5 - Measurement of Cardiac Output

Question 1

What is cardiac output?

Answer 1

The cardiac output (in volume blood per unit time, L/min) is the blood flow required to match the metabolic demands of the tissue, especially for oxygen delivery and is equal to the stroke volume multiplied by the heart rate

Question 2

How can you determine cardiac output?

Answer 2

• the sum total of the blood flows to individual tissues OR…
• venous return

Question 3

What is the equation for cardiac output?

Answer 3

CO = SV x HR

CO = cardiac output
SV = stroke volume
HR = heart rate

Question 4

What factors affect cardiac output?

Answer 4

Only factors which influence stroke volume or heart rate

Question 5

What factors influence heart rate?

Answer 5

• Parasympathetic and sympathetic stimulation
• Alterations in rate of diastolic depolarization
• Alterations in the slope of phase 4 in the SA nodal cell action potential

Question 6

What does sympathetic stimulation do to the heart rate?

Answer 6

Increase conduction velocity through the AV ode by increasing the slope of phase 4

Faster heart rate

Question 7

What does parasympathetic stimulation do to the heart rate?

Answer 7

Decrease conduction velocity through the AV by decreasing the slope of phase 4

Slower heart rate

Question 8

What factors regulate stroke volume?

Answer 8

• Inotropic state of myocytes
• Contraction and relaxation of myocytes
• Dimensions of the ventricles
• Structure of the ventricles

Question 9

How do we determine stroke volume?

Answer 9

We look at preload and afterload

Question 10

What is preload?

Answer 10

Diastolic load, that is, the load on the heart at the end of filling

The filling of the ventricle causes stretch, increasing the length of the sarcomere (contractile unit)

Question 11

What is afterload?

Answer 11

Systolic load

The amount of work it takes to move the contents of the ventricles into the aorta and beyond

The afterload is related to the contractile EFFICIENCY… That is the force generated and the rate at which it is generated to deal with the preload

Question 12

How would you theoretically determine the preload?

Answer 12

If we measure a single muscle fiber and add a weight, the muscle fiber will stretch corresponding to a particular tension… This is our preload

Question 13

What does the Frank-Starling Law of the Heart describe?

Answer 13

The Frank-Starling Law of the Heart denotes that the work done by the heart is a function of the length of the muscle fiber.

The amount of work done by the heart is directly proportional to end diastolic volume

Force of ventricular contraction is a function of ventricular end-diastolic muscle length - Related to Ca++ sensitivity, within limits

Question 14

Why would you measure the end diastolic volume?

Answer 14

End diastolic volume = the amount of blood back in the heart after filling

The more volume that returns to the heart, the more that can be expelled

If there is more end diastolic volume, the heart must generate a higher stroke volume to accommodate this larger volume

Question 15

Describe the relationship between end diastolic volume and stroke volume

Answer 15

Fairly linear… If the end diastolic volume goes up, the stroke volume goes up

This is the case until the end diastolic volume gets very large and the heart cannot keep up

Question 16

How does this linear relationship work? How does the heart know to increase stroke volume with increased end diastolic volume?

Answer 16

Changes in the end diastolic volume generate changes in the diastolic pressure because of the increase in tension

The increased tension is felt by the elastic components of the contractile apparatus, especially titin

Question 17

What is meant by venous return?

Answer 17

The RATE of volume returning to the heart

Question 18

What is the speed at which the blood returns to the heart dependent upon?

Answer 18

The pressure within the peripheral venous compartment and the pressure within the central venous compartement (superior and inferior vena cava and right atrium)

The difference in these pressures creates a “driving force” for the return

Question 19

What is the effect of increased pressure within the central venous compartment?

Answer 19

If central venous pressure (or right atrial pressure) increases only a few mmHg, venous return will greatly diminish (the driving force will decrease)

Question 20

When would the venous return cease?

Answer 20

When the central venous pressure equals the peripheral venous pressure

This point is called the MSFP - mean systolic filling pressure

Question 21

What factors influence the MSFP (mean systolic filling pressure)?

Answer 21

Blood VOLUME

Question 22

What influences venous return?

Answer 22

• Volume of blood

- Arteriolar tone

Question 23

How does blood volume affect venous return?

Answer 23

more blood will require a larger rate of return than a smaller
Measurement of Cardiac Output Page 5
volume

Question 24

How does arteriolar tone affect venous return?

Answer 24

increased arteriolar tone- or more vasoconstriction- will “hold onto” the volume and slow the rate of return; conversely, arteriolar dilation will allow more blood to flow into the venous circulation faster, thus increasing the rate it will come back to the heart

Question 25

Does arteriolar tone alone change the MSFP or mean systolic filling pressure?

Answer 25

NO Arteriolar tone usually does not change alone, meaning that it would usually change along with venous tone

Question 26

Does venous tone affect the amount of blood returning to the heart?

Answer 26

Yes because the venous circulation serves a “storage” function.

