Diagnostic Assessment of Supraventricular Tachycardia

Introduction to SVT Diagnosis

A cycle length of 320-330 ms corresponds to a heart rate of approximately 180-190 bpm, which in an adult with cardiac history most likely represents supraventricular tachycardia (SVT), specifically AVNRT or AVRT, though atrial flutter with 1:1 conduction and atrial tachycardia must also be considered 1, 2
The American Heart Association recommends that a cycle length of 320-330 ms translates to a rate of 180-190 bpm, characteristic of paroxysmal SVT mechanisms, particularly those involving the AV node 1, 2, 3

The American College of Cardiology recommends obtaining a 12-lead ECG during tachycardia, as this is the single most critical diagnostic step that determines all subsequent management 4
QRS duration <120 ms confirms SVT; ≥120 ms requires differentiation from ventricular tachycardia, with a strength of evidence level of IIa 3, 5, 4

Assessing P-wave relationship to QRS is crucial, as P waves buried in QRS (RP <90 ms) indicate typical AVNRT, while P waves in early ST segment (short RP) suggest AVRT or atypical AVNRT 2, 3, 4
Atrial rate exceeding ventricular rate indicates atrial flutter or atrial tachycardia, with a "sawtooth" pattern in inferior leads characteristic of atrial flutter 2, 3

Patient history red flags, such as syncope during episodes, warrant urgent cardiology referral regardless of mechanism, with a class I recommendation 4
"Neck pounding" sensation is highly specific for AVNRT due to cannon A-waves, with a strength of evidence level of IIb 4

If hemodynamically unstable, immediate synchronized cardioversion is indicated, with a class I recommendation 5
If stable, vagal maneuvers followed by IV adenosine is first-line acute treatment, with a strength of evidence level of IIa 1, 5
Catheter ablation has a 94.3-98.5% success rate and is preferred definitive treatment for recurrent symptomatic SVT, with a class I recommendation 4