Cardiac Cycle

Dr. Wasif Haq

Introduction• Cardiac events that occur from beginning of one

heartbeat to the beginning of the next.• Inversely proportional to heart rate.• Consists of systole & diastole.• Systole: Period of contraction.• Diastole: Period of relaxation, heart fills with blood.

Some Essential Concepts• Delay between the impulse passage and actual

contraction.• Whenever the pressure in one region falls, blood will

flow into lower region pressure from higher region pressure.

• Valve distal to high pressure region open.• All valves closed in “Iso” states.

Phases of Cardiac Cycle• Consists of 4 phases

• 1. Period of rapid filling of ventricles.• 2. Period of isovolumic/isometric contractions.• 3. Period of ejection.• 4. Period of isovolumic/ isometric relaxation.

1. Rapid Filling of Ventricles• Ventricular pressure falls after systole, pushing

blood from atria into ventricle.• A.V. valve open causing filling of ventricles with

blood.• Rapid filling consists of 3 portions/parts; 1/3 rapid

filling occurs( 80% of atrial blood without contraction of atria), 2/3 some quantity of blood flows, 3/3 atrial contraction occurs (causing remaining 20% of blood to flow as well)

2. Isovolumic/ Isometric Contractions

• All valves closed (A.V. and semilunar valves)• Some delay before opening of semilunar

(pulmonary & aortic valves) needed to build sufficient pressure to open these valves.

• Ventricular fibers are contracting, but no volume change inside the ventricle because of closure of both the valves.

3. Period of ejection• When ample pressure has been built in

ventricle(>80 mm Hg in left ventricle & > 8 mm Hg in right ventricle), contraction occurs to eject blood into arteries.

• Two phases; 1. Period of rapid ejection (70% of blood emptied), followed by 2. Period of slow ejection (remaining 30% blood ejected).

• Semilunar valves (aortic & pulmonary) open.

4. Isovolumic/Isometric relaxation• All valves closed.• After systole, increased pressure in arteries force

closure of semilunar (aortic & pulmonary) valves, A.V. valves are also closed at this time, preventing any volume change inside ventricle.

• Ventricular fibers relax during this phase.

Volumes• End systole volume: Residual volume of blood in

each ventricle at end of contraction/systole. Usually 50 ml.

• End diastole volume: Filled volume of blood prior to contraction 110-120 ml.

• Stroke volume output: Amount of blood pumped by heart with each heartbeat.70 ml.

• Stroke volume= End diastolic volume- End systolic volume

Electrocardiogram, Atrial Pressure Changes,

Ventricular Volume, Ventricular pressure

curve, aortic pressure curve & heart sounds.

Electrocardiogram • Graphic tracing of variations in electrical potential

caused by excitation of heart & detected at body surface.

• Consists of P, QRS complex, T and U wave.

Waves Significance• P waves represent ‘atrial depolarization’ , wave

appears before atria actually contract.• Q.R.S. Complex represent ‘ventricular

depolarization’, occurs slightly before ventricular depolarization.

• T wave represent ‘ventricular repolarization’, occurs before the termination of ventricular contraction.

• U wave represent ‘Purkinjee fibers repolarization’, not always present.

P wave= Atrial depolarization, Q.R.S. complex= Ventricular depolarization, T wave= Ventricular

repolarization, U wave = Purkinjee fibers repolarization

Atrial Pressure Changes

• a wave represents atrial contraction, increasing atrial pressure.

• c wave represents regurgitation of blood from ventricles into atrium due to sliding/closure of A.V. valves towards atrium due to increased ventricular pressure.

• v wave represents the flow of blood from atria to ventricle after ventricles stop contracting.

Aortic Pressure Changes

• Opening of aortic valve: Rise in left ventricular pressure upon ventricular contraction (Q.R.S.) causes aortic valve opening, pressure in aorta will rise to 120 mm Hg.

• Closing of aortic valve: After ventricular systole, aortic valve close, causing pressure dissipation slowly to 80 mm Hg before next ventricular contraction.

Ventricular Pressure Changes

• During ventricular systole, the ventricular pressure increases which causes closing of A.V. valve ( c wave in atrial pressure changes) and opening of aortic valve, pumping blood to the systemic circulation.

Ventricular Volume Changes

• The ejection of blood from ventricle during systole (aortic valve opening & A.V. valve closure) causes drop in ventricular volume, followed by rapid filling of ventricles (A.V. valve open & aortic valve closure).

Phonocardiogram

• 1st heart sound: A.V. valve close, semilunar valves open, systole begins. “Lub”

• 2nd heart sound: Semilunar valve close, A.V. valve opens, systole finishes. “Dub”.

• 3rd heart sound: Rapid flow of blood from atria into ventricles, mitral valve is open, normal in children but in adults, may mark pathology.

• 4th heart sound: Filling of ventricles by atrial systole, not normal.

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