READING AN EKG

1.      Rate – if regular, then count the number of large squares between R waves; 1 square = 300 bpm, 2 = 150 bpm, 3 = 100 bpm, 4 = 75 bpm, 5 = 60 bpm, 6 = 50 bpm.  Each small box = 0.04 s, each large box = 5 small boxes = 0.20 s.

2.      Rhythm – is it regular? (use calipers/ruler to make sure all R-R intervals are the same); are there P waves, and are they in front of every QRS? (in sinus rhythm, P waves will be upright in lead II); are P waves all identical?

3.      Intervals
˛         PR interval: normally 0.12 to 0.20 seconds (will not exceed a large box)
˛         QRS interval: normally 0.04 to 0.10 seconds (no larger than half a large box)
˛         QT interval: should be less than half the R-R interval (if HR < 100)

4.      Axis deviation – net QRS deflection should be positive in both leads I and aVF
˛         Right axis deviation: QRS negative in I, positive in aVF
˛         Left axis deviation: QRS positive in I, negative in aVF

5.      Hypertrophy

˛         Left ventricular hypertrophy: sum of deepest S in V1 or V2 and tallest R in V5 or V6 > 35 mm (patients > 35 yo); R in aVL > 12 mm indicative of “strain”.
˛         Left atrial enlargement: P waves are notched (M-shaped) in I, II, or aVL or a deep terminal negative component to P in V1
˛         Right atrial enlargement: tall, peaked P waves (> 2.5 mm) in II, III, aVF
˛         Right ventricular hypertrophy: right atrial enlargement, right axis deviation, incomplete RBBB, low voltage tall R wave in V1, persistent precordial S waves, right ventricular strain are all suggestive.

6.      Infarction

˛         Q waves: small, normal Q waves can be seen in lateral leads (I, aVL, V4 to V6), while moderate-large sized Q waves may be normal in leads III, aVF, aVL, and V1.  To localize the infarction, look for groupings of Q waves in the following leads…

Inferior

II, III, aVF

Anteroseptal

V1 to V3

Anterior

V3 and V4

Anterolateral

V4 and V6, I, aVL

Posterior

V1 and V2

˛         R wave progression : transition should occur between V2 and V4.
˛         ST segment elevation or depression: remember that ischemia is associated with ST depression, while infarction is associated with ST elevation.  Look for changes in two adjacent leads.
˛         T wave inversion: may be normally inverted in III, aVF, aVL, and V1.  T wave inversion indicates areas of ischemia in Q wave infarctions.

7.         Heart Block (AV block)

˛         1st Degree AV block: PR interval > 0.20 sec
˛         2nd Degree AV block
                                                                    i.      Mobitz I: (Wenkeback) PR interval progressively widens until a beat is dropped
                                                                  ii.      Mobitz II: PR interval is prolonged, randomly dropped beatŕNeeds Pacemaker
˛         3rd Degree AV Block: No connection (dissociation) between atrial and ventricular ratesŕNeeds Pacer

Other pearls

˛         Hypokalemia: ST depression, decreased or inverted T waves, U waves
˛         Hyperkalemia: peaked T waves, decreased P waves, short QT, widened QRS, sine wave
˛         Hypocalcemia: prolonged QT, flat or inverted T waves
˛         Hypercalcemia: short or absent ST, decreased QTc interval
˛         Hypomagnesemia: prolonged QT, flat T waves, prolonged PR, aFib, torsade
˛         Hypermagnesemia: short PR, heart block, peaked T waves, widened QRS0
˛         Digitalis toxicity: ST depression (scoop), flat T waves
˛         Quinidine: prolonged QT, widened QRS
˛        Pericarditis: diffuse ST elevation with PR interval depression