ECG / Rhythm Reference

15 common cardiac rhythms with recognition criteria and treatment pathways.

Normal Sinus Rhythm (NSR)

Rate: 60–100 bpm · Regularity: Regular

P wave: Upright, uniform before each QRS · PR interval: 0.12–0.20 sec · QRS: Narrow (<0.12 sec)

Key feature: Every P followed by a QRS; normal intervals throughout

Treatment pathway

  1. No intervention required
  2. Treat underlying cause if symptomatic

Sinus Bradycardia

Rate: <60 bpm · Regularity: Regular

P wave: Upright, uniform before each QRS · PR interval: 0.12–0.20 sec · QRS: Narrow (<0.12 sec)

Key feature: Same as NSR but rate <60 bpm

Treatment pathway

  1. Asymptomatic: observe, monitor
  2. Symptomatic (hypotension, AMS, chest pain): Atropine 0.5 mg IV, repeat q3–5 min, max 3 mg
  3. Atropine-refractory: TCP (transcutaneous pacing)
  4. Infusion: Dopamine 2–20 mcg/kg/min or Epinephrine 2–10 mcg/min

Sinus Tachycardia

Rate: >100 bpm · Regularity: Regular

P wave: Upright, may be buried in T wave at fast rates · PR interval: 0.12–0.20 sec (may shorten) · QRS: Narrow (<0.12 sec)

Key feature: Gradual onset/offset; identify and treat the underlying cause

Treatment pathway

  1. Treat the underlying cause (pain, hypovolemia, hypoxia, fever, anxiety)
  2. Do NOT cardiovert — this is a compensatory mechanism
  3. IV fluids if hypovolemic

Atrial Fibrillation (A-Fib)

Rate: Atrial 350–600 bpm; Ventricular 60–170 bpm (uncontrolled) · Regularity: Irregularly irregular

P wave: Absent — chaotic fibrillatory baseline · PR interval: Unmeasurable · QRS: Narrow (<0.12 sec) unless aberrant conduction

Key feature: No distinct P waves; irregularly irregular ventricular response

Treatment pathway

  1. Stable with rapid ventricular rate: rate control — diltiazem or metoprolol
  2. Unstable (hypotension, chest pain, AMS): synchronized cardioversion 120–200 J biphasic
  3. Anticoagulation per medical direction (stroke risk)
  4. New-onset <48h: consider rhythm control per protocol

Atrial Flutter

Rate: Atrial ~300 bpm; Ventricular 75–150 bpm (2:1, 3:1, or 4:1 block) · Regularity: Regular (if constant block ratio) or regularly irregular

P wave: Sawtooth flutter waves (F waves) — best seen in II, III, aVF · PR interval: Unmeasurable · QRS: Narrow (<0.12 sec) unless aberrant

Key feature: Classic sawtooth pattern at ~300 bpm atrial rate

Treatment pathway

  1. Stable: rate control with diltiazem or metoprolol
  2. Unstable: synchronized cardioversion — often converts with 50–100 J
  3. Vagal maneuvers may temporarily slow rate for diagnosis

Supraventricular Tachycardia (SVT / PSVT)

Rate: 150–250 bpm · Regularity: Regular

P wave: Often buried in or just after QRS (retrograde) · PR interval: Shortened or unmeasurable · QRS: Narrow (<0.12 sec) — narrow complex tachycardia

Key feature: Sudden onset/termination; regular narrow complex at 150–250 bpm

Treatment pathway

  1. Vagal maneuvers first (Valsalva, carotid massage)
  2. Adenosine 6 mg rapid IV push with 20 mL flush
  3. If no conversion: Adenosine 12 mg IV, repeat once
  4. Refractory stable: diltiazem or metoprolol
  5. Unstable: synchronized cardioversion 50–100 J

Ventricular Tachycardia (VT / Monomorphic VT)

Rate: >100 bpm (typically 120–200 bpm) · Regularity: Regular

P wave: May be absent or dissociated from QRS (AV dissociation) · PR interval: Unmeasurable · QRS: Wide (>0.12 sec), bizarre morphology

Key feature: Wide complex tachycardia with AV dissociation; fusion/capture beats

Treatment pathway

  1. Pulseless VT: Defibrillation 200 J biphasic → CPR → Epinephrine 1 mg IV q3–5 min
  2. Pulseless VT with refractory VF: Amiodarone 300 mg IV or Lidocaine 1–1.5 mg/kg
  3. Stable VT with pulse: Amiodarone 150 mg IV over 10 min
  4. Unstable VT with pulse: Synchronized cardioversion 100 J

Ventricular Fibrillation (VF)

Rate: Unmeasurable — chaotic · Regularity: Chaotic, no organized pattern

P wave: Absent · PR interval: Absent · QRS: No identifiable QRS — coarse or fine fibrillatory waves

Key feature: No organized electrical activity; always pulseless

Treatment pathway

  1. Immediate defibrillation 200 J biphasic (unsynchronized)
  2. CPR 2 min immediately after shock
  3. Epinephrine 1 mg IV/IO q3–5 min
  4. Amiodarone 300 mg IV after 3rd shock, then 150 mg
  5. Lidocaine 1–1.5 mg/kg IV alternative to amiodarone
  6. Address reversible causes: H's and T's