Question 27

Describe the generation of tension in the heart muscle

Answer 27

- The preload stretches the muscle fiber (left ventricle) - Muscle contracts and generates tension in the muscle fiber - The peak tension generated by the contraction is called the ACTIVE TENSION

Question 28

Describe the end systolic volume of a health heart

Answer 28

really efficient heart can eject more volume and reduce end systolic volume

Question 29

Describe the end systolic volume of a failing heart

Answer 29

A failing heart cannot efficiently remove as much of the starting (end diastolic) volume and therefore more is “leftover” in the heart after contraction- a larger end systolic volume

Question 30

What is intropy?

Answer 30

Intrinsic contractile state of the heart

Question 31

What is intropy influenced by?

Answer 31

Extrinsic factors - i.e. sympathetics Intrinsic factors - i.e. stiffness of the ventricles

Question 32

What is the effect of changing the contractility of the heart?

Answer 32

A INCREASED stroke volume This means that muscle efficiency increases the amount of blood propelled into the peripheral circulation

Question 33

When would you want to change the contractility of the heart?

Answer 33

When there is a need to increase blood (oxygen, nutrient) delivery when the body senses it is needed most (e.g. when the sympathetic nervous system (“fight-or-flight” response) is activated.)

Question 34

What are Frank-Starling curves? What are Sarnoff curves?

Answer 34

the relationship between stroke volume and contractility is very complex- there are an infinite number of Frank-Starling curves for the heart, demonstrated experimentally by Dr. Sarnoff (Sarnoff curves.)

Question 35

How does a change in heart rate affect cardiac output?

Answer 35

Changing heart rate will change cardiac output - Higher heart rate, higher cardiac output - Lower heart rate, lower cardiac output

Question 36

How does this relationship play out in real life?

Answer 36

Heart rate is never exclusively changed... The stroke volume is also affected by an increase or decrease in heart rate

Question 37

Give an example of how cardiac output stays constant during increases or decreases in heart rate

Answer 37

Dramatic changes in heart rate have little overall effect on cardiac output; if the heart rate increases, the reduction in stroke volume (the heart doesn’t have enough time to pump out the blood) will offset to buffer the change in CO. Conversely, if HR decreases, the heart has longer time to fill and thus a greater EDV will increase SV.

Question 38

How do vagal efferents affect heart rate and therefore cardiac output?

Answer 38

- Vagal efferents activate muscarinic receptors on nodal cells - This reduces the slope of diastolic depolarization - This increases the time between action potentials - This decreases the heart rate and therefore the cardiac output

Question 39

How does the activation of beta adrenergic receptors affect cardiac output?

Answer 39

- Activate beta adrenergic receptors on the nodal tissue - This increases the slope of diastolic depolarization - This decreases the time between action potentials - This increases the heart rate and therefore increases cardiac output

Question 40

Summarize the effect of muscarinic activation and adrenergic activation on cardiac output

Answer 40

Muscarinic activation = decreased HR and CO Adrenergic activation = increased HR and CO

Question 41

What is mass balance? What is it used to describe?

Answer 41

Mass balance (or conservation of mass) would describe that from adding a known quantity of something (dye) into an unknown volume of liquid, one can calculate the volume from the volume from the final concentration of the dye in the liquid This principle is used to measure cardiac output

Question 42

Describe the reasoning behind the Fick method of determining cardiac output

Answer 42

- The "material" used was oxygen - The rate of oxygen consumption by an organ (or person) is equal to the rate at which the oxygen comes to the organ (person) multiplied by the amount of oxygen in the tissue (estimated by the difference of what came in arterially and went out venously)

Question 43

How did Fick use this rationale to test for cardiac output?

Answer 43

- We can measure the oxygen consumption by having our patient inhale and exhale into a spirometer - We can sample the amount of oxygen exiting the lungs via the pulmonary vein (or MUCH easier in an arterial blood sample) and the amount of oxygen entering the lungs via a sampling of the right ventricle/ pulmonary artery Then we use this equation... Cardiac output (L/min) = (oxygen consumption [mL/min])/ (arterial-venous oxygen difference [mL/L])

Question 44

Describe the indicator dilution technique of determining cardiac output

Answer 44

- Injection of a known amount of indicator into the circulation (usually into the R ventricle, pulmonary artery or L ventricle) - A downstream blood sample is then taken - The timing and amount of indicator is used to calculate cardiac output

Question 45

Describe the thermal dilution technique

Answer 45

A common variation of the indicator dilution technique - Inject a known volume of ice-cold saline into the right ventricle - A temperature sensor is placed in the pulmonary artery - The solution reduces the blood temperature - The amount of temperature change is related to the volume of blood the cold saline mixes with - The cardiac output can then be calculated based on timing and temperature

Question 46

What is the last method that can be used to approximate cardiac output?

Answer 46

Electrocardiogram

Question 47

How does an ECG determine cardiac output?

Answer 47

- The ultrasound image can be quantified to assess chamber dimensions (during systole and diastole) and Doppler shift can assess the blood flow velocity through the semilunar valves - output is related to the conversion of blood velocity to stroke volume and heart rate

Question 48

Any other methods?

Answer 48

- Impedence cardiography - Pulse contour analysis - Flow probe placed in aorta (labs)