Cardiorespiratory Responses to Acute Exercise. Cardiovascular Responses to Acute Exercise Increases blood flow to working muscle Involves altered heart.

Cardiorespiratory Responses to Acute

Exercise

Cardiovascular ResponsesCardiovascular Responsesto Acute Exerciseto 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:Cardiovascular Responses:Resting Heart Rate (RHR)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:Cardiovascular Responses:Heart Rate During ExerciseHeart Rate During Exercise

• Directly proportional to exercise intensity

• Maximum HR (HRmax): highest HR achieved in all-out effort to volitional fatigue– Highly reproducible– Declines slightly with age

– Estimated HRmax = 220 – age in years

– Better estimated HRmax = 208 – (0.7 x age in years)

Accuracy of Predicting Max HRAccuracy 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 HRPredicting Maximal HR

• HRmax = (220-age)• SEE = 10 beats/min• Age = 24 years• HRmax = 220-24• 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:Cardiovascular Responses:Heart Rate During ExerciseHeart 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 HRmax

Figure 8.1Figure 8.1

Cardiovascular Responses:Cardiovascular Responses:Stroke Volume (SV)Stroke Volume (SV)

• With intensity up to 40 to 60% VO2max – 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


Cardiovascular Responses:Cardiovascular Responses:Factors That Increase Stroke VolumeFactors 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 Cardiovascular Responses: Stroke Volume Changes During ExerciseVolume 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:Cardiac Output and Stroke Volume:Untrained Versus Trained Versus EliteUntrained Versus Trained Versus Elite

Cardiovascular Responses:Cardiovascular Responses:Cardiac Output (Q)Cardiac Output (Q)

• Q = HR x SV

• With intensity, plateaus near VO2max

• Normal values– Resting Q ~5 L/min

– Untrained Qmax ~20 L/min

– Trained Qmax 40 L/min

• Qmax a function of body size and aerobic fitness


Cardiovascular Responses:Cardiovascular Responses:Fick PrincipleFick Principle

• Calculation of tissue O2 consumption depends on blood flow, O2 extraction

• VO2 = Q x (a-v)O2 difference

• VO2 = HR x SV x (a-v)O2 difference

Cardiovascular Responses:Cardiovascular Responses:Blood PressureBlood 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


Cardiovascular Responses:Cardiovascular Responses:Blood Flow RedistributionBlood 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:Cardiovascular Responses:Blood Flow RedistributionBlood 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


Cardiovascular Responses:Cardiovascular Responses:Cardiovascular DriftCardiovascular Drift

• Associated with core temperature and dehydration

• SV drifts – Skin blood flow – Plasma volume (sweating)– Venous return/preload

• HR drifts to compensate (Q maintained)


Cardiovascular Responses:Cardiovascular Responses:Competition for Blood SupplyCompetition 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:Cardiovascular Responses:Blood Oxygen ContentBlood Oxygen Content

• (a-v)O2 difference (mL O2/100 mL blood)

– Arterial O2 content – mixed venous O2 content

– Resting: ~6 mL O2/100 mL blood

– Max exercise: ~16 to 17 mL O2/100 mL blood

• Mixed venous O2 ≥4 mL O2/100 mL blood

– Venous O2 from active muscle ~0 mL

– Venous O2 from inactive tissue > active muscle

– Increases mixed venous O2 content


Central Regulation of Central Regulation of Cardiovascular ResponsesCardiovascular 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 Central Cardiovascular Control During ExerciseDuring Exercise

Cardiovascular Responses:Cardiovascular Responses:Integration of Exercise ResponseIntegration 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.)


Respiratory Responses:Respiratory Responses:Ventilation During ExerciseVentilation 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

– CO2, H+ sensed by chemoreceptors

– Right atrial stretch receptors

Respiratory Responses:Respiratory Responses:Ventilation During ExerciseVentilation 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, PCO2, temperature



Respiratory Responses:Respiratory Responses:Estimating Lactate ThresholdEstimating Lactate Threshold

• Ventilatory threshold as surrogate measure?– Excess lactic acid + sodium bicarbonate

– Result: excess sodium lactate, H2O, CO2

– Lactic acid, CO2 accumulate simultaneously

• Refined to better estimate lactate threshold– Anaerobic threshold

– Monitor both VE/VO2, VE/VCO2

Respiratory Responses:Respiratory Responses:Limitations to PerformanceLimitations to Performance

• Ventilation normally not limiting factor– Respiratory muscles account for 10% of VO2, 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:Respiratory Responses:Limitations to PerformanceLimitations to Performance

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


Cardiorespiratory Responses to Acute Exercise. Cardiovascular Responses to Acute Exercise Increases blood flow to working muscle Involves altered heart.

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