14. Cardiac Output & Contractility

Question 1

what is preload

Answer 1

• amount of blood ready to be pumped at the end of diastole = end diastolic volume
• wall tension in ventricle just before contraction is initiated
• fiber length at the end of diastole

Question 2

how is preload related to venous return

Answer 2

more blood returning → greater preload

• in a steady state, venous return = CO
• frank starling: volume of blood ejected depends on the volume present at end of diastole

Question 3

what is afterload

Answer 3

force opposing contraction, pressure required to eject blood (i.e. open aortic/pulmonary valve)

• increase in vessel pressure causes and increase in afterload

Question 4

how does afterload effect the velocity of muscle fiber contraction?

when is velocity the greatest?

Answer 4

• velocity of shortening decreases as afterload increases (inverse relationship)
• greatest if afterload were 0

Question 5

what effect does preload have on contractility and CO?

afterload?

Answer 5

↑ preload → ↑ CO and contractility

↑ afterload → ↓ CO
Heart must then ↑ contractility or HR to overcome

Question 6

Describe the relationship between heart rate and contractility in terms of the positive staircase effect.

Answer 6

↑ HR (+ chronotropic effect) → ↑ contractility (+ ionotropic effect)

Positive staircase effect: ↑ force of contraction in a stepwise manner d/t cumulative increase in intracellular Ca; more Ca enters cell and more is taken up into SR

also seen with premature or out of synch beat; weak beat will be followed by stronger beats

Question 7

how do you calculate SV and what is normal

Answer 7

SV: volume of blood ejected by ventricles with each beat
= HR x CO = EDV - ESV

Normal = 70 mL

Question 8

how do you calculate EF% and what is normal

Answer 8

Ejection fraction: fraction of EDV ejected in each SV, measures efficiency and contractility
= SV / EDV

Normal = 55%, reduced in heart failure patients

Question 9

how do you calculate CO and what is normal

Answer 9

CO: total volume of blood ejected by ventricle per minute
= SV x HR

Normal = 5L/min

Question 10

what effects does the sympathetic system have on contractility?

how?

Answer 10

positive ionotropic effect through B1 adrenergic activation → ↑ in contractility

Activation causes phosphorylation of…

• Sarcolemmal Ca channels → ↑ inward ICa during phase 2
• Phospholamban (stimulatory) → ↑ activity of Ca pump so more Ca is accumulated in the SR
• Troponin I (inhibitory, inhibition of inhibitor)

Question 11

what effect do cardiac glycosides have on contractility?

how?

Answer 11

Cardiac glycosides: ↑ contractility

Done by inhibiting Na-K-ATPase → ↑ in intracellular Na, diminishing gradient used by Na-Ca exchanger (more Ca stays in)

Question 12

what effect does the parasympathetic system have on contractility?

how?

Answer 12

negative ionotropic effect in atria only via muscarinic activation → ↓ in contractility

• ↓ inward ICa during phase 2
• ACh ↑ outward IK via K-ACh channel

Question 13

what is happening between points 1 and 2 on pressure volume loop?

Answer 13

1 → 2: isovolumetric contraction, all valves closed

• 1: EDV (preload), marks the end of diastole, pressure is low (EDV = 140 mL)
• 2: afterload, marks point where ejection of blood begins, aortic valve open

Question 14

what is happening between points 2 and 3 on pressure volume loop?

Answer 14

2 → 3: ventricular ejection

• max pressure between points 2 and 3
• SV = 70 mL (measured by width of loop)
• 3: ESV

Question 15

what is happening between points 3 and 4 on pressure volume loop?

Answer 15

3 → 4: isovolumetric relaxation

• 3: systole ends (ESV) and ventricles relax
• Pressure falls quickly by volume remains constant
• 4: AV valve opens

Question 16

what is happening between points 4 and 1 on pressure volume loop?

Answer 16

4 → 1: ventricular filling

Question 17

what effect does increased preload have on pressure volume loop?

Answer 17

↑ venous return, ↑ blood volume → ↑ EDV

Shifts point 1 and 2 to the right

Compensation: ↑ contractility

Question 18

what effect does increased afterload have on pressure volume loop?

Answer 18

↑ in aortic pressure (ventricle must eject blood against higher pressure) → ↓ SV, ↓ EF%
↓ SV → ↑ ESV

Shifts point 2 and 3 up

d/t aortic stenosis, hypertension

Question 19

what effect does increased contractility have on pressure volume loop?

Answer 19

greater tension in ventricle → ↑ SV, ↑ EF% (↑ SV → ↓ ESV)

Shifts points 3 and 4 to the left, 2 → 3 gets higher

d/t adrenergic stimulation

Question 20

what is stroke work and how does it effect function of the heart?

Answer 20

= SV x pressure work (aortic pressure)

Area within the pressure-volume loop

Heart has to work harder to overcome aortic pressure → cardiomegaly

Question 21

how does pressure work effect LV?

what does this have to do with O2 consumption?

how do you calculate CO based on O2 consumption

Answer 21

• LV must proportionally work harder than RV despite CO being similar because systemic pressure is greater than pulmonary pressure.
• Pressure work (fighting aortic pressure) requires the most O2 consumption
• CO = O2 consumption / ([O2] pulmonary a - [O2] pulmonary v)

Aortic stenosis and HTN increase LV pressure work

Question 22

describe the cardiac function curve

Answer 22

↑ venous return → ↑ RA pressure → ↑ EDV and end diastolic fiber length

Question 23

describe the vascular function curve

Answer 23

relationship between venous return and RA pressure

Question 24

what is the equilibrium point between the cardiac and vascular function curve

Answer 24

CO = 5 L/min

RA pressure = 2 mm Hg

Question 25

what effect will increased inotropy have on the cardiac fxn curve?

decreased?

Answer 25

↑ ionotropy → ↑ HR → ↓ afterload

↓ ionotropy → ↓ HR → ↑ afterload

Question 26

what is mean systemic pressure

Answer 26

pressure in atria when there is no blood flow

CO and VR are zero, pressure is equal throughout cardiovascular system

Question 27

how would mean systemic pressure be increased and what effect would this have on the graph?

decreased?

Answer 27

↑ MSP by ↑ in blood volume or ↓ in venous compliance
- shifts curve to the right

↓ MSP by ↓ in blood volume or ↑ in venous compliance
- shifts curve to left

Question 28

what effect does + inotropy have on cardiac fxn and how would this appear on the graph?

• inotrophy?

Answer 28

+ inotropic effect: ↑ contractility and CO

• slope of cardiac fxn curve ↑
• e.g. digitalis
• inotropic effect: ↓ contractility and CO
• slope of cardiac fxn curve ↓

Question 29

what effect does increased TPR have on cardiac fxn and how would this appear on the graph?

decreased TPR?

Answer 29

Increased: ↓ CO and venous return
- Shifts intersection point ↓

Decreased TPR: ↑ CO and venous return
- Shifts intersection point ↑

***No change in atrial pressure

Question 30

what effect does increased blood volume have on cardiac fxn and how would this appear on the graph?

decreased blood volume?

Answer 30

↑ blood volume: ↑ MSP, ↑ CO and RA pressure
- cardiac function curve moves up

↓ blood volume: ↓ MSP, ↓ CO and RA pressure
- cardiac function curve moves down

Question 31

what happens to inotropy, vascular compliance, blood volume, and TPR in heart failure?

Answer 31

↓ inotropy and vascular compliance

↑ blood volume and TPR