Directions of waves of the ECG
Depolarisation towards electrode - Positive
Repolarisation towards electrode - Negative
Depolarisation away from electrode - Negative Repolarisation away from electrode - Positive
If depolarisation is perpendicular, produces a biphasic curve
N.B. depolarisation spreads out across the wall of the ventricle, repolarisation spreads inwards
What determines wave amplitude of the signal on an ECG
Mass of myocardium being measured (larger muscles produce large signals)
Rate of depolarisation or repolarisation (depolarisation produces larger signals)
Which limb leads look at the inferior surface of the heart?
Lead II, aVF, (lead II)
Which limb leads look at the right side of the heart?
aVR, lead III
Which limb leads look at the left side of the heart?
aVL, lead I, lead II
Leads that look at the right atrium
V1, V2, aVR
What characteristic should you see in the ECG from V1-V6
Progression of the R wave from prominently downward (V1) to prominently upwards (V6)
Conduction from the atria to the ventricles (from SA node>AV node>bundle of His)
ventricular depolarisation and repolarisation
varies with heart rate
How to report an ECG
Description of QRS
Description of ST segments and T waves
First degree heart block
Prolonged PR interval
Delay in the pathway from SA node to ventricles
Second degree heart block
Failure of excitation to pass through the AV node or bundle of His
Mobitz type 2: constant PR interval with dropped QRS complex
Wenckebach: progressive lengthening of PR interval with dropped QRS
2:1 type : two pay waves per QRS, normal constant PR interval
Third degree heart block
Occurs when atrial contraction is normal but no beats are conducted to the ventricles.
No relationship between P waves and QRS complex
Right bundle branch block
No conduction down the right bundle branch but septum depolarised from left to right as normal.
RSR' pattern in V1, QRS complex with deep S wave in V6
Nrmal cardiac axis
Left bundle branch block
Conduction down the left bundle branch ffails. Septum becomes depolarised from right to left.
Broad M pattern QRS complex in V4-V6
Left axis deviation
Could be due to aortic stenosis or ischaemic disease
Which leads are used to derive the cardiac axis?
Leads I, II and III QRS complex.
Upward deflections seen in all three leads, most prominent in lead II in a normal axis.
RV hypertrophy causes right axis deviation, prominent upward deflection in lead III
LV hypertrophy causes left axis deviation. Results in negative deflection in lead II and lead III
What happens to the cardiac action potential in ischemia?
Shortened in ischemia because the compromised metabolic state opens K+ channels which cause hyperpolarisation.
Causes current flow between ischemic and normal myocardium during systole, giving rise to a signal during QRS and T
ECG changes in exercise induced ischemia
Ischaemic region returns to rest during the ST interval but the non-ischaemic region is still active.
Characteristic of acute MI.
Severe transmural myocardial ischemia. Ischemic region returns to rest in ST interval but unaaffected part of the heart is still depolarised.
Current flows away from electrode in lead II.
Regions of the heart in ECG
Anterior heart supplied by LAD artery
Anterolateral - diagonal branch of LAD
Lateral heart supplied by Left circumflex
Inferior heart supplied by right coronary
Acute posterior MI
ST depression in V1-V3 (look at anterior heart)
Tall R wave in V1 and V2
ECG changes seen with progression of STEMI
Before - normal ECG
Severe ischemia - ST elevation, Peak in T wakes, followed by T wave inversion
Infarction - Loss of R-wave progression, less ST elevation, development of pathological (wide) Q waves
Fibrosis - normal ST segment and T wave, wide W waves persist.
ECG changes seen with NSTEMI
Before - normal ECG
Ischemia - ST depression and T wave inversion
Infarction - ST depression and T wave inversion
Fibrosis - ST returns to baseline, T wave inversion persists.