ECG

How does an AP travel through the heart?

1. AP generated in the sino-atrial node - primary heart pacemaker

2. Atrial depolarisation (wave of depolarisation/contraction)

3. AP delayed at the atrioventricular node

4. AP travels down the septum, to the apex of the heart and around the ventricles

5. Wave of AP causes wave of contraction

6. Wave of repolarisation (relaxation) follows wave of depolarisation

What is the P wave?

Atrial depolarisation/contraction

Positive deflection, wave of depolarisation direction is parallel to lead II

what is the PR interval?

AVN delay

No deflection, no net current flow

What is the QRS wave?

ventricular depolarisation/contraction

Why is the Q wave negative?

Septal depolarisation, occurs left to right in body, so vector away from lead II

But small magnitude

What is the R wave?

Ventricular depolarisation towards apex, parallel to lead II so positive deflection

depolarisation towards positive electrode

Why is the S wave negative?

Last ventricular region to depolarise: from endocardium to epicardium

Starts pointing more towards the left, away from lead II, so slight negative deflection

What is the ST segment?

beginning of ventricular relaxation, no net current flow so no deflection

What is the T wave?

Ventricular repolarisation

Direction of repolarisation is away from electrode of lead II so positive deflection

Starts at apex, epicardium, endocardium

Describe T wave inversion in NSTEMI?

Normal T wave = repolarisation from epi to endo cardium = away from positive electrode = positive deflection

 

In NSTEMI, sub endocardium ischemia = open KATP channels = faster repolarisation

Repolarisation from endo to epi cardium = towards positive electrode = negative deflection