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The cardiac cycle

Describing the sequence of events in one heart beat

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Cardiac Contraction

Contraction is initiated byaction potentials thatnormally originates in SAnode in right atrium

Action potentialdepolarizes cells, causingcontraction

Sequence of contraction-

Atria and ventricles AV node delays impulses

by 0.1 seconds.

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Definitions

Systole= Begins with ventricular contractionand ends when ejection ceases.

Diastole= Begins when ejection ceases asventricles relax, ventricular filling starts after

sufficient relaxation occurs. Duration = Each Cardiac cycle - 0.80 sec,

systole 0.27,diastole 0.53 (HR = 75/MIN)

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Cardiac cycle

General Principles

Contraction of the myocardium generates pressurechanges which result in the orderly movement ofblood.

Blood flows from an area of high pressureto anarea of low pressure, unless flow is blocked by avalve.

Events on the right and left sides of the heart are thesame, but pressures are lower on the right.

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Phases of cardiac cycle

Phase 1 ATRIAL CONTRACTION

Phase 2 ISOVLUMETRIC CONTRACTION

Phase 3 RAPID EJECTION Phase 4 REDUCED EJECTION

Phase 5 ISOVOLUMETRIC RELAXATION

Phase 6 RAPID FILLING

Phase 7 REDUCED FILLING

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Atrial

systole

The heart is full of bloodand the ventricles arerelaxed (diastole)

Both the atria contract andblood passes down to theventricles

70% of the blood flowspassively down to theventricles so the atria do nothave to contract a great

amount Narrows the vena cava

orifices but some there issome regurgitation of bloodin to the veins.

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Atrial Diastole

Blood continuously enter

atria except when

transiently impeded during

atrial systole

Occurs throughout

ventricular systole and

diastole until p wave on

ECG.

AV valve suddenly openswhen ventricular pressure

falls below atria (creating v

wave)

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Ventricular

Systole

AV valve closes

Isovolumetric ventricularcontraction(0.05 sec), AV valvebulges.

Pressures in ventriclesexceeds than aorta(80 mmhg)and pulmonaryartery(10mmhg).

Aortic and Pulmonary valvesopen, rapid ejection phase

begins. Pressures in ventricles rises to

120 and 25 mmhg (left andright

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Ventricular Diastole

Aortic and pulmonary valvescloses.

Isovolumetric relaxation.

Ventricular pressures fallsbelow atrial and AV valves

opens, permitting ventricles tofill.

Filling is rapid at first, thenslows as next cardiaccontraction approaches.

Atrial pressures continues to

rise after vent. systole until AVvalves open and slowly risesagain until the next atrialsystole.

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Heart Sounds

FirstLow pitched, slightlyprolonged ``LUB, caused byvibrations due to suddenclosure of AV valves.

SecondHigh pitched, shorter``DUB caused by vibrationsassociated with closure ofsemi lunar valves.

ThirdSoft, low pitched dueto rapid ventricular filling

(normal in young) Fourthsome times heard

before first heard sound whenatrial pressures are high (rarein normal individuals)

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STROKE VOLUME

Ventricular stroke volume is

difference between

ventricular end diastolic

(EDV) and end systolic

volumes (ESV)

SV(70 to 90) = EDV(130) -

(50)ESV

In normal heart stroke

volume is the volume ofblood as ejected in to aorta

during each contraction.

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Ejection Fraction (EF)

EF is fraction of blood ejected by ventricle

relative to its filled volume (EDV)

EF = SV / EDV

EF is a measure of ability of heart to eject

blood.

Normally about .55 to .65 (55 to 65%)

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Length of systole and diastole

High heart rate- duration of systole decreases from 0.27 (65) to

0.16 (200 beats/min)

Duration of diastole decreases from 0.62 seconds to 0.14 (200

beats/min)

This fact has important clinical and physiological implicationsbecause during diastole heart muscle rests, coronary blood flow to

subendocardial portion of LV occurs and most of ventricle filling

occurs.

At heart rates up to 180, filling is adequate as long as there is ample

venous return and CO per minute is increased by an increase inrate.

At very high HR, filling may be compromised to such a degree that

CO per minute falls.

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TH NKS