The Cardiac Cycle

Question 1

What are the two main phases of the cardiac cycle?

Answer 1

Diastole:

Lasts approximately 2/3 of each beat

Ventricular relaxation (they fill with blood)

Split into 4 phases

Systole:

Lasts 1/3 of a beat

Ventricular contraction (eject blood into arteries)

3 phases

Question 2

What is the end diastolic volume?

Answer 2

The maximum volume of blood in the ventricles

At peak relaxation

Part of systole (isometric contraction), but just before the ventricles contract

Question 3

What is the end systolic volume?

Answer 3

The amount of blood in the heart after contraction

Residual volume

End of systole

Question 4

How can we calculate stroke volume?

Answer 4

End diastolic volume - end systolic volume = stroke volume (mL)

Eg. 120 - 50 = 70 ml

Question 5

What is and how do we calculate ejection fraction?

Answer 5

Clinical representation of cardiac function

(Stroke volume / end diastolic volume) X 100 = ejection fraction (%)

Normal range is 52-72%

Question 6

What is the order of events in the cardiac cycle?

Answer 6

(Diastole) Atrial systole ->

Systole: isovolumetric contraction -> rapid ejection -> slow ejection

Diastole: isovolumetric relaxation -> rapid passive filling -> slow passive filling

Repeat

Question 7

What is atrial systole?

Answer 7

P wave on ECG

Atria almost full from passive filling driven by pressure gradient. Atria contract to top up the volume of blood in the ventricles

This will have more of an effect during exercise when there isn’t as much time to fill ventricles from passive filling

If a patient has an abnormality (congestive heart failure, pulmonary embolism, tricuspid incompetence) there may be a 4th heart sound head

Question 8

What is isovolumetric contraction?

Answer 8

QRS complex (on ecg) marks the start of ventricular depolarisation

This is the interval between when the AV valves close and the semi lunar valves open

Contraction of ventricles with no change in volume

Volume in ventricles is at its highest

1st heart sound (lub) due to closure of AV valves

Question 9

What is rapid ejection?

Answer 9

Opening of aortic and pulmonary valves mark the start of this phase

As ventricles contract the pressure in them exceeds the pressure in the aorta and pulmonary arteries. The semi lunar valves open, blood is pumped out and the volume of the ventricles decreases

Pressure in aorta mirrors the pressure in the ventricles (hump)

No heart sounds

Question 10

What is reduced ejection (slow ejection?

Answer 10

Phase marks the end of systole

T wave on ecg (due to some level or repolarisation of the ventricles)

Reduced pressure gradient means aortic and pulmonary valves begin to close

Blood flow from ventricles decreases and ventricular volume decreases more slowly

As pressure in ventricles falls below that in arteries, blood begins to flow backwards causing the semi lunar valves to close

Question 11

What is isovolumetric relaxation?

Answer 11

The aortic and pulmonary valves shut. But the AV valves remain closed until ventricular pressure drops below aortic pressure

The rate of decrease of pressure in the ventricles (with constant volume) is determined by the rate of relaxation of the muscles (lucitropy). This is dependant on the SR calcium ATPases taking up the calcium back into the sarcoplasmic reticulum

Atrial pressure continues to rise (dichrotic notch) caused by rebound pressure against aortic valve as distended aortic valve relaxes

Second heart sound (dub) due to closure of the semi lunar valves

Question 12

What is rapid passive filling?

Answer 12

Occurs during isoelelctric (flat ecg) between cardiac cycles

AV valves open as ventricular pressure is lower than atrial

Blood rapidly flows into the ventricles

May be a 3rd heart sound (usually abnormal) (severe hypertension or mitral incompetence) due to turbulent ventricular filling

Question 13

What is reduced passive filling?

Answer 13

Often called diastasis

Ventricular volume fills more slowly

The ventricles are able to fill considerably without contraction of the atria

Question 14

What are pulmonary circuit pressures?

Answer 14

The pattern of pressure changes in the right heart are essentially identical to those in the left

Quantitatively The pressures in the right heart and pulmonary circulation are much lower

Despite lower pressures, right ventricle ejects the same volume of blood as the left (same quantity of blood into lower pressure circuit)

Question 15

What are pressure volume loops?

Answer 15

Left Ventricular volume on y

Left Ventricular pressure on x

Google a picture

Sorta like an anti-clockwise loop from end diastolic volume -> aortic pressure encountered (aortic valve opens) -> end systolic volume -> isovolumetric relaxation

Question 16

What determines the preload?

Answer 16

Blood fillin the ventricles during diastole, stretched the resting ventricular muscle

Question 17

What determines the afterload?

Answer 17

The blood pressures in the aorta and pulmonary artery represent the afterload

If the afterload increases, stroke volume decreases

Question 18

What is the end systolic pressure volume line?

Answer 18

ESPVR is the maximal pressure that can be developed by the ventricle at any given volume

Question 19

What is the equation for cardiac output?

Answer 19

Heart rate x stroke volume = cardiac output

Question 20

What factors affect stroke volume?

Answer 20

Preload (stretch/loading, intrinsic)

Afterload (diastolic blood pressure)

Contractility

Question 21

What is contractility?

Answer 21

Contractile capability (strength) of the heart

Increased by sympathetic stimulation

Extrinsic mechanism: changes ca2+ delivery to myofillaments (due to increased CAMP)

As contractility changes, a ‘family’ of frank-starling relations become apparent