Academic Day Seminars

ADS: Interpretation of ECG

Today felt like an ECG bootcamp – but in a great way, and quite timely as I am currently in ICU and have had patient’s w/ ECGs done (and no clue how to incorporate it in my workup) – so I’m looking forward to applying this when I’m back with my patients.

Step 1: Rate

  • Either look at the print out – what does it say the rate is?
  • Or look at how many QRS’ there are in the rhythm strip – then x 6 (because rhythm strip = 10 secs. Rate = 60secs)
  • Or measure it you’re self – baby boxes are 40msec, big boxes are 200msec (5xbaby boxes)

Step 2: Rhythm

  • Sinus rhythm vs. normal sinus rhythm
  • Any heart blocks?
    • 1st degree heart block is a PR interval >200msec —> can be caused by b-blockers (blocking AV node)

Step 3: Conduction:

  • PR normally <200ms; if more then some degree of heart block
  • QRS: 80-120msec
    • wide ie. V-tach, bundle branch
  • QT > 500msec = inc risk for torsades
    • QTc is corrected for heart rate

Step 4: Axis

  • Mean vector of depolarization of your ventricles
  • We care about the direction of the vector, not the amplitude
    • Normal axis is -30 to +120
    • On the ECG look at the R in the P-R-T axes
  • -30 to -90:  is L axis shift = L ventricular hypertrophy, R ventricular ischemia
  • +120 to +180: is R axis shift = R ventricular hypertrophy, L ventricular ischemia

Step 5: Hypertrophy – Atrial, LV, RV

  • Atrial: biphasic P wave in the V1 lead
  • LV: Scoring systems required and if any are positive then it is hypertrophy
    • One way of doing it is = 3 main voltage criteria: S waves in V1 and R wave in V5 >35mm = LVH
    • low sensitivity and high specificity
  • RV: Scoring systems required
    • poor R wave progression (not increasing/progressing from V1 to V3)

Step 6: Injury, ischemia, infarction

  • Inferior, anterior, lateral vs. posterior
  • T wave inversion: early indicator of ischemia
  • ST:
    • ST depression: don’t come back to baseline after S to the T, >1mm higher than baseline
    • ST elevation: S wave passes baseline and rolls into the T, >1mm higher than baseline
  • Q waves:
    • If you have Q waves on your ECG but no sx then they are old Q waves (e.g. previous ischemia)
    • Q waves are significant if they are greater than 1 box in width (longer than 0.04 msec) OR are larger than 1/4 of the R wave. (or >1/3 of the amplitude of the entire QRS complex)
    • A no positive deflection before the negative deflection to make sure it’s the Q wave vs the S wave
  • Bundle branch blocks – if they exist the rest of the ECG doesn’t follow the rest of rules, always QRS widening and S waves split/asymmetrical


Look at the inferior leads first and then the anterior leads. You have to see abnormalities in contiguous leads, not just one lead (ie. VII and III or III and aVF) to make a diagnosis.


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