What is the first‑line medication for an acute hemodynamically stable supraventricular tachycardia in an adult without contraindications?

First-Line Medication for Acute Hemodynamically Stable SVT

Adenosine is the first-line medication for acute treatment of hemodynamically stable supraventricular tachycardia in adults without contraindications, administered as 6 mg rapid IV push followed immediately by a 20 mL saline flush. 1, 2

Initial Management Algorithm

Step 1: Vagal Maneuvers First

Before any medication is administered, vagal maneuvers are the recommended initial intervention and should be attempted first in all hemodynamically stable patients 1, 2:

• Modified Valsalva maneuver: Patient bears down against a closed glottis for 10-30 seconds (generating ≥30-40 mm Hg intrathoracic pressure) while supine, with a success rate of approximately 43% 1, 2
• Carotid sinus massage: Apply steady pressure over the carotid sinus for 5-10 seconds after confirming absence of bruit 1, 2
• Ice-water facial immersion: Apply ice-cold wet towel to the face 1, 2
• Overall vagal maneuver success rate is approximately 27.7% across techniques 1

Critical safety warning: Never apply pressure to the eyeball—this technique has been abandoned due to danger 1, 2

Step 2: Adenosine Administration (When Vagal Maneuvers Fail)

Adenosine terminates 90-95% of AVNRT and 78-96% of AVRT cases, making it the most effective first-line pharmacologic agent 1, 2:

Standard Dosing Protocol

• Initial dose: 6 mg rapid IV push through a large proximal vein, followed immediately by 20 mL saline flush 1, 2
• Second dose: If no conversion within 1-2 minutes, give 12 mg IV push with saline flush 1, 2
• Third dose: If still no conversion, may repeat 12 mg IV push 1, 2
• Maximum total dose reported as safe is up to 24 mg 2

Modified Dosing for Special Circumstances

• Reduce initial dose to 3 mg in patients taking dipyridamole or carbamazepine, those with transplanted hearts, or if given by central venous access 2
• Increase dose in patients with significant blood levels of theophylline, caffeine, or theobromine 2

Administration Technique

Both the traditional double-syringe technique (adenosine followed by separate saline flush) and the newer single-syringe technique (adenosine diluted in up to 20 mL saline) are effective, with the single-syringe showing higher first-dose termination rates (odds ratio 2.87) 3, 4

Monitoring Requirements

• Continuous ECG recording during administration helps diagnostically and distinguishes between drug failure and successful termination with immediate reinitiation 2
• A defibrillator should be available when administering adenosine, particularly if Wolff-Parkinson-White syndrome is a consideration, due to risk of initiating atrial fibrillation with rapid ventricular rates 2

Common Side Effects

Most patients experience transient side effects lasting <60 seconds, including flushing, dyspnea, and chest discomfort—these are dose-dependent but self-limited 2, 5

Absolute Contraindication

Adenosine is absolutely contraindicated in patients with asthma due to risk of severe bronchospasm 2

Alternative First-Line Medications (When Adenosine is Contraindicated)

For Patients with Asthma or COPD

Intravenous diltiazem is the preferred alternative, with a 64-98% conversion rate 1, 2, 6:

• Diltiazem: 15-20 mg (0.25 mg/kg) IV over 2 minutes 1, 2, 6
• Verapamil: 2.5-5 mg IV over 2 minutes as an alternative 1, 2
• Intravenous beta-blockers (metoprolol or esmolol) may be used with caution in severe COPD, though they are slightly less effective than calcium channel blockers 1, 2

Critical Safety Warnings for Calcium Channel Blockers

Do NOT administer verapamil or diltiazem if 1, 2:

• Ventricular tachycardia cannot be excluded
• Pre-excited atrial fibrillation (e.g., Wolff-Parkinson-White syndrome) is present—risk of ventricular fibrillation
• Suspected systolic heart failure exists
• Patient is hemodynamically unstable

The FDA label for diltiazem specifically warns that it should not be used in patients with atrial fibrillation or flutter associated with an accessory bypass tract such as WPW syndrome 6

Post-Conversion Management

After successful conversion, immediate monitoring is critical 2:

• Watch for immediate recurrence: Patients commonly experience atrial or ventricular premature complexes that may trigger recurrent SVT within seconds to minutes 2
• If recurrence occurs: Treat with adenosine again or consider a longer-acting AV nodal blocking agent (diltiazem or beta-blocker) 2
• If adenosine unmasks another SVT mechanism (atrial flutter or atrial tachycardia): Consider treatment with a longer-acting AV nodal blocking agent 1, 2

When to Proceed Directly to Cardioversion

Synchronized cardioversion is indicated immediately (bypassing vagal maneuvers and medications) if the patient presents with 2, 7:

• Hypotension
• Altered mental status
• Signs of shock
• Chest pain
• Acute heart failure

Synchronized cardioversion achieves near-100% termination of hemodynamically unstable SVT 1, 7

Diagnostic Considerations

A 12-lead ECG must be obtained during tachycardia to differentiate SVT mechanisms and exclude ventricular tachycardia or pre-excited atrial fibrillation before administering any AV nodal blocking agent 2, 8