16. ECG Reading Made Easy

Be systematic and always compare to old EKG:  

  1. Rate, rhythm.
    1. Machine rate usually accurate. Caution: tall t waves in hyperkalemia, and tall p waves (e.g., bad COPD) can be read by computer as QRS complexes.
    2. Look at rhythm strip to eval for arrhythmias.
  2. Intervals, axis.
  3. Chamber size.
    1. p waves for atrial abnormalities.
    2. QRS for LVH.
  4. Ischemia: see ACS sections.

Rhythm

  • Sinus rhythm: normal p wave before each QRS. Normal p wave axis (upright in I, II, aVF, down in aVR).
  • Supraventricular tachycardia or narrow complex tachycardia:
    • Regular: sinus tachycardia, atrial tachycardia, AVNRT, AVRT, junctional tachycardia, atrial flutter).
    • Irregular: atrial fibrillation, MAT (multifocal atrial tachycardia), atrial flutter with variable conduction (atrial rate >240 and usually ~300).
  • Wide complex tachycardia (VT, SVT with LBBB): see section Diagnosis of Wide-Complex Tachycardia.
  • Polymorphic VT or Torsades de Pointes.
  • Ventricular fibrillation.

Differential Diagnosis of Axis Deviations (in order of likelihood).

Right Axis

Left Axis

1. RVH or Strain

1. Left anterior fascicular block

2. Lateral or anterolateral MI

2. Inferior MI

3. WPW with left freewall pathway

3. WPW with posteroseptal pathway

4. Left posterior fascicular block

4. LVH

Right atrial abnormality (only 1 criteria needed) 

lead II              P > 0.25 mV or > 25% QRS amplitude

lead V1             P > 0.15 mV

Left atrial abnormality (only 1 criteria needed)

lead II              P > 120 msec with notches separated by at least 1 small box

lead V1             P wave has a negative terminal deflection that is 40 msec by 0.1 mV

Left ventricular hypertrophy

There are numerous criteria; three useful ones below. All are specific but all are insensitive, so fulfillment of one set is sufficient for LVH (applies to > 35 y.o.)

R in aVL                                  >11 mm (men), >9 mm (women)

Rin aVL + S in V3                    >20 mm (women) and >25 mm (men)

Sin V1 + (R in V5 or R in V6)   >35 mm

Right ventricular hypertrophy

The following findings suggest RVH; there are several others.

Right axis deviation

R in V1 + S in V6 > 11 mm

R:S ratio > 1 in V1 (in absence of RBBB or posterior MI)

RBBB (Right Bundle Branch Block)

QRS > 120 msec

Wide S wave in I, V5, V6

Secondary R wave (R’) in right precordial leads with R’ greater than initial R (rsR’ or rSR’).

LBBB (Left Bundle Branch Block)

QRS > 120 msec, broad R in I and V6, broad S in V1 and normal axis or

QRS > 120 msec, broad R wave in I, broad S in V1, RS in V6, and left axis deviation.

LAFB(Left Anterior Fascicular Block)

There are several sets of criteria for LAFB

Axis is more negative than –45 degrees

Q in aVL, and time from onset of QRS to peak of R wave is >0.05 s.

Also helpful is QI, SIII pattern

LPFB (Left Posterior Fascicular Block; must rule out anterolateral MI, RVH, RBBB)

Axis >100 and QIII, SI pattern

Q Waves

Use the following screening mantra, courtesy of Tom Evans, M.D.  V1, V2, V3: "any, any, any"; V4, V5, V6: "20, 30, 30"; I, II, aVL, aVF: "30, 30, 30, 30"; V1, V2: "R > 40, R > 50".  Numbers refer to width of Q wave in milliseconds.