Internet Book of Critical Care (IBCC)

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ECG Guide

by

ALGORITHM🫀

Index🫀

[1] rhythm & rate

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rhythm

  • Look at every beat in the rhythm strip to confirm that they all are consistent with your rhythm diagnosis.
  • Don't settle for a diagnosis that doesn't truly explain the ECG.

rate

  • 300 – 150 – 100
  • 75 – 60 – 50
  • 43 – 37 – 33

(related)

[2a] PR interval

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PR <120 ms (3 boxes) 📖

  • Tachycardia with short PR (flutter, atrial tach with 2:1 block, AVRT, JT).
  • With inverted P-waves, consider:
    • Low ectopic atrial rhythm.
    • Junctional rhythm.
  • WPW.
  • Benign (asymptomatic, PR ~100-110).

PR >5 200 ms (5 boxes)

  • 1st degree AV block. 📖
  • Normally, PR decreases with tachycardia (so borderline PR may indicate 1st degree block in the context of tachycardia).
[2b] QRS >120 ms? 📖

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ddx of QRS >120 ms

ddx of QRS >200 ms

  • Hyperkalemia.
  • Acidosis.
  • Sodium channel blocker toxicity.
  • Shark-fin (STE with hyperacute T-wave that seems to widen the QRS complex).
  • Ventricular tachycardia.
[2c] QT interval

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⚠️ Be sure to look for the longest QT interval among various ECG leads.

long QT (>1/2 RR if HR<100) 📖

  • ? QRS prolongation.
  • ? Long ST segment: hypocalcemia, hypothermia, LQTS3.
  • ? T-wave with double-hump morphology: Hypokalemia, medications that inhibit the IKr channel, LQTS2.
  • ? Hypothermia.
  • ? Takotsubo CM.
  • ? Myocarditis.

short QT 📖

  • QT <360 ms or 9 boxes.
  • QTc <~350 ms.
[3a] vertical axis & abnormal S-I

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LAD (negative aVF and II) 🚀

  • Prior IMI.
  • LAHB, criteria: 📖
    • [1] QRS <120 ms (often ~100-120 ms).
    • [2] Frontal axis between -45° to -90°:
      • Positive in Lead I.
      • Negative in aVF.
      • Substantially net negative in Lead II.
      • S-wave in aVF > R-wave in Lead I.
    • [3] qR in aVL.
    • [4] R-peak time in aVL of ≧45 ms.
  • LVH.

RAD 📖

  • S-I prominent (terminal RAD)
    • S-I >1-1.5 mm is often regarded as abnormal.
    • S-I > R-I is definitely abnormal.
    • Coexisting S-wave in V6 supports abnormality.

[3b] P-wave axis

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normal P-wave axis is 0° to +75°: 

  • P-wave should be upright in I and II. (aVF will usually be positive but could be mute if the P-wave axis is close to 0°).
  • P-wave should be inverted in aVR.

rightward P-axis deviation (>75 degrees)

  • P-wave becomes negative in aVL.
  • Causes include:
    • RAA (right atrial enlargement).
    • Vertical heart configuration (e.g., emphysema without RAA).
    • Ectopic atrial rhythm or conduction abnormality.

leftward P-axis deviation (<0 degrees)

  • P-wave becomes negative in aVF.
  • Causes include:
    • LAA (left atrial enlargement).
    • Horizontal heart configuration (e.g., morbid obesity).
    • Ectopic atrial rhythm or conduction abnormality.
[3c] horizontal axis

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[4a] atria 

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LAA 📖

  • [1] V1 negative deflection >40 ms x 1 box. However, consider high lead placement if: 📖
    • V2 with negative/biphasic P-wave and/or TWI.
    • QRS in aVR looks similar to V1.
    • Rsr' in V1 and/or V2, often with saddleback STE.
  • [2] Broad P-wave in inferior leads (e.g., P-mitrale).
  • 💡 Look carefully for LVH or BiV hypertrophy.

RAA 📖

  • [1] >1.5 mm P-wave in V1 or V2 (insensitive).
  • [2] P-pulmonale (peaked & >2.5 mm in lead II): causes
    • Acute PE or chronic RVH.
    • Vertical heart 📖 (Lead I sign; S-waves in I and V6; low lateral precordial voltages).
    • Hypokalemia. 📖
    • LAA (“pseudo-P-pulmonale”).
[4b] LVH 📖

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voltage criteria for LVH

Romhilt-Estes (4 high prob; ≧5 def)

  • Voltage criteria (3 points): any
    • R or S in limb leads ≧20 mm.
    • S in V1 or V2 ≧30 mm.
    • R in V5 or V6 ≧30 mm.
  • LV strain pattern: 3 points (1 with digoxin).
  • LAA in V1 (terminal P-wave is >40ms and >1 mm): 3 points.
  • LAD: 2 points.
  • QRS >90 ms: 1 point.
  • Delayed intrinsicoid deflection in V5 or V6 (>50 ms): 1 point.

BiV hypertrophy

  • RVH features often include:
    • RAA.
    • Terminal RAD (S-I).
    • (R-V1 plus S-V5/6) > 10.5 mm.
  • LVH features often include:
    • LAA.
    • High voltages (including R or S in limb leads ≧20 mm).
  • Katz-Wachtel: >~50 mm biphasic (R~S) QRS amplitude in V2-V4.
[4c] low voltage 📖

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definition of low voltage

  • Whole QRS <5 mm in all limb leads.
  • Whole QRS <10 mm in all precordial leads.

(links)

  • Causes of low voltage: 📖
  • Low limb lead voltage: 📖
  • Low precordial voltage: 📖
[5a] pathological Q-waves 📖

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1

[5b] other morphological abnormalities

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reminders & general principles for clinical ECG analysis  
  • Always be systematic.
  • Avoid satisfaction of search (multiple problems often coexist & superimpose).
  • Identify and name specific ECG findings (e.g., tall RV1, diffuse STD with STE in aVR).
  • Include differential diagnoses rather than attempting to discern a single diagnosis.
  • Avoid relying on subjective impressions of morphology.
  • When in doubt, tend to err towards the more dangerous/acute diagnosis (“worst first”).

questions & discussion

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The Internet Book of Critical Care is an online textbook written by Josh Farkas (@PulmCrit), an associate professor of Pulmonary and Critical Care Medicine at the University of Vermont.


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