Cardiac Output

Question 1

What is cardiac output and how is it calculated?

Answer 1

Cardiac output is the volume of blood pumped by the ventricle per minute
Cardiac output = Stroke volume x Heart rate

Question 2

What is the average value of cardiac output?

Answer 2

5-5.5l

Question 3

How does exercise affect cardiac output?

Answer 3

Increases the cardiac output up to 25L/min

Question 4

How are minute volume and cardiac output related?

Answer 4

Minute volume = cardiac output at rest

Question 5

What is cardiac index?

Answer 5

Volume of blood pumped by each ventricle per meter square body surface area per minute (3.2 L/m2/min)

Question 6

What is venous return?

Answer 6

Venous return is the volume of blood returning to the right side of the heart in a minute
Equal to CO

Question 7

What is ejection fraction?

Answer 7

Percentage ratio of stroke volume to the end diastolic volume

Question 8

How is ejection fraction calculated?

Answer 8

Ejection Fraction (%) = (Stroke Volume/End DIastolic Volume) x 100

Question 9

Range of regular EF% values?

Answer 9

>60%

Question 10

What is the significance of EF%?

Answer 10

Indicator of myocardial contractility and it decreases in heart failure

Question 11

How can cardiac output be measured?

Answer 11

Echocardiography
Fick’s principle
Dye dilution method

Question 12

What is Fick’s principle?

Answer 12

Cardiac Output = (O2 consumption)/(Arterial PO2-Venous PO2)

Question 13

What are the types of variations in cardiac output?

Answer 13

Physiological variations
Pathological variations

Question 14

What physiological factors may influence cardiac output?

Answer 14

Posture: Cardiac output decreases in standing position
Digestion: Cardiac output decreases
Temperature: Increases in high temperature and very low temperature (shivering)
Excitation: Increases
Exercise: Increases
Pregnancy: Increases

Question 15

What pathological factors may increase cardiac output?

Answer 15

Anemias
Hyperthyroidism and fever
Aortic regurge

Question 16

What physiological factors decrease cardiac output?

Answer 16

Paroxysmal tachycardia
Complete heart block
Myocardial ischemia and infarction
Shock
Mitral stenosis or regurge & aortic stenosis
Heart failure

Question 17

Why is aortic regurge the only valve disease which increases cardiac output?

Answer 17

Aortic regurge increases stroke volume and causes blood to flow back to the ventricles from the aorta

Question 18

What are the factors affecting cardiac output

Answer 18

Preload
Afterload
Heart rate
Ventricular contraction

Question 19

How does venous return/preload affect cardiac output?

Answer 19

Preload determines the degree of stretch of the myocardium
Increased venous return increases the stroke volume (by Starling Law) and Heart Rate (by Bainbridge reflex)

Question 20

What are the factors affecting venous return?

Answer 20

Pumping action of the heart: Increase VR
Right atrial pressure (RAP): Decrease VR
Respiratory movement: Increase VR
Gravity: Decreases VR
Diameter of arterioles and veins and opening of capillary: Increases venous return
Skeletal muscle contraction: Increases venous return
Blood volume: Increases venous return

Question 21

How does arterial blood pressure/afterload affect the cardiac output?

Answer 21

Afterload affects the aortic pressure, arterial wall rigidity and blood viscosity (i.e. Arterial blood pressure/ABP )
Sudden rise in ABP
First systolic beat: Decreased stroke volume
Second systolic beat: EDV is increased → increase contraction (Starling law)

Question 22

How does heart rate between 50 and 200 bpm with constant venous return affect cardiac output?

Answer 22

No cardiac output effect

Question 23

How does heart rate below 50bpm with constant venous return affect cardiac output?

Answer 23

Complete heart block → marked decrease in heart rate

Question 24

How does heart rate above 200bpm with constant venous return affect cardiac output?

Answer 24

Similar to situation in paroxysmal tachycardia: shortening of the diastole → decrease filling → marked decrease in systolic volume

Question 25

How does increased heart rate with increased venous return affect cardiac output?

Answer 25

Increased cardiac output

Occurs during exercise

Question 26

How does the strength of ventricular contraction affect systolic volume?

Answer 26

Increased strength of ventricular contraction incraeses the systolic volume

Question 27

How does sympathetic stimulation affect systolic volume?

Answer 27

Increases sytolic volume

Question 28

How does cardiomyopathy affect systolic volume?

Answer 28

Decreases systolic volume

Question 29

Positive and negative inotropic agents effect on heart

Answer 29

Positive inotropic agents = increased contractility
Negative inotropic agents = decraesed contractility

Question 30

What is luisitropy?

Answer 30

Rate at which the heart relaxes

Question 31

Positive and negative luisitropic agents effect on heart

Answer 31

Positive luisitropic agents = increase rate of relaxation
Negative luisitropic agents = decreased rate of relaxation

Question 32

What further factors impact the cardiac output?

Answer 32

Instrinsic regulation (autoregulation)
Extrinsic regulation

Question 33

What are the types of intrinsic regulators?

Answer 33

Heterometric (preload)
Homometric (afterload)

Question 34

What are the types of extrinsic regulators?

Answer 34

Nervous
Chemical

Question 35

Which nervous impulses can affect cardiac output?

Answer 35

Sympathetic increases cardiac output
Parasympathetic decrease cardiac output

Question 36

Which chemical factors can affect cardiac output?

Answer 36

Hormones: Catecholamines increase cardiac output
Drugs: Thyroxine and digitalis increase cardiac output
Hypoxia and ischemia decrease cardiac output