How to distinguish supraventricular tachycardia (SVT) on an electrocardiogram (ECG)?

Distinguishing SVT on ECG

To distinguish supraventricular tachycardia (SVT) on ECG, first assess QRS width: narrow QRS (<120 ms) confirms SVT, while wide QRS requires differentiation from ventricular tachycardia using AV dissociation, fusion complexes, and specific algorithms like Brugada or Vereckei criteria. 1

Initial QRS Width Assessment

• Narrow QRS complex (<120 ms) almost always indicates SVT and is the most reliable initial distinguishing feature 1
• Wide QRS complex (>120 ms) could represent either VT or SVT with aberrant conduction, and should be treated as VT until proven otherwise due to higher mortality risk 1
• In infants, QRS duration may be normal or only slightly prolonged (<0.08 s), but morphology different from sinus rhythm indicates a ventricular origin 2

Distinguishing Narrow Complex SVT from Sinus Tachycardia

Rate Characteristics

• SVT typically presents at 260-300 beats/min in infants and >150 beats/min in adults, with extreme regularity after the first 10-20 beats 2, 3
• Sinus tachycardia is almost always <230 beats/min in infants and rarely exceeds 180 beats/min in adults, with gradual acceleration and deceleration over several seconds 2, 3
• The R-R interval in SVT shows extreme regularity like a metronome after initial beats, while sinus tachycardia demonstrates variation over several seconds 2, 4

P Wave Analysis

• In SVT, approximately 60% of cases show visible P waves, but these almost always have different morphology from sinus P waves 2
• Sinus tachycardia maintains the same P wave axis as normal sinus rhythm with clearly visible P waves 2
• In typical AVNRT, P waves are hidden within the QRS complex, creating a pseudo S wave in inferior leads (II, III, aVF) and pseudo R' in lead V1 2, 5
• In orthodromic AVRT, the P wave appears in the early ST segment, separated from QRS by ≥70 ms 1, 6

RP Interval Classification

• Short RP interval (RP < PR) is typical of AVNRT or AVRT, where the P wave is closer to the prior QRS than the subsequent QRS 2, 1
• Long RP interval (RP > PR) suggests atypical AVNRT, permanent form of junctional reciprocating tachycardia (PJRT), or atrial tachycardia 2, 1
• In atrial tachycardia, the P wave typically appears near the end of or shortly after the T wave with morphology different from sinus 2

Distinguishing Wide Complex Tachycardia (SVT vs VT)

Pathognomonic Features for VT

• AV dissociation with ventricular rate faster than atrial rate is diagnostic of VT 2, 1
• Fusion complexes (representing merger of conducted sinus impulses and ventricular depolarization) are pathognomonic for VT 2, 1
• QRS complexes in tachycardia that are identical to those in sinus rhythm strongly suggest SVT with aberrancy 2

Brugada Criteria for VT

• Absence of any R-S complexes in all precordial leads implies VT 1
• R-S interval >100 ms in any precordial lead suggests VT 1
• QRS concordance (all precordial leads showing either positive or negative deflections) indicates VT or pre-excitation 2, 1

Vereckei Algorithm (Lead aVR)

• Initial R wave in lead aVR implies VT 1
• Q wave >40 ms in lead aVR suggests VT 1
• Notch on descending limb at onset of predominantly negative QRS in aVR indicates VT 1

Additional VT Criteria

• QRS width >140 ms with RBBB pattern or >160 ms with LBBB pattern favors VT 1
• R-wave peak time ≥50 ms in lead II suggests VT 1

Clinical Context Integration

• Sinus tachycardia typically has an identifiable physiologic cause such as fever, sepsis, hypovolemia, pain, anxiety, or medications 3
• SVT often occurs in patients with otherwise normal hearts without obvious precipitating factors 2, 4
• In adults with history of myocardial infarction presenting with wide complex tachycardia, VT is the most likely diagnosis 7
• VT accounts for more than 80% of wide QRS complex tachycardias 7

Diagnostic Maneuvers

• Vagal maneuvers or adenosine administration can differentiate SVT from sinus tachycardia: sinus tachycardia will gradually slow while SVT will either abruptly terminate or transiently slow revealing underlying atrial activity 3
• A continuous rhythm strip must be obtained during adenosine administration and at termination of tachycardia to capture diagnostic information 8
• Comparing the tachycardia ECG with previous ECGs during normal sinus rhythm helps assess P wave morphology changes 3

Critical Pitfalls to Avoid

• Never assume hemodynamic stability rules out VT—stable vital signs do not distinguish between SVT and VT 1
• Avoid calcium channel blockers in undiagnosed wide complex tachycardias as they can precipitate hemodynamic collapse in VT 1
• When diagnosis of wide complex tachycardia is uncertain, treat as VT—this is the safest approach 1
• Pre-existing bundle branch block can make SVT appear as wide complex tachycardia 1
• SVT with accessory pathway conduction (antidromic AVRT) presents with wide QRS and requires prompt evaluation by an electrophysiologist 1, 4
• In infants, persistent aberration of SVT is exceedingly rare, so QRS morphology different from sinus almost always indicates VT 2
• If examining only a single lead, ventricular tachycardia may masquerade as SVT—always obtain a 12-lead ECG 6