12 lead ECG

Reading ECG
rate
rhythm
electrical axis
conduction blocks/intervals
electrolyte disturbances
prior MI/ischemia/infarction

ECG is snapshot.  Holter monitor is continuous recording ECG.

Sudden death in young athletes - some are due to channelopathies; e.g., long QT syndrome.


Leads I, II, III


Lead II can utilize left leg as well as right.
Memory tool
Lead I = RA - LA  (one L)
Lead II = RA - LL (two Ls) 
Lead III = LA - LL (three Ls)

How you go from Einthoven's triangle to hexaxial reference system

 
Precordial Leads

V1 through V6 look at different regions of the heart and relate to different branches of the coronary circulation

depolarization heading toward positive electrode = upward deflection.

depolarization heading away from positive electrode = upward deflection.

depolarization heading perpindicular to positive electrode = no net upward or downward deflection (isoelectric QRS). Very useful in identifying mean electrical axis.




R wave progression in leads V1 to V6 useful in identifying pathology; e.g., heart block, arrythmia.

ECG is a summary of all the action potentials of the different parts of the heart:

P wave = right and left atrium.  3 small boxes.  each box = 0.04 seconds = 0.12 seconds
PR interval = measure of atrial to ventricle conduction.  > 5 small boxes = 1st degree AV block.
QRS complex = depolarization of ventricles - should be 3 small boxes
T wave = repolarization of ventricles Why is repolarization also positive?  Because it goes in the opposite direction from depolarization (repolarization going away from + electrode)

Chart speed is 25 mm/sec.  25 mm/sec x 60 sec/min = 1500 mm/min.

Heart rate = 1500 mm/min /mm between R waves 

If there are 10 mm between R waves, HR is 1500/10 = 150 beats/min.

RV is closer to sternum than LV.  This is why pulmonic valve closure sounds as loud as aortic valve closure.

Q wave = dead myocardium

ST segment = ischemia or infarction

elevation = early (ischemia)

depression = late (infarction)

reciprocal changes in ST segment occur in leads on opposite side of event.


changes, if real, must show up in contiguous leads.