3rd Degree Heart Block (Complete Heart Block)

Rate: Atrial 60–100 bpm; Ventricular 20–40 bpm (idioventricular) or 40–60 bpm (junctional escape) · Regularity: Regular P-P, regular R-R — but they march through each other (AV dissociation)

P wave: Present, upright, regular — but no relationship to QRS · PR interval: Varies completely — no consistent PR (AV dissociation) · QRS: Wide if ventricular escape (>0.12 sec); may be narrow if junctional escape

Key feature: Complete AV dissociation — atria and ventricles beating independently

Treatment pathway

  1. Symptomatic: TCP (transcutaneous pacing) immediately
  2. Atropine 0.5 mg IV may be tried but often ineffective in complete block
  3. Dopamine 2–20 mcg/kg/min or Epinephrine 2–10 mcg/min if pacing unavailable
  4. Definitive: transvenous pacing in hospital

2nd Degree Type I (Wenckebach / Mobitz I)

Rate: Normal to bradycardic · Regularity: Irregular — grouped beating pattern

P wave: Present, upright, regular · PR interval: Progressively lengthens until a QRS is dropped · QRS: Narrow (<0.12 sec) when present; QRS is eventually dropped

Key feature: Progressive PR lengthening → dropped QRS → cycle repeats (Wenckebach pattern)

Treatment pathway

  1. Often benign (inferior MI, increased vagal tone, medications)
  2. Asymptomatic: observe and monitor
  3. Symptomatic: Atropine 0.5 mg IV, TCP if refractory

2nd Degree Type II (Mobitz II)

Rate: Normal to bradycardic · Regularity: Irregular (unexpected dropped QRS)

P wave: Present, upright, regular · PR interval: Constant (does NOT lengthen) — then QRS suddenly drops · QRS: Wide (>0.12 sec) — bundle branch pathology; QRS randomly drops

Key feature: Constant PR then sudden dropped QRS — no warning; high risk of progressing to complete block

Treatment pathway

  1. HIGH RISK — may progress to complete block suddenly
  2. TCP (transcutaneous pacing) — have it ready even if asymptomatic
  3. Atropine often ineffective — do not delay pacing
  4. Immediate transport; transvenous pacing in hospital

Junctional Rhythm

Rate: 40–60 bpm (accelerated junctional: 60–100 bpm) · Regularity: Regular

P wave: Absent, inverted, or retrograde (before/after/in QRS) · PR interval: Short (<0.12 sec) if P before QRS, or unmeasurable · QRS: Narrow (<0.12 sec) unless aberrant

Key feature: AV node acting as pacemaker — retrograde or absent P waves

Treatment pathway

  1. Asymptomatic: observe
  2. Symptomatic bradycardia: Atropine 0.5 mg IV
  3. Accelerated junctional (60–100 bpm): typically benign, treat cause

Pulseless Electrical Activity (PEA)

Rate: Variable — any organized rhythm without a pulse · Regularity: Variable

P wave: May be present (organized rhythm on monitor) · PR interval: Variable · QRS: May appear normal or wide

Key feature: Organized electrical activity on monitor but NO palpable pulse — treat as cardiac arrest

Treatment pathway

  1. High-quality CPR — do not interrupt for rhythm checks
  2. Epinephrine 1 mg IV/IO q3–5 min
  3. Search and treat reversible causes — H's and T's:
  4. H's: Hypoxia, Hypovolemia, Hydrogen ion (acidosis), Hypo/Hyperkalemia, Hypothermia
  5. T's: Tension pneumothorax, Tamponade, Toxins, Thrombosis (PE/MI)
  6. Point-of-care ultrasound if available to identify cause

Asystole

Rate: None · Regularity: None

P wave: Absent · PR interval: Absent · QRS: Absent — flat line (confirm in 2 leads)

Key feature: No electrical activity; confirm in two leads to rule out fine VF

Treatment pathway

  1. High-quality CPR immediately
  2. Epinephrine 1 mg IV/IO q3–5 min
  3. Do NOT defibrillate asystole
  4. Treat reversible H's and T's
  5. Consider early ROSC indicators: ETCO2 rising, signs of life

Paced Rhythm

Rate: Set by pacemaker (typically 60–80 bpm) · Regularity: Regular (demand pacing may be irregular)

P wave: May be present if atrial paced; absent if ventricular only · PR interval: Normal if atrial paced; variable if ventricular · QRS: Wide (>0.12 sec) with LBBB pattern if ventricular paced; preceded by pacing spike

Key feature: Pacemaker spike before QRS; wide bizarre morphology

Treatment pathway

  1. Assess for pacemaker capture and underlying rhythm
  2. If bradycardic despite pacing (failure to capture): TCP at higher output
  3. Pacemaker malfunction: treat underlying escape rhythm per bradycardia protocol
  4. If device shock inappropriate: contact medical direction, do not place magnet without guidance

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