Cardiovascular Response to Exercise in Children PDF

CARDIOVASCULAR RESPONSE TO EXERCISE IN CHILDREN PEDATRIC EXERCISE SCIENCE (KIN3382)  LEARNING OUTCOMES Describe the key differences in cardiovascular structure and function between children and adults Explain the acute CV response to exercise in children Discuss the mecha...

CARDIOVASCULAR RESPONSE TO EXERCISE IN CHILDREN PEDATRIC EXERCISE SCIENCE (KIN3382)  LEARNING OUTCOMES Describe the key differences in cardiovascular structure and function between children and adults Explain the acute CV response to exercise in children Discuss the mechanisms of lower SV and higher HR in children Explain cardiovascular drift Evaluate factors affecting CV response QUESTIONS TO CONSIDER How does the CV system respond to exercise in children vs. adults? How do SV and HR change during exercise? What is CV drift? How do peripheral adaptations support oxygen delivery? How do gender and growth influence the response? CARDIOVASCULAR SYSTEM CARDIOVASCULAR CHANGES OVERVIEW FRANK-STARLING MECHANISM FRANK-STARLING MECHANISM → Increased venous return → Increased end-diastolic volume →(EDV) → Greater myocardial stretch → Enhanced contractile force → Higher stroke volume → Stretching optimizes actin- myosin → cross-bridge formation → Improves contraction efficiency CV EXERCISE RESPONSE DOES POSITION MATTER? FROM SUPINE TO UPRIGHT POSITION When an individual stands up from a supine position, blood volume in the legs increases by 500- 1000mL from gravity The impact of this change in central blood volume is a decrease in cardiac output and stroke volume by 20% and 40% respectively  DOES POSITION MATTER? FROM SUPINE TO UPRIGHT POSITION Stroke index higher in supine position Increases at onset of exercise about 30-40% after increasing workload (sitting position) Increases until 50% VO2max then stable Above 50% VO2max no difference between sitting and supine STROKE VOLUME (SV) Men Increases at onset of exercise and then plateaus past 50% of maximal capacity Boys SV  volume of blood pumped out of the LV during each systolic contraction (SV = EDV – ESV)  LEFT VENTRICULAR END-DIASTOLIC AND SYSTOLIC DIMENSION (LVED & LVES) LVED  diameter of the left ventricle at the end of diastole (fully filled with blood) LVSD  diameter of the left ventricle at the end of systole (after blood has been ejected) SV = EDV – ESV EDV – end-diastolic volume ESV – end-systolic volume  LEFT VENTRICULAR END-DIASTOLIC AND SYSTOLIC DIMENSION (LVED & LVES) LVED  Stable during exercise Exception at beginning of exercise with slight increase LVSD Decreases significantly Increased LV shortening fraction LVED – LVSD LVSF = LVED x 100 Average LVSF increase  37% to 47% boys and 36% to 49% in men WHY THE LOWER SV RESPONSE IN KIDS? Adults nearly double SV between resting and maximal exercise Children only increase about 1.1 to 1.35 times (much lower) Smaller body size Left ventricle mass/size Lower blood volume Lower sympathetic adrenal activity (30% lower noradrenaline, 25% lower catecholamines) DETERMINAN TS OF CARDIAC OUTPUT VO2 Q CaO2 – CvO2 = Q HR x SV =  CARDIAC OUTPUT (Q) & EXERCISE RESPONSE Increases by ~3-4 times the resting state Children have lower Q compared to young adults but comparable cardiac index at all workloads Usually accompanied by a higher HR in children Increases linearly until exhaustion Q is ~10% higher in boys compared to girls SUBMAXIMAL EXERCISE & CV RESPONSE Q = HR x SV Remember Fick’s equation and how these measures are related  THE BIG PICTURE  KIDS VS. ADULTS Resting and Submaximal Measures Lower SV: smaller heart, lower blood volume Higher HR: almost compensates for low SV Slightly lower cardiac output (Q) than adults A-VO2 difference increases for further compensation OVERVIEW  CV RESPONSE TO ACUTE PROGRESSIVE EXERCISE  Stroke volume rises in the initial phase of upright exercise but then remains stable as intensity increases  Pattern occurs while heart rate and cardiac output increase linearly with work rate  Left ventricular end-diastolic dimension remains stable or gradually declines as intensity increases  Contractile force of the heart becomes accentuated as work rate increases  The onset of exercise is accompanied by a dramatic fall in peripheral vascular resistance, which continues to decline in a curvilinear fashion as intensity increases PERIPHERAL FACTORS  PERIPHERAL VASCULAR RESISTANCE Decrease rapidly at the onset Decreases in a curvilinear of exercise fashion with increase in exercise intensity No difference between standing and sitting positions As intensity increases peripheral vascular resistance decreases 50% decrease in slightly