Cardiac Output

Question 1

What is Cardiac Output?

What are the determinants/ equation of CO?

https://www.youtube.com/watch?v=hpQFToprlH8

Answer 1

The volume of blood ejected from the heart (by each ventricle) in 1 minute. (L/min)

• normal CO = 4-8L/min, increases with exercise and is specific to body size
• Determinants of CO = Stroke Volume (ml/beat) x Heart Rate (beats/min)
  SV X HR = CO

Question 2

What is contractility?

Answer 2

How hard the myocardium contracts for a given preload.

The ability of the heart’s myofibrils (muscle cells) to change their strength of contraction.

• As contraction occurs, contractility increases as a result of increased stretch of actin and myosin molecules in myofibril (as the diastolic volume increases and stretched them).
  However if stretch is excessive and causing actin and myosin to be stretched beyond their cross-bridging limits, contractility decreases.
  –> ex: left ventricle pumping ability may be impaired after a heart attack :/
  = less oxygenated blood to the body = decreased pulses, cool skin…

Question 3

What is Preload?

(filling) — preloading…

(filling the garbage truck)

Where is preload measured?

Answer 3

The amount of blood ENTERING your ventricles (right and left) from your atria (right and left), during diastole (ventricle filling).
–> influenced by venous return (how much blood is coming back from the peripheral body), blood volume, and atrial contraction

The volume of blood in the left ventricle at the end of diastole (when the heart muscle relaxes and allows the chambers to fill with blood.)

• -> the amount of ventricular stretch at the end of diastole.
• -> the heart loading up for the next big squeeze of the ventricles during systole.

–> preload is measured in the ventricles!!

Question 4

What is Afterload?

–> after filling… (dumping the garbage out of the dump truck)

Answer 4

When ventricles are contracting during systole, ejecting blood out of the aorta.

The force or the resistance against which the ventricles have to pump in order to eject/circulate blood.
–> the amount of resistance the heart must overcome to open the aortic valve and push the blood volume out into the systemic circulation.

Factors increasing afterload: atherosclerosis, vasoconstriction (these will increase resistance)

Question 5

What 3 factors influence stroke volume?

Answer 5

Contractility, Preload, Afterload

Question 6

What is Stroke Volume?

Answer 6

The volume of blood pumped out by each ventricle with each beat (ml/beat).

Question 7

What are the 4 chambers of the Human heart, and their functions?

Answer 7

The left and right atria (which receive blood from the vena cave and pulmonary arteries and pump blood into the ventricles), and the right and left ventricles (which receive blood from atria and pump blood into the peripheral beds of the systemic and pulmonary circulation within the body and lungs).

–> right atrium receives blood from the superior and inferior vena caves (veins) –> blood moves to the right ventricle where it is pumped to the lungs

–> the lungs re-oxygenate the blood and send it to the left atrium

–> blood moves from he left atrium to the left ventricle via the bicuspid valve

–> blood is pumped out of the left ventricle to the aorta (artery) which sends blood to the organs and muscles of the body.

Question 8

Why is the left ventricle considered the major pump of the heart?

Answer 8

Because of its thickness and the large force it must generate to eject blood into the aorta.

Question 9

What is the function of cardiac valves?

Answer 9

Cardiac valves are composed of flexible, fibrous tissue. They allow blood to flow in only one direction, and are able to open and close depending on the relative pressure gradients on either side.

Question 10

Arteries vs Ventricles

Answer 10

The arteries transport oxygenated blood from the left ventricle of the health to the rest of the body.

Question 11

What is the superior vena cava?

Answer 11

A large vein that carries deoxygenated blood from the upper half of the body to the right atrium of the heart

Question 12

What is the inferior vena cava?

Answer 12

A large vein that carries deoxygenated blood from the upper half of the body to the right atrium of the heart

Question 13

The hypovolemic pt has too ____ preload, whereas the pt with HF has too ____ preload.

Answer 13

little, much

–> hypovolemic pts have decreased CO due to blood loss of some sort, so the total blood volume is decreased = decreased preload (filling of atrium)

–> pts with HF (heart muscle does not pump blood as well as it should, due to CAD, high BP etc) have too much blood remaining in their atria that can’t be pumped properly into the ventricles, = increased preload.

