Exercise physiology: Cardiac

Cardiac Output

• Cardiac Output = Heart rate x stroke volume

• Heart rate on a average: 70 beats per min

• Stroke volume: 70-80ml (end diastolic volume - end systolic volume)

• Cardiac output at rest: 5L/min

• HR does not drive cardiac output. (Athletes have a lower resting and maximal HR).

End Diastolic Volume

Effected by:

• Filling time: HR

• Filling amount: Venous return, cardiac out and blood volume of the peripheral blood.

End Systolic Volume

Effected by:

1. Ventricular preload

2. Ventricular contractility

3. Ventricular afterload

Determinants of SV: Ventricular preload.

• filling volume, or the degree of stretching of the ventricle

• Preload is directly proportional to the EDV, the greater the EDV the greater the stretching of the ventricle.

Cardiac Output during exercise

• Resting Q: 5-6 L/min

• Sedentary Gmax: 20-22L/min

• Athlete Qmax: 35-40L/min

Cardiovascular Control - Nerves

• HR and oxygen uptake are linearly related

• Increase in sympathetic nerve stimulation, accelerates SA node and increases HR = tachycardia (noradrenaline)

• Parasympathetic (vagus) fibres, decreases heart rate via vagus nerves = bradycardia (acetylcholine)

Cardiovascular Control Centres

• Medulla Oblongata of the brain stem contains the control centre for the cardiovascular

• Afferent signals from baroreceptors

• Sends corrective signals via the nervous system and vi hormone release.

HR During Exercise

1. HR response is rapid due to anticipatory increase by central command

2. Parasympathetic tone decrease, Sympathetic output increae

3. As exercise continues, HR is increasingly controlled by catecholamine release.

Stroke volume during exercise

Rest:

• Untrained:70ml

• Trained: 100ml

Max Exercise:

• Untrained: 110ml

• Trained: 180ml

Increasing SV during exercise

1. Increased Venous return: increase ventricular filling during diastole - Frank Starling’s Law

2. Normal ventricular filling followed by powerful contractions

3. Training adaptions - increased load volume

Ventricular Cntractility

• Positive ion tropic effect: increases contractility

• Negative ion tropic effect: decreases contractility

Effected by

• ANS activity

• Hormones

• Changes in ion concentrations

Response to Prolonged Exercise - Cardiovascular drift

• SV gradually decreases

• HR gradually increase

Why:

• Blood flow redirected to skin - sweat

• Therefore plasma volume decreases (due to movement into interstitial fluid).

Capillaries

• 0.01mm in diameter

• 5% of total blood volume

• Around each capillary is a pre-capillary sphincter. A ring of smooth muscle that controls diameter

Blood pressure

• Normal SBP: 110-120mmHg

• Normal DBP: 70-80mmHg

Hypertension: SBP > 140, DBP > 90

Hypotension: SBP < 100, DBP < 60

BP = Q x Total peripheral resistance

Hypertension and exercise training

• Regular aerobic exercise reduces SBP and DBP: 6-10mmHg

• Exercise as (preventative) medicine:

• - reduced SNS ( decreased resistance)

• - Increased renal function (removal of sodium = decreased fluid) (less fluid = less pressure)

BP during Initial stage of exercise

• increased SBP during first few mins of steady-state exercise

• - SBP generally levels off: 140-160mmHg

• - DBP remains relatively unchanged

BP during maximal graded exercise

• Linear increase in SBP proportional to workload

• SBP can > 200mmHg of higher during max exercise = most likely sue to a large cardiac output (increased venous return = increased cardiac output = increased blood flow)

• DBP remains relatively stable during exercise

BP during steady state exercise

• rhythmic muscular activity = vasodilation to active muscles = decreased total peripheral resistance = increased blood flow

• Venous valves: one way flow

• Muscle pump: increased pressure

Rate Pessure Product

• Indirect index of myocardial oxygen consumption (VO2 of the myocardium

• RPP = SBP x Heart Rate

• Healthy = 20000mmHg per min or higher

• Insufficient = 16000mmHg per min or lower.

Thermoregulation - Body temperature

• Range of 36.1 - 37.8ºC (typically 37ºC)

• At 41ºC Brain Death Begins - Alberto Salazar 1978

Gaining Heat

1. Metabolism - can increase 20 times above resting levels

2. Environment - Solar radiation from objects that are warmer than the body

Heat Transfer - Conduction

Rate of conductive heat loss depends on

1. The temp gradient between skin and surrounding surfaces

2. Thermal qualities of the surrounding surfaces (water absorbed heat faster than air therefore heat is lost 25 times faster)

Heat Transfer - Convection

• Transfer of heat by the motion of a gas or liquid across a heat surface

• Conduction and convection accrounts for 10-20% heat lost to surrounding air.

