Normal Cardiac Mechanical Function

Question 1

what are the two parts of the cardiac cycle

Answer 1

1. ventricular systole: period of ventricular contraction
2. ventricular diastole: ventricular relaxation during which ventricle fills with blood

Question 2

draw and explain the cardiac cycle

Answer 2

1. left ventricular pressure: its low at the start of systole. contraction of muscles causes a rapid increase in pressure momentarily causing a back flow of blood from left ventricule to left atrium and this closes the mitral (left AV) valve. no immediate ejection of blood from the ventricle, however as the aortic valve remains closed until left ventricular pressure exceeds aortic pressure

2. iso-volumetric contraction: no change in ventricular volume (or aortic blood flow) in early systole despite an increase in ventricular pressure. once ventricular pressure > aortic pressure the aortic valve opens and blood is rapidly ejected. at ventricular pressure peak, blood ejection starts to slow and as pressure begins to decrease the blood ejection ends

3. relaxation: as pressure in ventricle drops just after peak the pressure in ventricle is slightly below aortic but blood flowing (momentum) keeps aortic valve open. as this slows there is a momentary back flow of blood from the aorta which closes the aortic valve. this begins ventricular diastole. muscles relax, pressure continues to decline although no refilling occurs at this point since pressure inside the ventricle is greater than atrial pressure. once it drops below atrial pressure the mitral valve opens

4. ventricular filling: once the mitral valve opens there is a phase of rapid ventricular filling followed by a period of slow filling until atrial contraction occurs (atrial systole)

5. atrial systole: the atrial systole doesn’t have a marked effect at rest, most of blood (90%) is already in the ventricle (atrial contraction just tops it up –> can survive atrial dysfunction ex. atrial fibrillation). atrial contraction becomes more important during exercise because the increased HR leaves less time for ventricular filling

Question 3

draw pressure volume loop and label where the different phases of the loop are

Answer 3

A. mitral valve opens: when mitral valve opens, blood comes in from atria to ventricles –> increase in volume (filling phase 40ml to 120ml). this is diastolic filling which finishes at B with only a small increase in pressure

B: mitral valve closes: when QRS kicks off –> mitral valve shuts and increase in pressure occurs but no change in volume (isovolumetric contraction B-C occurs at systole begins with no change in volume)

C. aortic valve opens: and in first phase (C-D) there is a large reduction in volume with a continued but less marked change in pressure. ventricle pressure eventually exceeds aortic pressure and causes aortic valve to open

D. ejection phase: during D-E there is a reduced ejection and a small drop in ventricular pressure. blood rushes into the aorta

E. aortic valve closes: ventricles relax and the pressure falls below aortic pressure and causes the aortic valve to close. continuing decrease in ventricular pressure but no change in volume as both valves are shut (isovolumetric relaxation). rapid drop in pressure with little change in volume

left ventricular pressure vs. ventricular volume goes anticlockwise

Question 4

what is end-diastolic volume

Answer 4

ventricles don’t empty in systole

just before ventricular systole the ventricles contain 40-60ml (dog) = end-diastolic volume (EDV)

when heart is fully relaxed –> time most of the blood is the ventricle

Question 5

what is the ejection fraction and what is it in the dog

Answer 5

at rest the dog ejects 50-65% of blood from left ventricle

Question 6

what is the end-systolic volume

Answer 6

after ejection of blood from the left ventricle –> what remains = end-systolic volume

Question 7

what is the volume of blood ejected from the left ventricle

Answer 7

stroke volume

difference between EDV and ESV = stroke volume (mls)

Question 8

how is cardiac output calculated

Answer 8

CO = SV x HR

(mls/min)

Question 9

what is a pressure-volume loop

Question 10

what is the peak pressure on the right side of the heart

Answer 10

20 mmHg

compared to 120 mmHg on left side

Question 11

what are the heart sounds

Answer 11

4 major sounds

1. first sound: linked to closure of AV valves (mitral and tricuspid) the sound being vibrations in blood when valve closes
2. aortic valve and pulmonary valve closure. reverberations in blood flow
3. rapid ventricular filling (when atria pressure > ventricle)
4. when atria contract

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