1OCT 2009 Dr Nyoman W / Dr DENNY AGUSTININGSIH

THE CARDIAC CYCLEWhat is the cardiac cycle.

The cardiac cycle is the sequence of events that occur when the heart beats. There are two phases of this cycle:

Diastole - Ventricles are relaxed. Systole - Ventricles contract.

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Electrocardiography

Atria

Ventricles

TWO SEPARATE PUMPS

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Heart Sounds• are Produced by the Closing of the Valves• Normal heart sounds are produced when valves snap

closed: LUB-DUP • LUB = closing of AV valves: beginning of systole • DUP = closing of semilunar valves: end of systole • Abnormal valve sounds: – Leakage of valve -> swishing sound (murmur) – Narrowing of valve (stenosis) -> high pitched sound

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Heart sounds:

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Ventricular Volumes

• End Diastolic Volume-(EDV)– volume in ventricles at the end of filling

• End Systolic Volume- (ESV)– volume in ventricles at the end of ejection

• Stroke volume (EDV-ESV)– volume ejected by ventricles

• Ejection fraction– % of EDV ejected (SV/EDV X 100%)– normal 50-60%

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Cardiac Output• Normally about 5 liters/min • The cardiac output per minute (CO) is the product of

the size of a single output, the stroke volume (SV), and the heat rate (HR) in beats/minute: – CO = HR X SV – = 70 beats/min X .07 liters/beat = 5 liters/min

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Factors Affecting Cardiac Output

• Rate:– Factors affecting HR– Last topics

• Rhythm– Sinus rhythm from SA node– Arrhythmia

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Factors Affecting Cardiac Output

• Preload– Equal to EDV– Depends on Venous Return– Frank-Starling Law– Contractility

• Afterload– TPR

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VENOUS RETURN

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Muscle pump

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Frank-Starling Law of the Heart

• Relationship between EDV, contraction strength, and SV.

• Within physiologic limits the heart will pump all the blood that returns to it without allowing excessive damming of blood in veins

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Mechanism of Frank-Starling

• Increased venous return causes increased stretch of cardiac muscle fibers. (Intrinsic effects)– increased cross-bridge formation– increased calcium influx• both increases force of contraction

– increased stretch on SA node• increases heart rate

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Extrinsic Control of Contractility

• Contractility:– Strength of contraction at

any given fiber length.

• Depends upon sympathoadrenal system:– NE and Epi produce an

increase in contractile strength. • + inotropic effect:

– More Ca2+ available to sarcomeres.

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TPR

• Total Peripheral Resistance:– Impedance to the ejection of blood from ventricle.– Afterload.

• In order to eject blood, pressure generated in the ventricle must be greater than pressure in the arteries.

– Pressure in arteries before ventricle contracts is a function of TPR.

• SV inversely proportional to TPR.– Greater the TPR, the lower the SV.

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RV wallcerscent

LV wall3x thickercylindricalInterventricular

septum

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Cardiac output distribution