Cardiac Cycle 2003

8/3/2019 Cardiac Cycle 2003

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Cardiac cycle


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y Defined as the cardiac events that occur from the

beginning of one heartbeat to the beginning of the next.

y Each cycle is initiated by spontaneous generation of an

action potential in sinus node.

y There is a delay of more than 0.1 second during passage

of the cardiac impulse from the atria and the ventricles.

y This allows the atria to contract ahead of the ventricles,thereby pumping blood into the ventricles before the

contraction of ventricles begin.


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y Consists of systole and diastole.

y Systole: period of contraction, ejection of blood occurs into

the aorta and pulmonary artery, respectively. 0.3 sec

y Dia

stole: period of relaxation during which the heart fillswith blood, 0.5 sec


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y 7 phases of cardiac cycle:

1. Phase 1 - Atrial Contraction

2. Phase 2 - Isovolumetric Contraction

3. Phase 3 - Rapid Ejection

4. Phase 4 - Reduced Ejection

5. Phase 5 - Isovolumetric Relaxation6. Phase 6 - Rapid Filling

7. Phase 7 - Reduced Filling


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Atrial Contraction (Phase 1)


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y A-VValves Open; SemilunarValves Closed

y This is the first phase of the cardiac cycle because it is

initiated by the p wave of the electrocardiogram (ECG)

y

Atrial depolarization then causes contraction of the atrialmusculature.

y As the atria contract, the pressure within the atrial chambers

increases, which forces more blood flow across the openatrioventricular (AV) valves, leading to a rapid flow of blood

into the ventricles.


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y Atrial contraction normally accounts for about 10% of left

ventricular filling when a person is at rest

y Volume of blood present on the ventricle at the end of

diastole is called the end diastolic volume (EDV). It is about

120 ml..


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Isovolumetric Contraction (Phase 2)


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y All Valves Closed

y This phase of the cardiac cycle begins with the

appearance of the QRS complex of the ECG,

which represents ventricular depolarization.

y Ventricular contraction begins


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y rapid increase in intraventricular pressure.

y The AV valves closes as intraventricular pressure

exceeds atrial pressure, producing 1st heart

sound.

y ventricular pressure rises rapidly without a

change in ventricular volume (i.e., no ejection

occurs)


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Rapid Ejection (Phase 3)


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y Aortic and Pulmonic Valves Open; AV Valves Remain Closed

y This phase represents the initial and rapid ejection of bloodinto the aorta and pulmonary arteries from the left and rightventricles, respectively

y Ejection begins when the intraventricular pressures exceedthe pressures within the aorta and pulmonary artery, whichcauses the aortic and pulmonic valves to open

y maximal (systolic) aortic and pulmonary artery pressures areachieved


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y No heart sounds are ordinarily noted during ejection because

the opening of healthy valves is silent.

y

The presence of sounds during ejection (i.e.,ejectionmurmurs) indicate valve disease or intracardiac

shunts


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Reduced Ejection (Phase 4)


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y Aortic and Pulmonic Valves Open; AV Valves Remain Closed

y Approximately 200 msec after the QRS and the beginning ofventricular contraction, ventricular repolarization occurs as

shown by theT-wave of the electrocardiogram

y Repolarization leads to a decline in ventricular active tensionand therefore the rate of ejection (ventricular emptying)

falls.

y Ventricular pressure falls slightly below outflow tract

pressure; however, outward flow still occurs due to kinetic

(or inertial) energy of the blood.


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Isovolumetric Relaxation (Phase 5)


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y All Valves Closed

y When the intraventricular pressures fall sufficiently at the

end of phase 4, the aortic and pulmonic valves abruptly close

(aortic precedes pulmonic) causing the second heart

sound (S2) and the beginning of isovolumetric relaxation

y Although ventricular pressures decrease during this phase,

volumes remain constant because all valves are closed.


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y The volume of blood that remains in a ventricle is called

the end-systolic volume and is ~50 ml in the left

ventricle

y The difference between the end-diastolic volume and the

end-systolic volume is ~70 ml and represents the stroke

volume.


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Rapid Filling (Phase 6)


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y A-V Valves Open

y As the ventricles continue to relax at the end of phase 5, the

intraventricular pressures will at some point fall below their

respective atrial pressures. When this occurs, the AV valves

rapidly open and ventricular filling begins


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Reduced Filling (Phase 7)


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y A-V Valves Open

y As the ventricles continue to fill with blood and expand, they

become less compliant and the intraventricular pressures

rise. This reduces the pressure gradient across the AV valves

so that the rate of filling falls.

y In normal, resting hearts, the ventricle is about 90% filled by

the end of this phase. In other words, about 90% of

ventricular filling occurs before atrial contraction (phase 1).


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Heart sounds


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y Produced by the closure of the valves

y It is caused by the vibration of the taut valves immediately

after the closure, along with the vibration of the adjacentblood, walls of the heart and major vessels around heart.

y 1st heart sound (S1)

y Produced by the closure of the AV valves

y Heard at the onset of the ventricular systole

y Duration of 0.14 sec

y Low pitched sound

y Best heard at the apex of the heart


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y 2nd heart sound (S2)

y Produced by the closure of the semilunar valves

y

Heard at the end of ventricular systoley Duration 0.11 sec

y High pitched sound

y Best heard at left sternal edge

y Splitting of second heart sound : aortic component heard earlier

then the pulmonary component because left ventricular

contraction preceeds that of right ventricular contraction

y 3rd heart sound (S3)

y Low pitched rumbling sound heard at the beginning of the

middle third of diastole

y It is produced by the oscillation of blood back and forth

between the walls of ventricles initiated by inrushing of the

blood from atria


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yDifficult to be heard by the ear

yNormal finding in children, young adults and

pregnancyy Pathological if heard above the age of 40 years

y F

our

th hear

t sound (a

tria

l hear

t sound / S4)y It is soft and low pitched

yAlways pathological and is caused by the forceful atrial

contraction against non compliant or stiff ventricle

yHeard just before first heart sound coinciding with theatrial contraction

yHeard with left ventricular hypertrophy

(hypertension, aortic stenosis)

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Cardiac Cycle 2003