peripheral Increased vasodilation  result resistance of chemical factors like nitric oxide and arteriolar dilations SKELETAL MUSCLE PUMPS First Mechanism Muscle contraction compresses intramuscular veins, providing kinetic energy to blood and propelling it centrally Second Mechanism Muscle contraction reduces intramuscular veinous pressure to low or even negative values causing an increased inflow gradient during the subsequent contraction (suction effect) BIG PICTURE  PERIPHERAL FACTORS Mostly applicable to acute progressive exercise response In order to maintain pre-load and SV, children have reduced peripheral resistance Children have lower peripheral resistance compared to adults Reduced peripheral resistances occurs through Vasodilation Increased skeletal muscle pump efficiency MECHANISMS OF CARDIOVASCULAR DRIFT CARDIOVASCULAR DRIFT Happens at constant load between 50-75% VO2peak (usually ~60%) Heart Rate: 15% increase Stroke Volume:15% decrease Dehydration, increase cutaneous blood flow, sympathetic nervous system (catecholamine), increase temperature RECOVERY CARDIAC DYNAMICS RECOVERY CARDIAC DYNAMIC First  Rate of Heart rate recovery is directly related to the fitness level.  People with higher fitness show a faster decrease in heart rate following exercise  Believed to be dues to a greater parasympathetic tone Second  Progressively slower rate of heart rate recovery is seen as children growth  This observation is independent of there aerobic fitness level or maximal heart rate  It is supposed to be faster in boys then in girl RECOVERY CARDIAC DYNAMIC 150 Heart rate 1 minutes post 145 140 exercise 135 130 125 1.12 Body surface area (m2) CHILDREN VS. ADULTS RECOVERY CARDIAC DYNAMIC Children vs. Adults: 10 children (7-11 yrs) and 12 adults (26-42 yrs) 3 intensities of exercise 80% of aerobic threshold(AT), 50% of the diff maximum O2 and 80% of AT, and 100 and 125% of maximal O2 uptake Results: Work recovery increase with intensity of exercise Markedly faster recovery in children compared to adults Decrease in heart rate is faster in children than in adults Dues to a greater parasympathetic modulation of the heart rate  FACTORS AFFECTING RESPONSE TO EXERCISE  SEX DIFFERENCE IN HEMODYNAMIC RESPONSE Adults (women compared to men) Higher HR Higher arterial venous difference Lower stroke volume Lower cardiac output Children (minor sex/gender effect) HR is higher in girls SV lower in girls No difference in Q or A-VO2 difference SEX DIFFERENCE IN HEMODYNAMIC RESPONSE Higher in boys = larger LV Higher because age & developmental stages O2 uptake, L.Min-1  KINE 3382 REMINDERS Guest Lecture  next two Thursdays (Feb 6th & 13th) Quiz #2  February 11th (next week) Midterm  February 27th  AGE AND SEX EFFECTS Age sub-max HR is always higher in kids vs. adults but decreases as they get older Sex/Gender girls have higher HR than boys for any given workload CLIMATE Climatic Stress: increase of 5° to 7 °C or 15% to 30% humidity increase compared to thermoneutral environment (23°-24°c, 50-60% humidity) results into a 10bpm increase For each 1°C above 22°C the HR increase is about 1.05bpm May lead to overestimation of energy expenditure by as much as ~10% ARTERIAL BLOOD PRESSURE SBP Independent of age, systolic blood pressure increase in a dose response manner with exercise intensity Independent of age, diastolic blood pressure remains stable or slightly DB decreases with exercise P For a given workload, smaller and younger children respond with lower increases in SBP PHARMACOLOGICAL Pharmacological agents that affect the autonomic nervous system or metabolic rate can affect HR response to exercise Propranolol: ↓ Methylphenidate: ↑ HR HR ↑ 10 bpm CENTRAL & PERIPHERAL HEMODYNAMIC RESPONSE TO EXERCISE Function Children’s response (Compared with adults Heart rate (sub-max) Higher especially in first decade Heart rate (max) Higher Stroke volume (sub-max & max) Lower Cardiac output (sub-max) Lower Blood flow to active muscle Higher Systolic/Diastolic blood pressure Lower (sub-max & max) Peripheral resistance Lower FACTORS AFFECTING HEART RATE RESPONSE TO EXERCISE Factors Submaximal HR Maximal HR Age Young > old No effect sex Female > males No effect Adiposity Obese > lean No effect Climatic stress  No effect Active muscle Small > large Large > small mass Training  No effect or slight  Fever  No effect Chronic fatigue   Muscle atrophy  No effect  THE BIG PICTURE  KIDS VS. ADULTS Resting and Submaximal Measures Lower SV: smaller heart, lower blood volume Higher HR: almost compensates for low SV Slightly lower cardiac output (Q) than adults A-VO2 difference increases for further compensation