Cardiorespiratory Responses to Acute Exercise

Cardiovascular Responses to Acute Exercise Increases blood flow to working muscle Involves altered heart function, peripheral circulatory adaptations –Heart rate –Stroke volume –Cardiac output –Blood pressure –Blood flow –Blood

Cardiovascular Responses: Resting Heart Rate (RHR) Normal ranges –Untrained RHR: 60 to 80 beats/min –Trained RHR: as low as 30 to 40 beats/min –Affected by neural tone, temperature, altitude Anticipatory response: HR  above RHR just before start of exercise –Vagal tone  –Norepinephrine, epinephrine 

Cardiovascular Responses: Heart Rate During Exercise Directly proportional to exercise intensity Maximum HR (HR max ): highest HR achieved in all-out effort to volitional fatigue –Highly reproducible –Declines slightly with age –Estimated HR max = 220 – age in years –Better estimated HR max = 208 – (0.7 x age in years)

Accuracy of Predicting Max HR All prediction equations have an SEE The SEE is a measure of the accuracy of the prediction SEE is based on the normal curve –There is a 67% probability that the actual value is within the range of the predicted value ± 1 SEE. –There is a 95% probability that the actual value is within the range of the predicted value ± 2 SEE.

Predicting Maximal HR HRmax = (220-age) SEE = 10 beats/min Age = 24 years HRmax = HRmax = 196 There is a 67% probability that true HRmax is 196 ± 10 or 186 – 206. There is a 95% probability that true HRmax is ± 20 or 176 – 216. This is the 95% Confidence Interval.

Cardiovascular Responses: Heart Rate During Exercise Steady-state HR: point of plateau, optimal HR for meeting circulatory demands at a given submaximal intensity –If intensity , so does steady-state HR –Adjustment to new intensity takes 2 to 3 min Steady-state HR basis for simple exercise tests that estimate aerobic fitness and HR max

Figure 8.1

Cardiovascular Responses: Stroke Volume (SV)  With  intensity up to 40 to 60% VO 2max –Beyond this, SV plateaus to exhaustion –Possible exception: elite endurance athletes SV during maximal exercise ≈ double standing SV But, SV during maximal exercise only slightly higher than supine SV –Supine SV much higher versus standing –Supine EDV > standing EDV

Figure 8.3

Cardiovascular Responses: Factors That Increase Stroke Volume  Preload: end-diastolic ventricular stretch –  Stretch (i.e.,  EDV)   contraction strength –Frank-Starling mechanism  Contractility: inherent ventricle property –  Norepinephrine or epinephrine   contractility –Independent of EDV (  ejection fraction instead)  Afterload: aortic resistance (R)

Cardiovascular Responses: Stroke Volume Changes During Exercise  Preload at lower intensities   SV –  Venous return   EDV   preload –Muscle and respiratory pumps, venous reserves Increase in HR   filling time  slight  in EDV   SV  Contractility at higher intensities   SV  Afterload via vasodilation   SV

Cardiac Output and Stroke Volume: Untrained Versus Trained Versus Elite

Cardiovascular Responses: Cardiac Output (Q) Q = HR x SV  With  intensity, plateaus near VO 2max Normal values –Resting Q ~5 L/min –Untrained Q max ~20 L/min –Trained Q max 40 L/min Q max a function of body size and aerobic fitness

Figure 8.5

Cardiovascular Responses: Fick Principle Calculation of tissue O 2 consumption depends on blood flow, O 2 extraction VO 2 = Q x (a-v)O 2 difference VO 2 = HR x SV x (a-v)O 2 difference

Cardiovascular Responses: Blood Pressure During endurance exercise, mean arterial pressure (MAP) increases –Systolic BP  proportional to exercise intensity –Diastolic BP slight  or slight  (at max exercise) MAP = Q x total peripheral resistance (TPR) –Q , TPR  slightly –Muscle vasodilation versus sympatholysis

