Cardiac cycle physiology_4_dpt

Cardiac Cycle

By

Mubin Mustafa Kiyani

Cardiac cycle

The Conducting System

• Heartbeat

– A single contraction of the heart

– The entire heart contracts in series

• First the atria

• Then the ventricles


• Structures of the Conducting System

– Sinoatrial (SA) node - wall of right atrium

– Atrioventricular (AV) node - junction between

atria and ventricles

– Conducting cells - throughout myocardium

The Conducting System• Conducting Cells

– Interconnect SA and AV nodes

– Distribute stimulus through myocardium

– In the atrium

• Internodal pathways

– In the ventricles

• AV bundle and the bundle branches


Figure : The Conducting System of the Heart


• Heart Rate

– SA node generates 80–100 action potentials per

minute

– AV node generates 40–60 action potentials per

minute


• The Sinoatrial (SA) Node

– In posterior wall of right atrium

– Contains pacemaker cells

– Connected to AV node by internodal pathways

– Begins atrial activation (Step 1)


Figure : Impulse Conduction through the Heart


• The Atrioventricular (AV) Node

– In floor of right atrium

– Receives impulse from SA node (Step 2)

– Delays impulse (Step 3)

– Atrial contraction begins


Figure: Impulse Conduction through the Heart


• The AV Bundle

– In the septum

– Carries impulse to left and right bundle branches

• Which conduct to Purkinje fibers (Step 4)

– And to the moderator band

• Which conducts to papillary muscles


• Purkinje Fibers

– Distribute impulse through ventricles (Step 5)

– Atrial contraction is completed

– Ventricular contraction begins

An Overview of Cardiac Physiology

Cardiac Cycle

The cardiac events that occur from the

beginning of one heartbeat to the beginning

of the next are called the cardiac cycle.

• The cardiac cycle consists:

– Period of relaxation - diastole,

• during which the heart fills with blood,

– Followed by a period of contraction called systole.

• during which heart ejects blood

Cardiac Cycle

• During systole there is contraction of the cardiac muscle and pumping of blood from the heart through arteries.

• During diastole, there is relaxation of cardiac muscle and filling of blood.

• Various changes occur in different chambers of heart during each heartbeat. These changes are repeated during every heartbeat in a cyclic manner.

• Each cycle is initiated by spontaneous generation of action potential in the S.A node.

Divisions of cardiac cycle• The cardiac cycle consists of a period of relaxation called

diastole, during which the heart fills with blood this period is followed by a period of contraction called systole.

• The contraction and relaxation of atria are called atrial systole and atrial diastole respectively.

• The contraction and relaxation of ventricles are called ventricular systole and ventricular diastole respectively.

Requirements for Efficient Cardiac Contraction

1. Atrial excitation and contraction need to be complete before ventricular contraction occurs.

2. Excitation of cardiac muscle fibers should be coordinated so that each chamber contracts as a unit.

3. Pair of atria and pair of ventricles should be coordinated so that both members of the pair contract simultaneously.

• First assembled by Lewis in 1920 but first conceived by Wiggers in 1915

• It is named after Dr. Carl J. Wiggers, M.D. • A Wiggers diagram is a standard diagram used in cardiac physiology • The X axis is used to plot time, while the Y axis contains all of the

following on a single grid: • Blood pressure Aortic pressure

Ventricular pressure Atrial pressure

• Ventricular volume• Electrocardiogram• Arterial flow (optional)• Heart sounds (optional)

Cardiac cycle

Wiggers diagram

Phases of Cardiac cycle

1. Atrial systole

2. Isovolumic contraction

3. Ejection

4. Isovolumic relaxation

5. Rapid inflow

6. Diastasis

Phases of the Cardiac Cycle

Figure 20.16

The Cardiac Cycle

• Cardiac Cycle and Heart Rate

– At 75 beats per minute

• Cardiac cycle lasts about 800 msecs

– When heart rate increases

• All phases of cardiac cycle shorten, particularly diastole

The Cardiac CycleEight Steps in the Cardiac Cycle

1. Atrial systole • Atrial contraction begins

• Right and left AV valves are open

2. Atria eject blood into ventricles• Filling ventricles

3. Atrial systole ends • AV valves close

• Ventricles contain maximum blood volume

• Known as end-diastolic volume (EDV)

The Cardiac Cycle

Eight Steps in the Cardiac Cycle

4. Ventricular systole

• Isovolumetric ventricular contraction

• Pressure in ventricles rises

• AV valves shut

5. Ventricular ejection

• Semilunar valves open

• Blood flows into pulmonary and aortic trunks

• Stroke volume (SV) = 60% of end-diastolic volume

The Cardiac Cycle


6. Ventricular pressure falls

• Semilunar valves close

• Ventricles contain end-systolic volume (ESV), about 40% of end-

diastolic volume

7. Ventricular diastole

• Ventricular pressure is higher than atrial pressure

• All heart valves are closed

• Ventricles relax (isovolumetric relaxation)

The Cardiac Cycle


8. Atrial pressure is higher than ventricular

pressure

• AV valves open

• Passive atrial filling

• Passive ventricular filling

• Cardiac cycle ends

• a, c and v atrial pressure waves are noted in atria

• The ‘a’ wave is caused by the atrial contraction, during right atrial contraction pressure increases 4-6 mm of Hg, while in left atrium it is 7-8 mm of Hg.

• The ‘c’ wave occur when the ventricles begin to contract, it is caused partially by slight back flow of blood when there is ventricular contraction, but mainly by bulging the A-V valves backward toward the atria because of increasing pressure in ventricles

• The ‘v’ wave occur toward the end of ventricular contraction, this is caused by the slow flow of blood from great veins while the A-V valves are closed, when A-V valves opens allowing the store blood into ventricle this ‘v’ wave disappear.

Pressure changes in the atria

Wiggers diagram

Summary: Pathway of Heartbeat• Begins in the sinoatrial (S-A)

node• Internodal pathway to

atrioventricular (A-V) node • Impulse delayed in A-V node

(allows atria to contract before ventricles)

• A-V bundle takes impulse into ventricles

• Left and right bundles of Purkinje fibers take impulses to all parts of ventricles

KEY Red = specialized cells;

all else = contractile cells

Impulse Conduction through the Heart

Time Intervals

Total ventricular systole 0.3 sec• Isovolumic contraction (b) 0.05 sec (0.015sec for RV)

• Maximal ejection (c) 0.1 sec• Reduced ejection (d) 0.15 secTotal ventricular diastole 0.5 sec• Isovolumic relaxation (e) 0.1 sec• Rapid filling phase (f) 0.1 sec• Slow filling (diastasis) (g) 0.2 sec• Atrial systole or booster (a) 0.1 sec GRAND TOTAL (Syst+Diast) = 0.8 sec

Cardiac cycle

Cardiac cycle physiology_4_dpt

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