ECG Interpretation USMLE HIGH YIELD

1. ECG Interpretation: Overview
The electrocardiogram (ECG) is a graphic representation of the electrical activity of the
heart recorded from the surface of the body
Approach to the ECGs:
Rate
Normal = 60-100bpm (atrial rate: 150-250bpm = paroxysmal tachycardia, 250-350bpm
= atrial flutter, >350bpm = atrial fibrillation)
Regular Rhythm
To calculate the rate, divide 300 by number of large squares between 2 QRS complexes
(there are 300 large squares in 1minute: 300×200 msec = 60sec)
Or remember 300-150-100-75-60-50-43 (rate falls in this sequence with the number of
additional large squares between QRS)
Rhythm
Regular = R-R interval is the same across the tracing
Irregular = R-R interval varies across the tracing
Regularly-irregular = repeating pattern of varying R-R intervals
Irregularly-irregular = R-R intervals vary erratically
Normal Sinus Rhythm (NSR)
P wave precedes each QRS; QRS follows each P wave
P wave axis is normal (positive in leads I, II, aVF)
Rate between 60-100bpm
Axis indicates the direction of the mean vector
Can be determined for any waveform (P, QRS, T)
The standard ECG reported QRS axis usually refers to the mean axis of the frontal
plane; it indicates the mean direction of the ventricular depolarization forces
QRS axis transition from negative to positive is usually in lead V3

2. QRS axis in the frontal plane:
Normal axis: -30 degrees to 90 degrees (I.e. positive in leads I and II)
Left Axis deviation (LAD): Axis <-30 degrees
Right Axis deviation (RAD): axis >90 degrees
Intervals of the various waveforms (P, QRS, T) may indicate pathology.
Normal PR interval = 120-200 ms (<1 large square)
Normal QRS interval = 0.075-0.11 ms (<3 small squares)
Normal QT interval = <440 ms (or <1/2 of the R-R interval)
Ischemic Changes: ST depression/T wave inversion (most commonly in V1-V6)
Injury:
- Transmural (involving the epicardium) – ST Elevation in the leads facing the area
injured/infarcted
- Subendocardial – marked ST depression in the leads facing the affected area
ST ELEVATION IN CONTIGUOUS LEADS = Acute MI. Distribution of the ST pathology
on the EKG corresponds with location of lesion along coronary distribution.
Relation of ECG Changes to Location of Infarct
ECG Leads with ST
Changes
Area of
Infarct
Coronary Artery Branch
II, III, aVF Inferior
Posterior Descending or Marginal
Branch
I, aVL, V4, V5, V6 Lateral
Left Anterior Descending or
Circumflex
V1, V2, V3 Septal Left Anterior Descending
V2, V3, V4 Anterior Left Anterior Descending
V1, V2 Posterior Posterior Descending

3. “Typical” sequential changes of evolving MI
Hyperacute T waves (tall, symmetric T waves) in the leads facing the infarcted area
w/wo SST elevation
ST elevation in contiguous leads (usually in the first hours post infarct)
Significant Q waves (>40msec or >1/3 of total QRS) hours to days post-infarct
Inverted T waves one day to weeks after infarct
Completed infarction results in abnormal Q waves (again Q/QRS ratio is >33%)