Question 14

True or False:

Both sides of the heart (left and right) are working at the same time.

Answer 14

TRUE!

Both right and left atria fill and contract at the same time, and both ventricles fill and contract at the same time.

Question 15

Contractility is primarily influenced by…

Answer 15

Preload.

Question 16

What cardiac output determinants are mainly affected by compensation?

Answer 16

Afterload and heart rate

Question 17

What is MAP?

Answer 17

Mean Arterial Pressure.
- MAP indicated how the body is perfusing the organs and tissues. It is the average pressure during systole and diastole.

• -> the average pressure within the arterial vessels during one full cardiac cycle (one contraction)
• -> the average pressure in the arteries over the course of systole and diastole.

Question 18

During systole, the ventricles of the heart ____

Answer 18

contract. It is the left ventricle that ejects blood into the body.
- -> the large amount of force that the left ventricle exerts, and the force that is carried in the volume of blood (which exerts a pressure on the vessels) = systolic blood pressure

–> low systolic pressure= your ventricles are not contracting as hard as they should be.

Question 19

During diastole, the heart _____ and the ventricles ____ before it contracts again.

Answer 19

relaxes, fill.

–>The blood that is ejected into the aorta during systole flows into the arteries, then the arterioles. As the blood volume dissipates through these vessels, the pressure decreases, and the lowest measurable pressure is created by the “tone” of the artery. Although the heart is “relaxing” during diastole, the arterial vessels are always in a state of tension or mild constriction. Just like any muscle in the body, it is not completely flaccid when not specifically in use.

Diastolic blood pressure (DBP) is a measurement of the tone in the arterial vessels at its lowest level, just before the heart contracts again.

Question 20

Diastole is _____ as long as systole.

Answer 20

Twice.

–> which is why MAP is calculated as (systolic BP + (2 x diastolic BP) / MAP = 3

Question 21

What is pulse pressure?

Answer 21

The difference between systolic and diastolic blood pressure (in mmHg).

• A narrow pulse pressure (<= 25mmHg) signifies a pt is vasoconstrictor
• A wide pulse pressure usually identifies that a pt is vasodilated

Question 22

What does a low total circulating volume mean for preload?

Answer 22

Low total circulating volume = low preload

- TCV is related to the pts overall fluid status

Question 23

A pt with peripheral edema is pressured to have too ___ preload. Why?

Answer 23

Too much. Because there is too much blood volume in the vascular system, and it will pool and move into the interstitial space, causing edema.

**pts with edema but have increased capillary permeability (sepsis, anaphylaxis) may not have an increased preload….

Question 24

What is venous return?

How does it affect preload?

Answer 24

The flow of blood through the venous system back to the right side of the heart. (veins –> right atrium)

Increased venous return = increased preload

Decreased venous return = decreased preload

**think of preload like a garbage truck filling,,, emptying = aferload

Question 25

What is venous return mainly affected by?

Answer 25

The dilation or constriction of the arterial vessels.

Vasodilation–> decreased venous return = decreased preload

• -> vasodilation decreases venous return and therefore decreased preload
• decreased pressure provides less push of blood to move into venous circulation
  (causes: increased temp, allergic responses, various meds)

Vasoconstriction –> increased venous return = increased preload

• -> vasoconstriction increases venous return
• vasoconstriction increases the blood flow from the arterial to venous system, thus increasing venous return to increase preload

Question 26

What is ventricular compliance?

Answer 26

The distensibility of the heart muscle. When the ventricle is less stretchy, it is less compliant = increased preload.
–> force necessary to eject blood cannot be generated, the preload that is left behind after each “ineffective” cycle of systole increases, = increased preload over time.

Question 27

Vasoconstriction = _____ afterload

Vasodilation = ____ afterload

Answer 27

increased, decreased

Question 28

What are some signs of increased afterload? (vasoconstriction)
–> decreased blood flow

Answer 28

Cool, pale skin, delayed capillary refill, difficult to palpate peripheral pulses, higher than baseline diastolic BP, T2DM…

Question 29

What are some signs of decreased afterload (vasodilation)?

–> increased blood flow

Answer 29

Flushed skin, bounding pulses, lower than baseline diastolic BP.