Lecture 3 - Electrical Conducting System of the Heart

Question 1

What are the different pacemakers of the heart?

Answer 1

Main one - Sinoatrial node
Seondary pacemaker - atrioventricular node
Backup pacemaker - all other electrically active myocytes

Question 2

Where is the action potential generated and how and where does it propagate?

Answer 2

Initiated in the SA node
Propagated to the AV node
Via - Inter nodal pathways in the atria

Question 3

How do cells of the AV node transmit action potential?

Answer 3

More slowly and delay the impulse by 100 ms

Question 4

Which is the only electrical connection between the atria and the ventricles. The impulse spreads down to the ventricles from this

Answer 4

Bundle of His

The AV node then branches off into Left and right bundle branches supplying LV and RV

Question 5

How do the impulses spread?

Answer 5

They spread through contractile cells of the ventricles through an extensive network of Purkinje fibres.

Question 6

What allow spread of AP throughout the myocardium like a functional syncitium?

Answer 6

Gap junctions in the intercalated discs.

Question 7

What happens in the different phases of a heart beat?

Answer 7

Phase 0 - Na+ channels open
Phase 1 - Na + channels close
Phase 2 - Ca2+ channels open, fast K+ channels close
Phase 3 - Ca2+ channels close - slow K+ channels open
Phase 4 - Resting potential

Question 8

What is depolarization and repolarization?

Answer 8

Depolarization - when sodium channels are open and sodium ions rapidly pass in.
Repolarization - when calcium channels open

Question 9

positions of pacemaker potentially altered by

Answer 9

sleep, exercise, emotions etc.

Question 10

Ventricular refractory period

Answer 10

200-300 ms

Question 11

Depolarization to an electrode has a …..

Answer 11

positive deflection

Question 12

Depolarization away from electrode has a …..

Answer 12

negative deflection

Question 13

Limb (Einthoven’s) leads are located where

Answer 13

right arm - left arm and both arms pass to leflt leg (triangle)

Question 14

Chest (Virchov’s) leads

Answer 14

6 leads
1 in 4th intercostal space to the right of the sternum, 2 in the 4th intercostal space to the left of the sternum, 4th in 5th intercostal space in midclavicular line, 3rd between the 2nd and 4th, 5th in midaxillary line in line with V4,
6th in line with V5 at midaxillary line lateral to it

Question 15

Where would you view the electrical activity of the heart

Answer 15

Frontal cardiac axis

90 degrees to most isoelectric limb lead. effective dominant ventricular dipole at V6

Question 16

PR interval

Answer 16

– Atrial depolarisation and conduction through the AV nodes

Question 17

QRS Interval

Answer 17

Ventricular depolarization and atrial repolarisation

Question 18

QT interval

Answer 18

Ventricular depolarization and repolarisation

Question 19

ST segment

Answer 19

Ventricular depolarization

Question 20

What is sinus arrhythmia

Answer 20

Often seen in children and young adults
Physiological slowing of the SA node rate during expiration caused by an increase in vagal activity
Tachycardia during inspiration and bradycardia during expiration
increase in heart rate during inspiration helps to preserve cardiac output

Question 21

What happens at T?

Answer 21

Ventricular repolarisation