 CF patients whose forced expiratory volume in 1 second (FEV1), range from normal to 37% predicted. Our results have shown that in healthy children end-tidal based-estimates of PaCO2 tended to overestimate Q, whereas PaCO2 values derived by the Bohr equation and assuming normal dead space tended to underestimate Q, compared with Q calculated from directly measured PaCO2. Our prediction equation resulted in good agreement compared with directly measured PaCO2 when used to calculate Q (mean difference, +1.3%; range, +9% to -13%). CF patients with little or no airway obstruction had results similar to healthy controls, but those with severe airway obstruction had lower values for Q when PetCO2 was used instead of directly measured PaCO2. We conclude that estimates of PaCO2 from PetCO2 are not reliable in patients with moderately severe pulmonary disease due to CF. In healthy children, the prediction equation for PaCO2 from PetCO2 derived in the present study gives results superior to other bloodless methods currently in use for computation of Q by the indirect Fick (CO2) method.

Responses of normal children and young adults to controlled bicycle exercise

Exercise responses were obtained from 149 children and young adults (average age 14.5 years) and divided by sex and body surface area (BSA): children with BSA less than 1 m2; children with BSA 1--1.19 m2; males with BSA greater than or equal to 1.2 m2; and females with BSA greater than or equal to 1.2 m2. Total work, mean and maximal power outputs were more affected by body size (height) than age in children with BSA less than 1 m2 and in males and females with BSA greater than or equal to 1.2 m2. Mean systolic pressure increased up to 64% above the preexercise supine value at peak effort, with the level of mean maximal systolic pressure having a positive relationship with body size (height), power output and preexercise sitting systolic pressure in all subgroups except children with BSA 1--1.19 m2. Mean diastolic pressure increased up to 24% above the preexercise supine value at peak effort. ST-segment depression of 1--2 mm was recorded in 12.1% (18 of 149) of the population at peak exercise. These changes occurred in 8.9% of all males and in 16.9% of all females (p greater than 0.1). The data from this study reveal the importance of sex and body size in the clinical interpretation of exercise responses in growing subjects, provide a reference for objective evaluation of subjects with or without cardiac abnormalities and provide a guide for careful monitoring of subjects during an exercise study.

Oxygen transport system during exhaustive exercise in Japanese boys

The study was designed to investigate the age-related increase in maximal oxygen uptake with special reference to the other physiological parameters (ventilation, pulmonary diffusing capacity, cardiac output etc.). 77 normal boys aged 10-18 years were tested by the progressive loading method on a bicycle ergometer. VO2 max increased by approximately 55% with the age from 10-18 years. The increase in VO2 max was accompanied with increase in ventilation (49%), pulmonary diffusing capacity (45%) and cardiac output (5%).