Electrophysiology and the ECG

Question 1

What occurs in the heart during the P wave, QRS Complex and T wave

Answer 1

P wave = Depolarisation of the Atria
QRS = Depolarisation of ventricles
T wave = Repolarisation of the Ventricle

Question 2

Where does the action potential originate

Answer 2

The action potential originates at the SA node

Question 3

Where does the action potential spread to from the SA node

Answer 3

The action potential spreads throughout the right and left atria

Question 4

How does the AP pass from the atria into the ventricles

Answer 4

The AP can only pass from the atria into the ventricles via the AV node after a brief delay

Question 5

Why is there a brief delay of the action potential from the atria into the ventricles

Answer 5

The delay allows atrial contraction to complete ventricular filling before ventricular contraction begins

Question 6

Describe the different waves of the ECG trace, link them to the changes in the membrane potential of cardiac cells and the physical changes of the heart

Answer 6

1. P wave = SA node fires sending action potential towards AV node causing the depolarisation and contraction of the atria (pumping blood into the heart)
2. QRS, first the Q wave is caused by depolarisation of the left to right ventricle, R wave = is due to depolarisation of the Left Ventricle, S = wave is then due to the depolarisation moving to the upper, posterior part of the left ventricular, 2. T wave = repolarisation of ventricle occurring in the opposite direction to depolarisation causing a change from + to - away from the electrode resulting in a positive T wave

Question 7

Define common arrythmias, how they occur and identify them from ECG traces

Answer 7

1. Extrasystole beat,
2. Sinus arrhythmia
3. Heart block (1st degree) = PR interval >200ms
4. Heart block (2nd degree) Mobitz type 1 = PR interval increases eventually skips QRS, rarely causes serious symptoms
5. Heart block (2nd degree) Mobitz type 2 = Multiple P waves for QRS, Treatment with pacemaker, often progresses to 3rd degree heart block then leading to MI or heart disease

Question 8

Define what a ventricular vector is, draw Einthoven’s triangle, and draw a vector

Question 9

Identify intervals, PR, QRS, QT