Heart Function Curves and Work of the Heart

Question 1

what is the relationship between cardiac output and venous return of the heart?

Answer 1

they should be the same

Question 2

what is venous return?

Answer 2

the rate at which the blood returns to the RA of the heart from the body systems

Question 3

what is cardiac output?

Answer 3

the rate at which blood is ejected from the LV and moves through the aorta and systemic circulation

Question 4

what is the central venous compartment?

Answer 4

the volume of blood enclosed by the right atrium and the great veins of the thorax

Question 5

how can you measure …?

Answer 5

see slide 99

Question 6

what is the cardiac function curve?

Answer 6

= CO curve
- demonstrates the direct relationship between the central venous pressure (RAP) on cardiac output
- Frank-Starling Law of the Heart

Question 7

what is the vascular function curve?

Answer 7

= venous return curve
- demonstrates the inverse (negative effect) of central venous pressure on venous return

Question 8

what is mean systemic (circulatory) pressure?

Answer 8

the volume of blood in the total systemic circuit when the heart is in cardiac arrest, no blood is flowing through the system
≈ 7mmHg

Question 9

what is the equilibrium point?

Answer 9

= steady state
- the point at which cardiac output and venous return are always equal and stabilized

Question 10

what does the vascular function curve define?

Answer 10

• the inverse relationship between venous return and right atrial pressure
• as you increase P, CO/venous return decreases

Question 11

the vascular function curve:
- depends on?
- shows?

Answer 11

• changes in the vascular system, dependent on the pressure gradient
• shows that as RA pressure increases, the venous return decreases
• if the RAP falls below 0, the intrathoracic pressure is higher which will cause collapse of the veins and a plateau (constant?) of the venous return

Question 12

venous return is determined by?

Answer 12

the pressure difference between the peripheral and central venous compartments

Question 13

how is venous return affected by decreases in peripheral venous pressure?
- leads to what?
- caused by what?

Answer 13

• leads to decreased BP
• caused by: blood loss via hemorrhage,
  loss of bodily fluid by sweat, diarrhea, or vomiting,
  venodilation (less sympathetic stimulation)

Question 14

how is venous return affected by increase in peripheral venous pressure?
- leads to what?
- caused by what?

Answer 14

• leads to increased BP
• caused by: blood transfusion,
  fluid retention in the kidneys,
  increases in circulating blood volume,
  venoconstriction, (sympathetic)
  venous compression,
  exercise,
  wearing elastic stockings

Question 15

what does the cardiac function curve define?
- based on?
- demonstrates?

Answer 15

the dependence of cardiac output on the preload of the heart
- based on the frank-starling relationship
- demonstrates that as RAP increases, there is a linear increase in cardiac output and at ~4mmHg pressure there is a plateau

Question 16

mean systemic (circulatory filling) pressure is determined by what? can be affected by what?

Answer 16

• the volume of blood in the total systemic circuit during cardiac arrest
• affected by changes in circulating blood volume, venous reserve, and venous tone (venous constriction or dilation)

Question 17

how is the cardiac function curve altered by positive and negative inotropic agents?

Answer 17

Question 18

how is the vascular function curve altered by blood volume and venous reserve?

Answer 18

Question 19

how are both curve affected by the changes in total peripheral resistance (TPR)?

Answer 19

Question 20

what is stroke work?

Answer 20

the work done by the ventricles to eject the blood into the aorta
- in a normal heart, the work done by the heart = stroke work
- work is done in units per time = power
- power in heart = cardiac minute work

Question 21

what consumes more energy: pressure work or volume work?

Answer 21

pressure work

Question 22

what is myocardial oxygen consumption (MVO2)?

Answer 22

the energy consumption of the heart
- amount of oxygen consumed per minute
- directly correlates with stroke work and cardiac minute work

Question 23

what is mechanical efficiency?

Answer 23

the ratio between work of the heart and oxygen consumption

Question 24

what happens to mechanical efficiency in a scenario of hypertension or aortic stenosis?

Answer 24

• there is an increased afterload
• which means high pressures are required to eject the stroke volume
• O2 consumption increases
• therefore mechanical efficiency decreases!!!

Question 25

what is the effect of strenuous exercise on cardiac output?

Answer 25

• oxygen consumption increases (less dramatic)
• so the mechanical efficiency is reduced (again.. less dramatic)

Question 26

compare the work of the right heart to the work of the left heart

Answer 26

• LVP work > RVP work
• pressure work increases the ventricular wall thickness
• right side work is about 1/6 of the left side

Question 27

what is the law of laplace?

Answer 27

when the heart has an increased pressure or volume work, there is an increased wall stress (tension) on the heart (thus thicker wall)
- so with increased pressure or volume load, the heart will undergo hypertrophy to reduce the wall stress (to help reduce the wall tension)

Question 28

what causes cardiac hypertrophy?

Answer 28

• an excessive workload placed on the heart