The cardiac cycle

the cardiac cycle

• encompasses all events which coordinate passage of blood through the heart during one single beat

• includes electrical, mechanical and pressure changes in a process transitioning from a relaxed heart to a contracted heart

wiggers diagram components

• electrophysiological events

• pressure events

• clinical findings

• LV pressure events

• mechanical events and blood flow

route of blood during the cardiac cycle

1. superior and inferior vena cava

2. right atrium

3. bicuspid valve

4. right ventricle

5. pulmonary valve

6. pulmonary trunk→ pulmonary arteries

7. lungs

8. pulmonary veins

9. left atrium

10. mitral valve

11. left ventricle

12. aortic valve

13. aorta

purpose of cardiac valves

• prevent backflow of blood

how many leaflets do each valve have

• mitral→ 2

• tricuspid→ 3

• aortic→ 3

• pulmonary→ 3

valve anatomy

mechanical events occurring in the cardiac cycle

1. late diastole

2. atrial systole

3. isovolumic ventricular contraction

4. ventricular ejection

5. isovolumic ventricular relaxation

late diastole

• both sets of chambers relaxed

• ventricles filled passively

atrial systole

• atrial contraction forces small amount of additional blood into ventricles

isovolumic ventricular contraction

• first phase of ventricular contraction

• pushes AV valves closed

• does NOT create enough pressure to open semilunar valves

ventricular ejection

• ventricular pressure rises and exceeds pressure in the arteries

• semilunar valves open and blood is ejected

isovolumic ventricular relaxation

• ventricles relax

• pressure in ventricles falls

• blood falls back into cusps of semilunar valves and snaps them closed

electrophysiological anatomy of the heart

myogenic contraction of the heart

1. sino-atrial node initiates electrical depolarisation across both atria

2. atria have completed contraction and there is a short pause before AVN depolarises

3. AV nod depolarises and sends electrical activity down the bundle of His and Purkinje fibres→ ventricular depolarisation

4. ventricular depolarisation has completed

5. ventricular repolarisation phase→ prepares cell for next phase of depolarisation

ECG trace

p-wave

• generally positive deflection on ECG

• smaller wave that QRS→ atria create smaller

PR interval

• pause in electrical activity before ventricular depolarisation

• delay caused by AV node

QRS complex

• formed from direction of ventricular depolarisation goes down heart and back up ventricular walls

ST segment

• demonstrates post-ventricular contraction

• phase prior to repolarisation

T wave

• ventricular repolarisation

• usually demonstrated by positive deflection

heart sounds

• lubb→ closure of antrioventricular valves

• dupp→ closure of semilunar valves

murmurs

• chnage in nature of normal heart sounds

• not always pathological

• e.g. valve stenosis (narrowing), regurgitation (leaky)

stroke volume

blood volume ejected from left ventricle during each heartbeat

cardiac output

• total blood volume pumped from heart in 1 minute

• stroke volume x heart rate

ejection fraction

• percentage of blood pumped from LV that enters systemic circulation