Cardiorespiratory Responses to Acute Exercise. CHAPTER 8 Overview Cardiovascular responses to acute...
-
Upload
tracey-walton -
Category
Documents
-
view
268 -
download
4
Transcript of Cardiorespiratory Responses to Acute Exercise. CHAPTER 8 Overview Cardiovascular responses to acute...
CHAPTER 8 CHAPTER 8 OverviewOverview
• Cardiovascular responses to acute exercise– Cardiac responses– Vascular responses– Integration of exercise responses
• Respiratory responses to acute exercise– Ventilation (normal exercise, irregularities)– Ventilation and energy metabolism– Respiratory limitations– Respiratory regulation of acid-base balance
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)
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
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 PressureBlood Pressure
• Rate-pressure product = HR x SBP– Related to myocardial oxygen uptake and
myocardial blood flow
• Resistance exercise periodic large increases in MAP– Up to 480/350 mmHg– More common when using Valsalva maneuver
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
Cardiovascular Responses:Cardiovascular Responses:Plasma VolumePlasma Volume
• Capillary fluid movement into and out of tissue– Hydrostatic pressure– Oncotic, osmotic pressures
• Upright exercise plasma volume– Compromises exercise performance
– MAP capillary hydrostatic pressure– Metabolite buildup tissue osmotic pressure– Sweating further plasma volume
Cardiovascular Responses:Cardiovascular Responses:HemoconcentrationHemoconcentration
• Plasma volume hemoconcentration– Fluid percent of blood , cell percent of blood – Hematocrit increases up to 50% or beyond
• Net effects– Red blood cell concentration – Hemoglobin concentration – O2-carrying capacity
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
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:Breathing IrregularitiesBreathing Irregularities
• Dyspnea (shortness of breath)– Common with poor aerobic fitness
– Caused by inability to adjust to high blood PCO2, H+
– Also, fatigue in respiratory muscles despite drive to ventilation
• Hyperventilation (excessive ventilation)– Anticipation or anxiety about exercise
– PCO2 gradient between blood, alveoli
– Blood PCO2 blood pH drive to breathe
Respiratory Responses:Respiratory Responses:Breathing IrregularitiesBreathing Irregularities
• Valsalva maneuver: potentially dangerous but accompanies certain types of exercise– Close glottis
– Intra-abdominal P (bearing down)
– Intrathoracic P (contracting breathing muscles)
• High pressures collapse great veins venous return Q arterial blood pressure
Respiratory Responses:Respiratory Responses:Ventilation and Energy MetabolismVentilation and Energy Metabolism
• Ventilation matches metabolic rate
• Ventilatory equivalent for O2
– VE/VO2 (L air breathed/L O2 consumed/min)
– Index of how well control of breathing matched to body’s demand for oxygen
• Ventilatory threshold– Point where L air breathed > L O2 consumed
– Associated with lactate threshold and PCO2
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)
Respiratory Responses:Respiratory Responses:Acid-Base BalanceAcid-Base Balance
• Metabolic processes produce H+ pH
• H+ + buffer H-buffer
• At rest, body slightly alkaline – 7.1 to 7.4– Higher pH = Alkalosis
• During exercise, body slightly acidic– 6.6 to 6.9– Lower pH = Acidosis
Respiratory Responses:Respiratory Responses:Acid-Base BalanceAcid-Base Balance
• Physiological mechanisms to control pH– Chemical buffers: bicarbonate, phosphates,
proteins, hemoglobin
– Ventilation helps H+ bind to bicarbonate– Kidneys remove H+ from buffers, excrete H+
• Active recovery facilitates pH recovery– Passive recovery: 60 to 120 min– Active recovery: 30 to 60 min
Respiratory Responses:Respiratory Responses:Air PollutionAir Pollution
• Carbon monoxide (CO)– Derived from burning fuel, tobacco smoke
– Hemoglobin’s affinity for CO much greater than for O2 VO2
• Ozone (O3)– Eye irritation, tight chest, dyspnea, cough, nausea
– Transfer of O2 at lung alveolar PO2
• Sulfur oxide (SO2)– Upper airway and bronchial irritant
– Aerobic exercise performance