Supraventricular Tachycardia (SVT)

A 28 year old female presents to the emergency department with sudden onset palpitations that began 30 minutes ago while studying. She describes the sensation as her heart racing and reports lightheadedness, but denies chest pain, dyspnea, or syncope. She has had similar self-resolving episodes in the past. She drinks 2 to 3 cups of coffee daily. On examination, pulse rate is 180 beats per minute, blood pressure is 110/70 mm Hg, respiratory rate is 16 breaths per minute, temperature is 98.2°F, and oxygen saturation is 96 percent on room air. She appears anxious but alert and oriented. ECG shows a regular narrow QRS complex tachycardia with absent visible P waves. Diagnosis?

Diagnosis is Supraventricular Tachycardia (SVT), most likely AVNRT.

1. Definition

Supraventricular tachycardia is a group of tachyarrhythmias originating at or above the atrioventricular node, characterized by a narrow QRS complex (< 120 ms) and heart rate typically 150 to 220 bpm.

2. Types

1. Atrioventricular nodal reentrant tachycardia (AVNRT) most common in young adults
2. Atrioventricular reentrant tachycardia (AVRT)
3. Atrial tachycardia

3. Etiology and Triggers

1. Caffeine, alcohol, and stimulants
2. Emotional or physical stress
3. Medications such as beta agonists
4. Electrolyte abnormalities or structural heart disease

4. Pathophysiology

1. Reentry circuits involving the AV node or accessory pathways
2. Leads to rapid, regular tachycardia

5. Clinical Features

1. Sudden onset palpitations
2. Lightheadedness or dizziness
3. Anxiety
4. May include chest discomfort, dyspnea, or syncope

6. Diagnostic Evaluation

6.1 ECG Findings

1. Regular narrow complex tachycardia
2. Absent or hidden P waves
3. May show pseudo R' in V1 or pseudo S waves in inferior leads
4. Heart rate typically 150 to 220 bpm

6.2 Clinical Assessment

1. Assess for hemodynamic instability such as hypotension, altered mental status, ischemia, or shock

7. Key Diagnostic Insight

Regular narrow complex tachycardia with absent P waves strongly suggests AVNRT

8. Management

8.1 Hemodynamically Unstable

1. Immediate synchronized cardioversion

8.2 Hemodynamically Stable

1. Vagal maneuvers, preferably modified Valsalva maneuver
2. Adenosine is first-line pharmacologic therapy
3. Alternatives include beta blockers or calcium channel blockers

8.3 Important Pharmacologic Insight

1. Adenosine acts by transient AV node blockade
2. Effective only in AV node dependent tachycardias
3. Does not terminate atrial tachycardia, atrial flutter, or atrial fibrillation

8.4 Additional Considerations

1. Avoid carotid massage in patients with carotid disease
2. Caffeine may reduce adenosine effectiveness

8.5 Definitive Treatment

1. Radiofrequency catheter ablation

9. Indications for Ablation

1. Recurrent symptomatic episodes
2. Medication intolerance or failure
3. Accessory pathway mediated tachycardia

10. Complications

1. Tachycardia-induced cardiomyopathy if prolonged
2. Rare progression to hemodynamic instability

11. Key Clinical Insight

Paroxysmal episodes of rapid, regular palpitations in a young patient strongly suggest AVNRT

12. Exam Level Pearls

1. AVNRT is the most common SVT in young adults
2. Absent or hidden P waves are characteristic
3. Pseudo R' and pseudo S waves are classic ECG findings
4. Adenosine terminates AV node dependent tachycardia
5. Do not delay cardioversion in unstable patients