Lecture 9: Ventricular Function

Question 1

Define cardiac output and write an equation for its calculation

Answer 1

Cardiac output = volume of blood ejected by one ventricle in one minute

CO = HR * SV

Question 2

Define cardiac index (CI) and write an equation for its calculation

Answer 2

Cardiac index = relates resting CO to body size

CI = CO / BSA

Question 3

Briefly explain importance of cardiac index (CI) and state approx. normal value

Answer 3

Important because it reflects metabolic rate - continually changes to meet changes in demand

Normal value = 3.4 +/- 0.8 L/min/m2

Question 4

List four determinants of cardiac performance (AKA four determinants of ventricular function)

Answer 4

Preload

Afterload

Contractility

Heart rate

Question 5

Identify 3 determinants that also affect isolated cardiac muscle performance

Answer 5

SV depends on interplay of:

Diastolic stretch

Contractility

Arterial pressure

Question 6

State the most precise definition of ventricular PL

Answer 6

Diastolic filling pressure

Question 7

List two parameters that can be used to estimate preload

Answer 7

End diastolic volume

End diastolic pressure

Question 8

Describe 3 potential mechanisms that explain Starling’s law

Answer 8

Increased fraction of crossbridges formed between actin + myosin

Filament lattice spacing

Muscle stretch influences tension development and calcium sensitivity (stretching the muscle requires less calcium to get same amount of tension)

Question 9

Which potential mechanism is the most important to explain Starling’s law

Answer 9

Increased fraction of crossbridges formed between actin and myosin

Question 10

How does a ventricular function curve illustrate Starling’s law?

Answer 10

Depicts how with increased muscle fiber length, the stroke volume increases

Question 11

List examples of conditions or interventions that can alter ventricular PL

Answer 11

Hemorrhage (or severe dehydration)

Posture (move from supine to upright)

Neural tone

Muscle compression

Respiration

Question 12

Define afterload

Answer 12

Wall stress produced by properties of ventricle itself combined with additional properties of the vasculature during systole

Question 13

Explain why AL changes throughout ventricular ejection

Answer 13

SV varies inversely with wall stress during ejection

Wall stress itself varies during the rapid and slow ejection phases of the cardiac cycle

Question 14

Define contractility

Answer 14

Intrinsic force-generating capacity of the cardiac cell under fixed PL and AL

Question 15

Describe the normal regulation of ventricular IS

Answer 15

Contractility and relaxation can be enhanced by catecholamines (act via cAMP/PKA)

Most important inotropic = sympathetic noradrenaline

Question 16

Illustrate the effect of an increase in IS on ventricular function curve

Answer 16

Greater AL = the less it shortens during systole (reduced shortening also reduces contractile force)

Increase in sympathetic stimulation (inc. BP) = decreased SV

Question 17

Illustrate the mechanism for the effect of cardiac glycosides on IS

Answer 17

Digitalis inhibits sodium-potassium ATPase = intracellular sodium accumulates

Calcium extrusion via Na-Ca exchanger is inhibited = accumulation of calcium in cytoplasm and SR

= increase in contractility because of extra calcium hanging around

Question 18

List three estimates of IS

Answer 18

Echocardiography (estimate most commonly used clinically)

Using dP/dt (estimate obtained from Wiggers diagram)

Calculation of ejection fraction from EDV and ESV (estimate from PV loop)

Question 19

Define the end systolic pressure volume relationship (ESPVR) and explain significance

Answer 19

Can be used to estimate contractility

At IVC, the ventricle is filled = higher volume means higher pressure

Higher volume decreases contractility because then it is harder for ventricle to contract as adequately

Question 20

Compare relative systolic and diastolic times at a HR of 60 bum and 180 bpm

Answer 20

Diastolic times decrease by 1/3

Question 21

What influences SV?

Answer 21

Energy of contraction

Aortic pressure

Question 22

How can energy of contraction be increased?

Answer 22

Raising EDP to stretch myocardium = Sterling’s law

Inc. in contractility = inc. strength of contraction by sympathetic nerves and adrenaline

Question 23

What factors regulate stroke volume?

Answer 23

Filling pressure (preload) = Starling’s law of heart

Arterial pressure opposing ejection (afterload)

Contractility: sympathetic nerves + circulating agents

Total peripheral resistance

Question 24

What is Starlings law of the heart?

Answer 24

Increase in contractile energy with stretch

Where stretch of muscle fibers set by diastolic filling pressure (PL)

Question 25

What is a ventricular function curve?

Answer 25

Plot of SV versus filling pressure

Or any plot where
X-axis = index of resting fiber length
Y-axis = measure of contractile energy

Question 26

What does a ventricular function curve depict via the descending limb?

Answer 26

SV declines in over-distended hearts