THE CARDIAC CYCLE
What 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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Dr Nyoman W / Dr DENNY AGUSTININGSIH
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Electrocardiography
TWO SEPARATE PUMPS
Atria
Ventricles
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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 wall
cerscent
LV wall
3x thicker
cylindrical
Interventricular
septum
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Cardiac output distribution
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