ECG Findings – Study Notes

1. Sinus Rhythm

• Normal rhythm: regular P waves followed by QRS complexes.

• PR interval constant.

• QRS = ventricular depolarization, normally narrow (80–100 ms).

• Sinus bradycardia: rate < 60 bpm.

• Sinus tachycardia: rate > 100 bpm.

2. Atrial Fibrillation (AF)

• No distinct P waves, chaotic atrial activity.

• QRS complexes not preceded by P waves.

• Irregularly irregular rhythm (variable R–R intervals).

• Substrate: atrial dilation.

• Trigger: ectopic foci (often pulmonary veins).

• AF with RVR: rate > 100 bpm.

• AF with slow VR: rate < 60 bpm.

3. Atrial Flutter

• Re-entry circuit in atria → coordinated atrial contractions.

• Atrial rate ≈ 300 bpm.

• Narrow complex tachycardia.

• ECG: Sawtooth waves (inverted P waves in inferior leads).

• Conduction ratio: usually 2:1 (ventricular rate ≈ 150 bpm).

  • Higher ratios = more AV block.

  • 1:1 conduction → unstable, risk of VF.

• May mimic AF if conduction ratio varies.

4. Premature Contractions

Premature Ventricular Contractions (PVCs)

• Origin: His–Purkinje system.

• Wide QRS (>120 ms).

• Compensatory pause follows.

• Benign if isolated; risk if frequent (>10–30/hr).

• Patterns:

  • Bigeminy (every other beat).

  • Runs of PVCs = ventricular tachycardia.

Premature Atrial Contractions (PACs)

• Abnormal P wave morphology (different from sinus).

• Narrow QRS follows.

• May appear as a pause but timing matches expected P wave.

• Usually benign.

5. Bundle Branch Blocks (BBB)

• General: QRS >120 ms.

• Left BBB:

  • Deep S in V1 (“W”).

  • Broad R in V6 (“M”).

  • Mnemonic: WiLLiaM (W in V1, M in V6).

• Right BBB:

  • RSR’ in V1 (“M”).

  • Deep S in V6 (“W”).

  • Mnemonic: MaRRoW (M in V1, W in V6).

• Note: ST segments altered → difficult ischemia interpretation.

6. AV Blocks

First Degree

• PR interval > 200 ms.

• Usually benign.

Second Degree

• Mobitz I (Wenckebach): Progressive PR prolongation → dropped beat.

• Mobitz II: Dropped beat without PR prolongation.

  • Structural issue.

  • Risk of progression to complete block.

Third Degree (Complete Heart Block)

• No association between P waves and QRS.

• Atrial rate > ventricular rate.

• Ventricular escape rhythm → bradycardia.

• Regular P waves + regular QRS, but independent.

7. Ventricular Tachycardia (VT)

• Broad complex tachycardia (>120 ms).

• Monomorphic VT: uniform QRS morphology.

• Polymorphic VT: varying QRS morphologies.

• Torsades de Pointes: polymorphic VT in prolonged QT.

• Features suggestive of VT:

  • Very broad complexes (>160 ms).

  • Extreme axis deviation.

  • No BBB morphology.

• Dangerous: can lead to VF or cardiac arrest.

8. Ventricular Fibrillation (VF)

• No P waves or QRS complexes.

• Chaotic electrical activity, varying amplitudes.

• Minimal cardiac output → lethal without defibrillation.

• Shockable rhythm in cardiac arrest.

9. Cardiac Arrest Rhythms

• Shockable: VT, VF.

• Non-shockable: Pulseless electrical activity (PEA), asystole.

10. ST Elevation

• ST segment: between end of S wave and start of T wave.

• Elevation > 1 mm (except V2–V3 need >1.5 mm).

• Causes:

  • STEMI (most important).

  • Pericarditis, LBBB, LVH, benign early repolarization.

• Localization:

  • Lateral = I, aVL, V5–V6.

  • Inferior = II, III, aVF.

  • Anterior/septal = V1–V4.

  • Posterior = V7–V9.

• Reciprocal changes: ST depression in opposite leads.

High-Yield Mnemonics

• BBB: WiLLiaM (LBBB), MaRRoW (RBBB).

• AF: “Irregularly irregular.”

• Flutter: “Sawtooth.”

• Torsades: Polymorphic VT + long QT.