Cardiovascular response to exercise

The Heart

Outline

• General cardiac responses to exercise– Control of heart rate– Control of stroke volume– Blood pressure– Distribution of blood flow– Barroreceptors

• Adaptations to training• Impact of the environment

Examples of work (METS)

• Rest 1• Cycling <10 mph 6• Cycling >20 mph 16• Running (10 min/mile) 10• Running (6 min/mile) 16

METS=metabolic equivalent tasks1 MET=resting energy expenditure= 3.5 mlO2kg-1 min-1= 1 Kcalkg-1 min-1

O2

aO2 content=20 ml O2/100 ml blood

vO2 content=15 ml O2/100 ml blood

CO=5 L/min

Effects of Exercise on Blood Pressure

MABP-RAP = CO TPR

80

120

Pressure(mmHg)

systolic

diastolic

mean

Impact of Dynamic and Isometric Exercise on Arterial Blood Pressure

Time (sec)

0 20 40 60 80 100 120 140

Blo

od

Pre

ssu

re (

mm

Hg

)

0

50

100

150

200

250

SystolicDiastolic

Work (W)

0 50 100 150 200 250

Blo

od

Pre

ss

ure

(m

mH

g)

0

50

100

150

200

250

SystolicDiastolic

Dynamic Static (isometric)

Comparison of BP Response Between

Arm and Leg Ergometry

Why is blood pressure going up? I thought we had sensors that control blood pressure.

Arterial Baroreceptors

Cardiovascular Physiology. Berne and Levy 1972

Activation of Barroreceptor reflex

Response to an increase in arterial pressure• Withdrawal of sympathetic tone• Activation of parasympathetic tone• Results:

– Decrease heart rate and contractility– Arterial vasodilation– Increase in venous compliance

Afferent nerve firing responds to absolute pressure and rate of change in pressure

Baroreceptors adapt

0 100 2000

25

50

75

100

(im

pu

lses

/sec

)

FIR

ING

RA

TE

CA

RO

TID

NE

RV

E

ARTERIAL PRESSURE (mmHg)

LONG TERM

ACUTE

Afferent and efferent neural baroreflex responses

Arterial Baroreflex Control of the Peripheral Vasculature in Humans: Rest and Exercise.FADEL, PAUL Medicine & Science in Sports & Exercise. 40(12):2055-2062, December 2008

Afferent and efferent neural baroreflex responses to the application of neck pressure (NP) and neck suction (NS)

Arterial Baroreflex Control of the Peripheral Vasculature in Humans: Rest and Exercise.FADEL, PAUL Medicine & Science in Sports & Exercise. 40(12):2055-2062, December 2008

3Heart rate control during exercise by baroreceptors and skeletal muscle afferents.OLEARY, DONAL Medicine & Science in Sports & Exercise. 28(2):210-217, February 1996.

Schematic illustration of the effect of exercise on arterial baroreflex control of heart rate. Exercise resets the relationship between arterial pressure and heart rate upward and to the right (OP = hypothetical arterial baroreflex operating point).

Oxygen Demand

O2

aO2 content=20 ml O2/100 ml blood

vO2 content=15 ml O2/100 ml blood

CO=5 L/min

Meeting oxygen needs during exercise

Fick equation VO2 = Q (CaO2 – CvO2)

VO2 = [HR SV] (CaO2 – CvO2)

VO2 = [BP TPR] (CaO2 – CvO2)

Oxygen Extraction (E)

CaO2 – CvO2

CaO2

Fick equation VO2 = Q (CaO2 – CvO2)

VO2 = Q CaO2 x E

E=

Arterial and venous oxygen during exercise

O2Extraction=(19-12)/19=0.33

O2 Extraction=(19.5-2)/19.5=0.90

Heart rate and stroke volume

(% Vo2 max)

20 30 40 50 60 70 80 90 100

Heart rate (bpm)

60

80

100

120

140

160

180

200

(% Vo2 max)

0 20 40 60 80 100

Stroke volume (ml/beat)

60

80

100

120

140

(% Vo2 max)

0 20 40 60 80 100

Cardiac Output(liters/min)

4

6

8

10

12

14

16

18

20

22

Heart rate

Estimate Maximal heart rate=208-0.7 x (age year)

