Systematic ECG rhythm analysis (always follow this order rather than pattern-matching): electrical activity → rate → regularity → QRS width → P waves → P-QRS relationship

300 ÷ big boxes between R waves = heart rate (bpm)

Quick landmarks: 1 box = 300, 2 boxes = 150, 3 boxes = 100, 4 boxes = 75, 5 boxes = 60, 6 boxes = 50

Regular rhythm = consistent R-R intervals (use calipers or paper method)

Regularly irregular = predictable pattern (e.g., Wenckebach, bigeminy)

Irregularly irregular = no pattern = think AF

NSR criteria:

• Rate 60-100 bpm
• Regular rhythm
• Upright P waves with consistent morphology
• PR interval 0.12-0.20 seconds
• Narrow QRS (<0.12s)

Sinus bradycardia = sinus rhythm with rate under 60 bpm

Causes: Athletes, sleep, beta-blockers, sick sinus syndrome, hypothyroidism, raised ICP

Sinus tachycardia = sinus rhythm with rate over 100 bpm

Always ask: Why is the heart rate up? (Pain, fever, hypovolaemia, PE, anxiety, sepsis, hyperthyroidism)

Sinus arrhythmia = sinus rhythm with R-R variation with breathing (normal in young people).

Sinus arrest = SA node fails to fire, causing a pause over 2 seconds without P waves

If frequent or prolonged, may need pacemaker.

PAC features:

• Early beat interrupting the regular rhythm
• P wave present but different morphology from sinus P waves
• Narrow QRS (usually)
• Often followed by a compensatory pause

Atrial tachycardia:

• Rate 150-250 bpm
• Regular rhythm
• P waves present but abnormal morphology
• May have 1:1 or 2:1 AV conduction

Atrial flutter features:

• Sawtooth flutter waves (F waves) at ~300/min
• Ventricular rate depends on AV conduction (2:1 = 150 bpm, 3:1 = 100 bpm, 4:1 = 75 bpm)
• Regular or regularly irregular ventricular response

AF features:

• Irregularly irregular R-R intervals (hallmark finding)
• No P waves - replaced by fibrillatory (f) waves
• Baseline looks wavy/chaotic
• Variable QRS rate

Junctional rhythm features:

• Rate 40-60 bpm (junctional escape rate)
• No P waves visible OR inverted P waves (before, during, or after QRS)
• Narrow QRS (unless pre-existing BBB)
• Regular rhythm

Junctional rates:

• Junctional escape: 40-60 bpm
• Accelerated junctional: 60-100 bpm
• Junctional tachycardia: >100 bpm

1st degree block = PR interval >0.20 seconds (>5 small squares), but every P wave is followed by a QRS

Not a true "block" - just delayed conduction through AV node.

Mobitz I (Wenckebach) features:

• Progressive PR prolongation until a QRS is dropped
• Grouped beating pattern
• PR interval resets after the dropped beat
• Usually benign, block is at AV node level

Mobitz II features:

• Constant PR interval (unlike Mobitz I)
• Intermittent dropped QRS complexes (can be fixed ratio like 2:1, 3:1)
• QRS often wide (block at bundle branch level)
• More dangerous than Mobitz I - can progress to complete heart block

3rd degree (complete) block features:

• P waves present but no relationship to QRS complexes
• Atria and ventricles beat independently (AV dissociation)
• Ventricular rate depends on escape rhythm:
  • Junctional escape: 40-60 bpm, narrow QRS
  • Ventricular escape: 20-40 bpm, wide QRS

PVC features:

• Wide QRS (>0.12s) and bizarre morphology
• No preceding P wave
• Usually followed by compensatory pause
• Unifocal = same morphology (one focus)
• Multifocal = varying morphology (multiple foci - more concerning)

Dangerous PVC patterns:

• R-on-T phenomenon (PVC landing on T wave)
• Frequent (>6/min)
• Multifocal
• Runs of 3+ (non-sustained VT)
• Bigeminy/trigeminy patterns

VT features:

• Rate >100 bpm (usually 150-250)
• Wide QRS (>0.12s)
• Regular rhythm (monomorphic) or irregular (polymorphic)
• AV dissociation may be visible (P waves marching through)
• Fusion beats and capture beats are diagnostic

Torsades features:

• Polymorphic VT with QRS complexes that "twist" around the baseline
• Associated with prolonged QT interval
• Triggered by: drugs (antiarrhythmics, antipsychotics, antibiotics), electrolyte abnormalities (↓K+, ↓Mg2+)

Torsades treatment:

• IV Magnesium 2g (even if Mg normal)
• Correct K+ to high-normal (4.5-5.0)
• Overdrive pacing (increases heart rate, shortens QT)
• Stop QT-prolonging drugs
• Avoid standard antiarrhythmics (may worsen)

Source: ANZCOR ALS / local resuscitation guideline.

VF features:

• Chaotic, irregular waveforms
• No identifiable P waves, QRS, or T waves
• Coarse VF = higher amplitude, better prognosis
• Fine VF = lower amplitude, may look like asystole

Idioventricular rhythm:

• Rate 20-40 bpm (ventricular escape rate)
• Wide QRS complexes
• No P waves
• Regular rhythm
• Occurs when higher pacemakers (SA node, AV junction) fail

Asystole = no electrical activity (flat line)

Always check: leads attached? gain turned up? check in multiple leads? (Protocol: "Check pulse, check leads, check gain")

Non-shockable rhythm - CPR + adrenaline + reversible causes