Figure 8.7

Cardiovascular Responses: Blood Flow Redistribution  Cardiac output   available blood flow Must redirect  blood flow to areas with greatest metabolic need (exercising muscle) Sympathetic vasoconstriction shunts blood away from less-active regions –Splanchnic circulation (liver, pancreas, GI) –Kidneys

Cardiovascular Responses: Blood Flow Redistribution Local vasodilation permits additional blood flow in exercising muscle –Local VD triggered by metabolic, endothelial products –Sympathetic vasoconstriction in muscle offset by sympatholysis –Local VD > neural VC As temperature rises, skin VD also occurs –  Sympathetic VC,  sympathetic VD –Permits heat loss through skin

Figure 8.8

Cardiovascular Responses: Cardiovascular Drift Associated with  core temperature and dehydration SV drifts  –Skin blood flow  –Plasma volume  (sweating) –Venous return/preload  HR drifts  to compensate (Q maintained)

Figure 8.9

Cardiovascular Responses: Competition for Blood Supply Exercise + other demands for blood flow = competition for limited Q. Examples: –Exercise (muscles) + eating (splanchnic blood flow) –Exercise (muscles) + heat (skin) Multiple demands may  muscle blood flow

Cardiovascular Responses: Blood Oxygen Content (a-v)O 2 difference (mL O 2 /100 mL blood) –Arterial O 2 content – mixed venous O 2 content –Resting: ~6 mL O 2 /100 mL blood –Max exercise: ~16 to 17 mL O 2 /100 mL blood Mixed venous O 2 ≥4 mL O 2 /100 mL blood –Venous O 2 from active muscle ~0 mL –Venous O 2 from inactive tissue > active muscle –Increases mixed venous O 2 content

Figure 8.10

Central Regulation of Cardiovascular Responses What stimulates rapid changes in HR, Q, and blood pressure during exercise? –Precede metabolite buildup in muscle –HR increases within 1 s of onset of exercise Central command –Higher brain centers –Coactivates motor and cardiovascular centers

Central Cardiovascular Control During Exercise

Cardiovascular Responses: Integration of Exercise Response Cardiovascular responses to exercise complex, fast, and finely tuned First priority: maintenance of blood pressure –Blood flow can be maintained only as long as BP remains stable –Prioritized before other needs (exercise, thermoregulatory, etc.)

Figure 8.12

Respiratory Responses: Ventilation During Exercise Immediate  in ventilation –Begins before muscle contractions –Anticipatory response from central command Gradual second phase of  in ventilation –Driven by chemical changes in arterial blood –  CO 2, H + sensed by chemoreceptors –Right atrial stretch receptors

Respiratory Responses: Ventilation During Exercise Ventilation increase proportional to metabolic needs of muscle –At low-exercise intensity, only tidal volume  –At high-exercise intensity, rate also  Ventilation recovery after exercise delayed –Recovery takes several minutes –May be regulated by blood pH, PCO 2, temperature

Figure 8.13

Figure 8.14

Respiratory Responses: Estimating Lactate Threshold Ventilatory threshold as surrogate measure? –Excess lactic acid + sodium bicarbonate –Result: excess sodium lactate, H 2 O, CO 2 –Lactic acid, CO 2 accumulate simultaneously Refined to better estimate lactate threshold –Anaerobic threshold –Monitor both V E /VO 2, V E /VCO 2

Respiratory Responses: Limitations to Performance Ventilation normally not limiting factor –Respiratory muscles account for 10% of VO 2, 15% of Q during heavy exercise –Respiratory muscles very fatigue resistant Airway resistance and gas diffusion normally not limiting factors at sea level Restrictive or obstructive respiratory disorders can be limiting

Respiratory Responses: Limitations to Performance Exception: elite endurance-trained athletes exercising at high intensities –Ventilation may be limiting –Ventilation-perfusion mismatch –Exercise-induced arterial hypoxemia (EIAH)

Figure 8.16