=208-0.7x54=170 bpm

Autonomic Nervous SystemNerves Neurotra

nsmitterDistribution

Effect

SNS CervicalThoracic

Pregang:ACHPostgang:NE

Heart,Arteries &Most veins

1(HR)

12

PNS Vagus and lumbar

Pregang:ACHPost gangACH

HeartVessels ofGenitalia & colon

HR

Heart rate(bpm)

40

50

60

70

80

90

100

110

120

Atropine Atropine

Propranolol

Propranolol

The heart is under net vagal tone

Epinephrine

• Source: Adrenal medulla

• Increase heart rate and contractility (1),

• low concentrations vasodilation (2)

• high concentrations vasoconstriction (1), decrease

venous compliance (1)

Norepinephrine

• Source: Adrenal medulla

• Increase heart rate and contractility (1),

• Limited effect on 2

• At all concentrations vasoconstriction (1), decrease

venous compliance (1)

Determinants of Cardiac Output

• Heart rate• Stroke volume

– Ventricular end-diastolic volume– Contractility– Afterload (aortic pressure)

Venus Blood Return to Heart

• muscle pump• one-way venous valves • breathing

Return of blood to heart

Increase Preload

Increase Afterload

Increase Contractility

Stroke volume

• End diastolic volume– End-diastolic volume (Starling’s Law)

• End systolic volume– Afterload– Contractility

Time between beats

2Heart rate control during exercise by baroreceptors and skeletal muscle afferents.OLEARY, DONAL Medicine & Science in Sports & Exercise. 28(2):210-217, February 1996.

Increase in heart rate decreases filling time

Time b

etwee

n bea

ts

What would happen if you could not increase adrenergic tone to the heart during exercise?

Metaprolol= 1 antagonist

J Appl Physiol 106: 486-493, 2009

Ο average-trained

endurance trained

Training and the CV system

VO2 = Q CaO2 x E

= SV x HR x CaO2 x E

Training

Benefits seen after 3x week for 6 weeks• 70% of max heart rate

> 30 min/bout

• Maintenance 2x week but maintain intensity

College Students World Class Athletes

Control Bedrest Trained

VO2 max (L/min) 3.3 2.4 3.9 5.3

Max Ventilation (L/min)

191 201 197 219

Arterial O2 (mlO2/100 ml blood)

21.9 20.5 20.8 22.4

Art-Ven O2 (mlO2/100 ml blood)

16.2 16.5 17.1 18

Max. cardiac Output (L/min)

20 14.8 22.8 30.4

Max Heart rate (bpm)

192 197 190 182

Stroke Volume (ml)

104 74 120 167

Directly measured cardiac pressure–volume curves for athletes and non-athletic controls Note the marked improvement in both static and dynamic compliance in the endurance

athletes

Levine B D J Physiol 2008;586:25-34

Winder et al JAP 45:370,1978

Heart Adaptations to Training

Endurance trained Sedentary

Resistance trained

1995 Marathon Training Data (females)

VO2 Pre-training Post-training 5 mph 30.7 29.8 6 mph 35.5 34.6

RER 5 mph 0.92 0.88* 6 mph 0.95 0.92*

HR 5 mph 168 151* 6 mph 182 167*

VO2max 54.4 58.5* HRmax 206 198*

*P < 0.05

Adaptations to Training

Resting bradycardia• Increased stroke volume• Increased cardiac size and compliance

Increased blood volume• Lower vascular resistance at any work load• Improved flow distribution• Improved oxygen extraction• Improved heat tolerance

Training Heart Rates

• Low range– Between 60-70% of maximal heart rate– ~50-60% of VO2 max

• High range– 90% of max heart rate– 85-90% of VO2max

Thermal stress and exercise

Duration of exercise ( min)

0 20 40 60 80 100

60

80

100

120

140

160

180

heart rate (BPM)

Stroke Volume (ml/beat)

cardiac output (%)

Impact of duration of exercise

Rowell LR Ann Rev Physiol 54:75,1976

Rowell LR. Ann Rev Physiol 54:75,1976

Rowell LR. Ann Rev Physiol 54:75,1976