Electrical activity of the Heart 2

Question 1

Cardiac Conduction System

Answer 1

Purpose: To ensure that the propagation of action potentials (and therefore contraction of cardiac muscle) happens in the coordinated sequence and direction required for optimum ejection of blood

Question 2

Coordination of Action Potentials

Answer 2

Conduction of action potentials through the heart must be coordinated so that:
• Atrial excitation and contraction are complete before ventricular contraction begins
• Excitation of cardiac muscle fibres is coordinated so each atrium and each ventricle contracts as a unit
• Each pair of atria and pair of ventricles needs to be coordinated so that each pair contract simultaneously

Question 3

Sinoatrial node (SA node)

Answer 3

Sinoatrial node (SA node) in right atria near opening of superior vena cava pacemaker of the heart - *pacemaker*
- DEPOLARISES FASTEST IN HEART

Question 4

Atrioventricular node (AV node)

Answer 4

Atrioventricular node (AV node) small bundle of cells located at base of right atrium near septum

Question 5

Bundle of His (atrioventricular bundle

Answer 5

Bundle of His (atrioventricular bundle) cells originate at AV node and enters interventricular septum

Question 6

Purkinje fiber

Answer 6

Purkinje fibers terminal fibers that extend from bundle of His and spread throughout ventricular myocardium

Question 7

Electrically nonconductive fibrous tissue

Answer 7

TISSUE DOESN’T ALLOW AP TO SPREAD THROUGH

THEREFORE AP CAN ONLY SPREAD THROUGH NODES

Question 8

Cardiac Conduction System: pattern of AP spread

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Answer 8

1. Cardiac action potential originates at the SA node
2. AP spreads throughout right and left atria
3. AP can only pass from atria into ventricles via AV node, does so after a brief delay (which allows atrial contraction to complete ventricular filling before ventricular contraction begins)

Question 9

Cardiac Conduction System: pattern of AP spread
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2

Answer 9

4. AP travels rapidly down interventricular septum via Bundle of His, then rapidly throughout myocardium through Purkinje fibres (ventricle begins contracting from base upward)
5. Remainder of ventricular cells activated by APs moving through gap junctions (completes ventricular contraction and ejection)

Question 10

The electrocardiogram (ECG)

Answer 10

The ECG is sum of the all the electrical activity as it spreads through the heart
P wave represents atrial depolarization
QRS complex represents ventricular depolarization
T wave represents ventricular repolarization
PR segment represents the AV node delay\

Question 11

ECG

Answer 11

TP interval = Time during which ventricles are relaxing and filling
ST segment =
Time during which ventricles are contracting and emptying
PR segment = AV nodal delay

Question 12

ECG leads

Answer 12

Each ‘lead’ gives a different viewpoint, but with the same pattern: P,QRS,T…

• ECG allows us to identify abnormalities