L4 The Electrical Conducting System of the Heart

Question 1

where does the action potential start to cause an electrical impulse?

Answer 1

Heartbeat starts in specialised SAN to internodal pathways to AV node (which transmit AP more slowly + delay impulse by 100ms) to bundle of His to left and right bundle branches then to purkinje fibres

Question 2

what is an action potential?

Answer 2

Transient depolarization of a cell as a result of activity of ion channels

Question 3

what are the two main types of cardiac action potentials?

Answer 3

nonpacemaker and pacemaker action potentials

Question 4

what are the phases of no pacemaker action potentials?

Answer 4

Phase 0 Na+ channels open

Phase 1 Na+ channels close

Phase 2 Ca2+ channels open; fast K+ channels close

Phase 3 Ca2+ channels close; fast K+ channels open

Phase 4 Resting potential

Question 5

where are pacemaker cells found and what is pacemaker activity?

Answer 5

found in SAN

Pacemaker activity refers to: the intrinsic, spontaneous time dependent depolarisation of a cell membrane that leads to an action potential. Any cardiac cell with pacemaker ability can initiate a heartbeat

Question 6

what is the Primary Pacemaker defined as?

Answer 6

the tissue with the highest ‘firing’ frequency, in other words the fastest pacemaker sets heart rate and overrides all slower pacemaker tissues.

Question 7

what is the hierarchy of the pacemakers?

Answer 7

primary functional pacemaker - SAN w/intrinsic freq of 100bpm (although is under constant vagal stimulation suppressing frequency to approx. 60 bpm)

secondary functional pacemaker - AV node w/ 40bpm

tertiary functional pacemaker - purkinje fibres at 20bpm

Question 8

what are the conduction velocities of the atrial muscle, AV node, bundle of His, l+r bundle branches, purkinje fibers and ventricular muscle?

Answer 8

atrial muscle - 0.5m/sec 
AV node - 0.05m/sec
bundle of His - 2m/sec
l+r bundle branches - 2m/sec
purkinje fibers - 4m/sec
ventricular muscle - 0.5m/sec

Question 9

where are the chest leads of an ECG placed?

Answer 9

V1 – 4th ic space at right margin of sternum
V2 – 4th ic space at left of sternum
V4 – 5th ic space, midclavicular
V3 – in between V2 and V4
V5 – 5th ic space, anterior auxhillary line
V6 – 5th ic space midauxhillary space

Question 10

where are the limb leads of an ECG placed?

Answer 10

bony prominence
either shoulders and tops of pelvis OR
arms and legs

Question 11

what causes positive and negative deflections on an ECG?

Answer 11

Electrical activity towards an electrode results in a positive deflection on ECG

Electrical activity away from an electrode results in a negative deflection on ECG

Question 12

what do the boxes on an ECG represent?

Answer 12

1 small box
horizontal = 0.04 sec
vertical = 1mm and 0.1 mV

large box (5x5 boxes)
horizontal = 0.20 sec
vertical = 5mm and 0.5 mV

Question 13

what do the P wave, PR segment, QRS complex and T wave mean on an ECG?

Answer 13

P wave – atrial depolarisation
PR segment – conduction through AV node and AV bundle
QRS complex – depolarisation of ventricle (this depolarisation masks atrial repolarisation)
T wave – ventricle repolarisation

Question 14

what does ST elevation show?

Answer 14

myocardial ischaemia

ST Elevated Myocardial Infaction - STEMI

Question 15

what happens during atrial fibrillation?

Answer 15

atrium has irregular contraction. there is no P wave but there is still is a QRS complex as the ventricles contract normally due to the AV node delay

Question 16

what happens during atrial flutter?

Answer 16

not as disorganised as atrial fibrillation but still more contraction. ventricles look fine as QRS complex is normal

Question 17

what happens during ventricular fibrillation?

Answer 17

ventricles are fibrillating and is very disorganized. ECG looks like a scribble. patient loses consciousness

Question 18

what happens during ventricular tachycardia?

Answer 18

ventricles are contracting - too many too fast

not enough time for blood to fill in during diastole so hardly any blood pushed out. patients are sick and unconsciousness and sometimes don’t have a pulse

Question 19

what happens during 1st degree heart block?

Answer 19

prolonged PR interval and delay between atrial contraction and ventricular contraction.
(not uncommon - seen in older age)

Question 20

which rhythms are shockable with a defib?

Answer 20

ventricular tachycardia and ventricular fibrillation

Question 21

do the ventricles still beat during complete heart block?

Answer 21

yes as there are other cells along the conduction pathway capable of creating an action potential. HR will be slow. causes you to collapse

Question 22

what happens during 2nd degree AV block?

Answer 22

prolongation of PR complexes and missed beats

Question 23

what happens during 3rd degree AV block?

Answer 23

no connection between atria and ventricles so they do what they want when they want. inserting a pacemaker can repair it