How do you differentiate between Atrial Fibrillation (AFIB), Supraventricular Tachycardia (SVT), and Ventricular Tachycardia (VT) on electrocardiogram (EKG) rhythm strips?

Differentiating AFIB vs SVT vs VT on EKG Rhythm Strips

The most critical initial step is to assess QRS width and regularity: narrow regular QRS suggests SVT, irregularly irregular rhythm without discrete P waves indicates AFIB, and wide QRS different from baseline sinus morphology should be treated as VT until proven otherwise. 1

Key Distinguishing Features by Rhythm Type

Atrial Fibrillation (AFIB)

• Irregularly irregular R-R intervals with no discernible pattern of variation 2
• Absence of organized P waves; instead, chaotic fibrillatory waves may be visible 3
• Narrow QRS complexes (unless pre-existing bundle branch block or aberrant conduction) 3
• Variable ventricular rate, typically 100-180 bpm when untreated 3

Supraventricular Tachycardia (SVT)

• Extremely regular R-R intervals after the first 10-20 beats 4
• Rate typically 260-300 bpm in infants, >150 bpm in adults (rarely exceeds 180 bpm in sinus tachycardia) 2
• P waves visible in only 60% of cases, but when present, they have different morphology from sinus P waves 4
• Narrow QRS complexes in >90% of cases; QRS morphology almost always same as baseline sinus rhythm 4
• Pseudo r' deflection in V1 or pseudo S waves in inferior leads suggest AVNRT 1, 5
• P waves separate from QRS (visible in ST segment, separated by ≥70 ms) suggest AVRT 1, 5

Ventricular Tachycardia (VT)

• Wide QRS complexes (>120 ms in adults), though in infants may be <90 ms but different from sinus QRS morphology 6, 1
• QRS morphology different from patient's baseline sinus rhythm is the hallmark feature 6
• Rate 200-500 bpm with slight R-R interval variation over several beats 4
• AV dissociation (independent P waves continuing unrelated to QRS) is pathognomonic for VT 6, 1
• Fusion beats (combination of supraventricular and ventricular complexes) strongly indicate VT 6, 1
• QRS concordance (all precordial leads positive or all negative) strongly suggests VT 6, 1
• R-S interval >100 ms in any precordial lead implies VT 6, 1
• Initial R wave in aVR or Q wave >40 ms in aVR suggests VT 6, 1

Algorithmic Approach to Differentiation

Step 1: Assess Regularity

• Irregularly irregular → AFIB 2
• Regular or nearly regular → Proceed to Step 2

Step 2: Measure QRS Width

• Narrow QRS (<120 ms) → Almost always SVT 1
• Wide QRS (>120 ms) → Proceed to Step 3 (assume VT until proven otherwise) 1

Step 3: For Wide QRS Tachycardia - Look for VT Criteria

• AV dissociation present? → VT (pathognomonic) 6, 1
• Fusion beats present? → VT (pathognomonic) 6, 1
• QRS concordance in precordial leads? → VT 6, 1
• R-S interval >100 ms in any precordial lead? → VT 6, 1
• QRS width >140 ms (RBBB pattern) or >160 ms (LBBB pattern)? → Favors VT 1
• If none of above present → Could be SVT with aberrancy, but treat as VT for safety 1

Step 4: For Narrow QRS Tachycardia - Differentiate SVT Type

• Extreme regularity (after first 10-20 beats) + rate 260-300 bpm? → SVT 4, 2
• Pseudo r' in V1 or pseudo S in inferior leads? → AVNRT 1, 5
• P waves visible after QRS (RP ≥70 ms)? → AVRT 1, 5
• Gradual rate variation + identifiable cause (fever, sepsis)? → Sinus tachycardia 2

Critical Pitfalls to Avoid

• Never assume hemodynamic stability rules out VT; stable vital signs do not distinguish SVT from VT 1
• When diagnosis is uncertain with wide QRS tachycardia, always treat as VT—this is the safest approach given higher mortality risk 1
• Never use calcium channel blockers in undiagnosed wide complex tachycardia; they can precipitate hemodynamic collapse in VT 1
• Pre-existing bundle branch block can make SVT appear as wide complex tachycardia; compare with previous ECGs during sinus rhythm 2, 1
• Do not rely solely on QRS width >140 ms or monophasic R in V1 as these are not sufficiently specific 7
• MCL1 lead cannot substitute for V1 when applying morphological criteria 7
• In infants, persistent aberration with SVT is exceedingly rare; wide QRS different from sinus almost always indicates VT 4

Additional Diagnostic Maneuvers

• Vagal maneuvers or adenosine administration: Sinus tachycardia will gradually slow, SVT will either abruptly terminate or transiently slow revealing underlying atrial activity, VT typically will not respond 2, 1
• Compare with previous ECGs during normal sinus rhythm to assess baseline QRS morphology and P wave characteristics 2
• Record 12-lead ECG during vagal maneuvers to reveal underlying mechanism 2