Differentiating SVT from VT
When differentiating between Supraventricular Tachycardia (SVT) and Ventricular Tachycardia (VT), the QRS width is the initial distinguishing feature, with VT being the presumed diagnosis in wide complex tachycardias until proven otherwise due to its higher morbidity and mortality risk. 1
Initial Assessment Based on QRS Width
- Narrow QRS Complex (<120 ms): Almost always indicates SVT 1
- Wide QRS Complex (>120 ms): Could be either VT or SVT with aberrant conduction; should be treated as VT if diagnosis is uncertain 1
Diagnostic Criteria for Wide QRS Complex Tachycardia
ECG Criteria Favoring VT:
- AV Dissociation: Presence of AV dissociation with ventricular rate faster than atrial rate is pathognomonic for VT 1
- Fusion Complexes: Represent a merger between conducted sinus impulses and ventricular depolarization; pathognomonic for VT 1
- QRS Morphology (Brugada Criteria):
- Lack of any R-S complexes in precordial leads implies VT
- R-S interval >100 ms in any precordial lead implies VT 1
- QRS in Lead aVR (Vereckei Algorithm):
- Initial R wave or Q wave >40 ms implies VT
- Notch on descending limb at onset of predominantly negative QRS implies VT 1
- QRS Width: >140 ms with RBBB pattern or >160 ms with LBBB pattern favors VT 1
- QRS Concordance: All precordial leads showing either positive or negative deflections suggests VT 1
- R-wave Peak Time: ≥50 ms in lead II suggests VT 1
Clinical History Factors Favoring VT:
- History of prior myocardial infarction (positive predictive value >95% for VT) 2
- History of congestive heart failure (positive predictive value >95% for VT) 2
- History of recent angina pectoris (positive predictive value >95% for VT) 2
- Age >35 years (sensitivity 92%, positive predictive value 85% for VT) 2
Diagnostic Criteria for Narrow QRS Complex Tachycardia (SVT)
P Wave Characteristics:
- Hidden P waves: In AVNRT, P waves may be partially hidden within QRS complex, creating pseudo-R wave in V1 and/or pseudo-S wave in inferior leads 1
- Visible P waves after QRS: In AVRT, P wave is visible in ST segment and separated from QRS by ≥70 ms 1
- RP Relationship:
Response to Interventions:
- Adenosine Response: Different SVT mechanisms have characteristic responses to adenosine, which can aid diagnosis 1
- Vagal Maneuvers: May terminate AVNRT and AVRT or temporarily slow atrial tachycardias 1, 3
Diagnostic Algorithm
Assess QRS width:
- If <120 ms: SVT is likely
- If >120 ms: Consider both VT and SVT with aberrancy
For wide QRS tachycardias:
- Look for AV dissociation (most specific for VT)
- Apply Brugada criteria and Vereckei algorithm
- Check for fusion beats
- Assess QRS concordance in precordial leads
For narrow QRS tachycardias:
- Assess regularity (irregular suggests AF, atrial flutter with variable conduction, or MAT)
- Look for P wave location and RP relationship
- Consider response to adenosine (if hemodynamically stable)
Important Pitfalls to Avoid
- Never assume stable vital signs rule out VT: Hemodynamic stability does not distinguish between SVT and VT 1
- Avoid calcium channel blockers in undiagnosed wide complex tachycardias: These can precipitate hemodynamic collapse in VT 1
- Don't rely solely on single-lead monitoring: Always obtain a 12-lead ECG for accurate diagnosis 4
- When in doubt, treat as VT: If diagnosis of wide complex tachycardia is uncertain, safest approach is to treat as VT 1
- Beware of pre-existing bundle branch block: This can make SVT appear as wide complex tachycardia 1
- Consider SVT with accessory pathway conduction: Can present with wide QRS, especially with antidromic AVRT 1
Remember that accurate differentiation between SVT and VT is crucial for appropriate management and to prevent adverse outcomes, as misdiagnosis of VT as SVT can lead to inappropriate treatment and potentially fatal consequences 1, 3.