Physiology of the Cardiac Electrical System

Question 1

Electrical conducting system of the heart

Answer 1

• composed of a system of fast conducting specialized cardiac muscle cells
• divided into atrial conducting system and ventricular conducting system
• separated by the cardiac skeleton

Question 2

Cardiac skeleton

Answer 2

Band of fibrous tissue that does not conduct electrical activity

Question 3

What structure pierces the cardiac skeleton?

Answer 3

The bundle of his (therefore this carries the electrical signal into the ventricular conducting system)

Question 4

Property of all conducting cells

Answer 4

Capable of self-depolarizing

Question 5

Inherent rate of self depolarization in different conducting cells

Answer 5

-inherent rate of self depolarization slows the further away they are from the SA node
60-100 bpm = SA
45-50 bpm = AV
35-40 bpm = perkinje fibers

Question 6

After passing through the conducting system where does the electrical current go

Answer 6

-the electrical current enters the cardiac muscles and moves cell to cell through gap junctions (this is a relatively slow process)

Question 7

Basis of Electrocardiogram recording

Answer 7

-as the wave of depolarization spreads from cell to cell across the entire heart, electrical forces are generated in each cell and if you put recording electrodes at the surface of the skin you can record the sum of these forces as an ECG

Question 8

Einthoven’s triangle

Answer 8

-position of bipolar limb leads
Lead I: LA + RA-
Lead II: LL + RA -
Lead III: LL + LA-

Question 9

Unipolar limb leads arrangment

Answer 9

RA + LA&LL - (aVR)
LA + RA&LL - (aVL)
LL+ RA&LA - (aVF)

Question 10

What do bipolar and unipolar limb leads measure

Answer 10

Both measure ECG in the frontal plane

Question 11

Precordial chest leads

Answer 11

Measure ECG in the horizontal plane

Question 12

How electrical activity is translated into an ECG

Answer 12

1. No electrical activity detected by the recording electrodes = pen moves along a flat line (iso-electric line)
2. If wave of depolarization moves towards the positive end of the lead the pen deflects upwards (away from the isoelectric line)
3. If the wave of depolarizatin moves away from the positive end of the lead the pen deflects downwards (away from the isoelectric line)

Question 13

ECG waveforms + what correspond to

Answer 13

P = atrial depolarization
QRS = ventricular depolarization
T = ventricular repolarization

Question 14

Atrial depolarization in leads I, II, III

Answer 14

lead I: +–> - LA to RA
-depolarizing towards LA therefore + deflecton
lead II + —> - LL to RA
-depolarizing towards LL therefore + deflection
lead III + —> - LL to LA
depolarizing towards LL therefore + deflection

For the simple waveform p

Question 15

Ventricular depolarization (QRS)

Answer 15

Lead I LA --> RA
positive deflection
Lead II LL --> RA
positive deflection
Lead III LL --> LA
positive deflection

Question 16

Ventricular repolarization - why does it always result in an upward pen deflection (if it is an opposite electrical event to depolarization)

Answer 16

-the last cells to depolarize (epicardium) are the first to repolarize
SINCE WAVE OF REPOLARIZATION IS IN THE OPPOSITE DIRECTION TO DEPOLARIZATION the QRS will be the same direction as T