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)
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| 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.
