What is the best initial treatment for a patient with a history of hypertension, presenting with acute onset palpitations, tachycardia (170/min), stable blood pressure (120/70 mmHg), normal cardiac and chest examinations, and oxygen saturation of 91%?

Adenosine is the Best Initial Treatment

For this hemodynamically stable patient with acute onset palpitations and narrow-complex tachycardia at 170/min, adenosine 6 mg rapid IV push (followed by 12 mg if needed) is the treatment of choice. 1

Clinical Reasoning

This patient presents with:

• Acute onset palpitations with heart rate 170/min
• Hemodynamically stable (BP 120/70 mmHg, no altered mental status, no chest pain, no acute heart failure)
• Normal cardiac and chest examinations
• Oxygen saturation 91% (mildly reduced but not critically low)

The presentation is most consistent with paroxysmal supraventricular tachycardia (PSVT), likely AVNRT or AVRT, which are the most common narrow-complex tachycardias requiring acute treatment. 1

Treatment Algorithm

Step 1: Initial Stabilization

• Provide supplementary oxygen (given O2 sat 91%) 1
• Establish IV access 1
• Obtain 12-lead ECG if available, but do not delay treatment 1

Step 2: Vagal Maneuvers (if time permits)

• Attempt Valsalva maneuver or carotid sinus massage first 1
• Success rate approximately 27.7% 1
• However, given the acute presentation in the ER, proceeding directly to adenosine is reasonable

Step 3: Adenosine Administration

Adenosine is recommended as first-line pharmacologic therapy for acute treatment of AVNRT and narrow-complex SVT 1

Dosing:

• First dose: 6 mg rapid IV push followed immediately by saline flush 1
• Second dose: 12 mg if first dose ineffective 1, 2
• Success rate: 93-95% for terminating PSVT 1, 3, 4
• Onset of action: Within 30 seconds 3

Why NOT the Other Options?

B. Cardioversion - Incorrect for initial management

• Reserved for hemodynamically unstable patients (acute altered mental status, ischemic chest pain, acute heart failure, hypotension, shock) 1
• This patient is stable with BP 120/70 mmHg 1
• Cardioversion is only indicated if adenosine and vagal maneuvers fail or are not feasible in unstable patients 1

C. Amiodarone - Not first-line

• Only considered when other therapies are ineffective or contraindicated 1
• Slower onset of action compared to adenosine 1
• Reserved as a second-line or third-line agent 1

D. Observation - Inappropriate

• Active treatment is indicated for symptomatic tachycardia at 170/min 1
• Observation alone risks deterioration and prolonged symptoms 1

Safety Considerations with Adenosine

Common transient side effects (lasting <1 minute): 2, 3, 4

• Flushing, chest discomfort, dyspnea (occur in >10% of patients)
• These resolve spontaneously due to adenosine's half-life of <10 seconds 2, 3

Contraindications to avoid: 2

• Second- or third-degree AV block (not present here)
• Sick sinus syndrome (not present here)
• Asthma or active bronchospasm (not mentioned in this case)
• Known hypersensitivity to adenosine

Important caveat: The patient's hypertension history is not a contraindication to adenosine, though adenosine can cause transient hypertension or hypotension 2. The blood pressure should be monitored closely during and after administration 5.

Expected Outcome

• 85-95% conversion rate to sinus rhythm within 30 seconds 1, 3, 6, 7
• If unsuccessful, can repeat 12 mg dose 1
• If still unsuccessful after maximum dosing, consider alternative diagnosis (atrial flutter, atrial fibrillation) or proceed to calcium channel blockers (diltiazem/verapamil) or synchronized cardioversion 1

Post-Conversion Management

After successful conversion with adenosine:

• Monitor for recurrence (occurs in minority of patients within minutes) 4
• Consider beta-blocker or calcium channel blocker for ongoing management if recurrent episodes 1
• Optimize blood pressure control given hypertension history 5
• Consider electrophysiology referral if episodes become frequent or sustained 8