Heat Transfer - Radiation

• TRansfer of heat in the form of infrared rays

• Primary heat loss mechanism at room temperature

Heat Transfer - Evaporation

• Heat loss during the “phase-shift” of a liquid to a gas

• Primary source of heat loss when exercising

Heat lost at rest

• 60% through radiation

• 40% through, conduction, convection and evaporation

Heat loss at Exercise

During intense exercise:

• Most cooling via evaporative sweat loss (80%)

• 1L of sweat evaporation = 580kcal heat loss

Evaporation - Rate of Heat loss

Effect by:

1. Surface area exposed to air

2. Temperature of ambient air

3. Convective air movement around the body

4. Relative humidity of air

Relative Humidity

• The amount of water in them being air compared tot the total quanta of moister it can hold

• Expressways a percentage

• When humidity is high the ability for evaporation to occur diminishes.

Exercise in Hot and/or humid environments

• 15-25% of cardiac output passes through the skin during heat stress.

• Sweating increases to cool the skin and blood = plasma loss = lower SV = Higher HR to maintain cardiac output.

• Max sweating rate =. 3L/hr

Fluid replacement

• Pre-exercise: 400-600ml to H2O 60min before

• During exercise: 150ml per 15-20min

• Post-exercise: 1.5 x sweat loss (change in wieght)

Blood Flow during exercise

• During Exercise, blood is redirected to the areas where is is needed. And restricted in areas it is not needed.

• During heavy exercise, muscles receive 80-85%

Regulation of vessel diameter

• resistance depends on radius of thee blood vessel

• Double radius = increased volume + flow

• Halving diameter = increased resistance x 16

Effect of Exercise - Blood Flow

1. Autoregulation - local metabolites act on arterial wall

2. Extrinsic neural control - stiffin veins

Blood Flow exercise effect - Autoregulation

• Response in changes in the local chemical and gas environment

vasodialtion occurs when:

• Decrease in PO2

• Increase in PCO2

• In crease in temperature

• Decrease in pH

• Increase in nitric acid

• Increases in adenosine, ATO, K+

Blood flow Exercise effect - Exrinsic Neural Control

• SNS stimulation = release of hormones (epinephrine and norepinephrine) that cause a generalised vasoconstriction

• Secondary response to autoregulation

• In areas that need extra blood SNS stimulation decreases

Functional Sympatholysis

• SNS activity Lo increases to skeletal muscle during exercise by the action is inhibited by the local metabolites to blunt the contraction.

Venous Valaves

• One way flow

• Break up the continuous colum into smaller sections

Muscle pump

• generates: 90mmHg pressure

Countercurrent Flow Mechanism

• As Pulsatile artery receives blood it expands and pushes on veins, allowing blood to be pushes in one direction as the venous valves don’t allow blood flowing back

Baroreceptors

• Pressure-sensitive sensors located in aortic and carotid bodies

• Send Afferent signals to the medulla

Baroreceptor Feedback

• Pressure = (HR x SV) x TPR

• PNS inhibition: HR, SV decreases

• SNS activiation: increase HR and SV, thereby Q as well as increases TPR

Chemoreceptors

• Monitor changes in chemicals in the blood: PO2, PCO2 and H+

• Located in aortic arch and carotid sinus, ventrolateral medulla

Lab - Aerobic Power Index

• THR - 220-age x 0.75

1. Calculate the difference between HR at the nd of the last workload and the second last workload

2. Calculate the difference between THR and the HR recorded at the second last workload

3. Apply these two figures (second step/ first step)

4. Times step 3 fraction by 25 watts and add to the watts of the second last workload.

5. Divide total watts by participants body-mass to find watts/kg

Lab - Submaximal exercise test

VO2 max:

• RER > 1.15

• Blood lactate of > 8mM

• Maximal HR reached

• Increased VO2 of < 150ml between the averages of the last two workload (last min).

Blood Lactate during test

• Sharp increase in blood lactate measurements

Lab - ECG

12 lead ECG Placement;

• RL (right leg): just above the right iliaccrest on midclavicularline

• LL (left leg): just above the left iliac crest on midclavicularline

• RA (right arm); just below right clavicle medial to deltoid muscle

• LA (left arm): just below left clavicle medial to deltoid muscle

• V1: on right eternal border in 4th intercostals space

• V2: on left eternal border in 4th intercostals space

• V3: at midpoint of a straight line between V2 and V4

• V4: on mid-clavicular line in 5th intercostal space

• V5: on anterior axillary line, 5th intercostal space

• V6: on midaxillary line, 5th intercostal space