What is the likely diagnostic rhythm issue in an adult patient with a history of cardiac issues presenting with telemetry evidence of short runs of regular tachycardia at a cycle length of 320-330 ms?

Diagnostic Assessment of Regular Tachycardia at 320-330 ms Cycle Length

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

Rate Calculation and Initial Classification

• A cycle length of 320-330 ms translates to a rate of 180-190 bpm (calculated as 60,000 ms/minute ÷ cycle length) 1
• This rate range is characteristic of paroxysmal SVT mechanisms, particularly those involving the AV node 1
• The "short runs" description suggests paroxysmal episodes with spontaneous termination, which is highly characteristic of AVNRT or AVRT 1, 2

Most Likely Diagnoses in Order of Probability

Primary Considerations

AVNRT (Atrioventricular Nodal Reentrant Tachycardia):

• This is the most common mechanism for regular SVT at this rate in adults with cardiac history 1, 3
• Typical AVNRT commonly presents with rates of 150-250 bpm, making 180-190 bpm well within the expected range 1
• The paroxysmal nature ("short runs") strongly supports this diagnosis 1, 2

AVRT (Atrioventricular Reciprocating Tachycardia):

• Orthodromic AVRT is the second most common cause of regular narrow-complex tachycardia at this rate 1, 4
• This mechanism involves an accessory pathway conducting retrogradely 4
• Rates are typically similar to AVNRT 1, 4

Atrial Flutter with 1:1 Conduction:

• Typical atrial flutter has an atrial rate around 300 bpm (cycle length ~200 ms), which would produce 2:1 conduction at 150 bpm 1
• However, 1:1 conduction can occur, particularly in younger patients or those on antiarrhythmic drugs that slow the flutter rate 1
• A cycle length of 320-330 ms could represent slowed atrial flutter with 1:1 AV conduction 1

Atrial Tachycardia:

• Focal atrial tachycardia can present at rates of 180-190 bpm with 1:1 AV conduction 1
• Less common than AVNRT/AVRT but must be considered 1, 3

Critical Diagnostic Steps Required

Immediate ECG Analysis Needed

Obtain 12-lead ECG during tachycardia:

• This is the single most critical diagnostic step that determines all subsequent management 5
• QRS duration <120 ms confirms SVT; ≥120 ms requires differentiation from ventricular tachycardia 1, 5

Assess P-wave relationship to QRS:

• P waves buried in QRS (RP <90 ms) → typical AVNRT 1, 5
• P waves in early ST segment (short RP) → AVRT or atypical AVNRT 1, 5
• Atrial rate exceeding ventricular rate → atrial flutter or atrial tachycardia 1
• "Sawtooth" pattern in inferior leads → atrial flutter 1

Exclude Dangerous Mimics

Rule out ventricular tachycardia if wide-complex:

• VT accounts for >80% of wide-complex tachycardias and misdiagnosis can be fatal 1, 6
• Look for AV dissociation, fusion complexes, or concordance in precordial leads 1
• Apply Brugada or Vereckei criteria if QRS ≥120 ms 1

Assess for pre-excitation:

• Review baseline ECG for delta waves indicating accessory pathway 5, 4
• Pre-excitation with palpitations requires immediate electrophysiology referral due to sudden death risk 5

Clinical Context Considerations

Patient History Red Flags

• Syncope during episodes: Warrants urgent cardiology referral regardless of mechanism 5
• "Neck pounding" sensation: Highly specific for AVNRT due to cannon A-waves 5
• Sudden onset/termination: Characteristic of AVNRT and AVRT 1, 2
• Episodes while driving: Occurs in 57% of SVT patients; 14% experience syncope while driving 1, 5

Age and Structural Heart Disease

• Elderly patients with AVNRT are more prone to syncope despite slower rates 5
• Structural heart disease assessment via echocardiography is recommended for all documented sustained SVT 5

Common Diagnostic Pitfalls

• Mistaking regular SVT for irregular rhythm: Rapid atrial fibrillation can appear regular and be misdiagnosed as SVT 1
• Assuming narrow-complex = benign: Even narrow-complex SVT requires proper diagnosis and may need ablation 5, 3
• Delaying ECG documentation: Repeated attempts at capturing the rhythm may be unnecessary if history is classic for SVT 2
• Over-reliance on stability criteria: In patients with structural heart disease, VT can show cycle length variability and be misclassified as SVT 7

Management Implications

• If hemodynamically unstable, immediate synchronized cardioversion is indicated 1
• If stable, vagal maneuvers followed by IV adenosine is first-line acute treatment 1
• Catheter ablation has 94.3-98.5% success rate and is preferred definitive treatment for recurrent symptomatic SVT 5, 3
• Long-term pharmacotherapy with calcium channel blockers, beta-blockers, or antiarrhythmics is an alternative if ablation is declined 1, 3