What are the recommended initial medications for Supraventricular Tachycardia (SVT) according to Advanced Cardiovascular Life Support (ACLS) guidelines?

ACLS Medications for SVT at 175 bpm

Adenosine 6 mg rapid IV push is the first-line medication for hemodynamically stable SVT, followed by 12 mg doses if needed, with vagal maneuvers attempted first. 1, 2

Initial Assessment and Hemodynamic Status

• Hemodynamically unstable SVT (hypotension, altered mental status, shock, chest pain, acute heart failure) requires immediate synchronized cardioversion, though adenosine can be considered first if the rhythm is regular narrow-complex 1
• Hemodynamically stable SVT should be treated with vagal maneuvers first, followed by pharmacologic therapy 2, 3

First-Line Pharmacologic Treatment: Adenosine

Adenosine is the definitive first-line drug with 85-100% success rates for terminating AV node-dependent SVT (AVNRT and AVRT). 1, 2

Dosing Protocol

• Initial dose: 6 mg rapid IV bolus through a large proximal vein, followed immediately by 20 mL saline flush 1, 2, 3
• Second dose: 12 mg if no conversion within 1-2 minutes 1, 3
• Third dose: 12 mg can be repeated once more 1, 3
• Maximum reported safe dose up to 24-30 mg in refractory cases 4, 5

Critical Administration Technique

• Must be given as rapid IV push (over 1-2 seconds) followed immediately by saline flush 1, 2
• Use the most proximal IV access possible 1, 3
• Have defibrillator readily available 2, 3
• Continuous ECG monitoring during and after administration 2, 3

Dose Modifications

• Reduce to 3 mg for patients taking dipyridamole or carbamazepine, heart transplant recipients, or central venous access 3
• Increase dose for patients on theophylline, caffeine, or theobromine 3

Expected Side Effects (Transient, <1 minute)

• Flushing, dyspnea, chest discomfort occur in ~30% of patients 1, 3
• Brief AV block, PVCs/ventricular tachycardia 1

Contraindications

• Asthma or bronchospastic lung disease (risk of bronchospasm) 1, 3
• Second or third-degree AV block or sick sinus syndrome 1, 2
• Pre-excited atrial fibrillation (can cause decompensation) 1

Second-Line Options When Adenosine Fails or Is Contraindicated

Calcium Channel Blockers (Class IIa)

Diltiazem or verapamil are effective alternatives with 64-98% conversion rates in stable patients. 1

• Diltiazem: 0.25 mg/kg IV bolus over 2 minutes, followed by infusion at 5-15 mg/h 1
• Verapamil: 5-10 mg (0.075-0.15 mg/kg) IV bolus over 2 minutes 1
• Slow infusion up to 20 minutes may reduce hypotension risk 1
• Critical warning: Do NOT use if VT or pre-excited AF suspected—can cause hemodynamic collapse or ventricular fibrillation 1
• Contraindicated in systolic heart failure 1

Beta Blockers (Class IIa)

Reasonable alternative but less effective than calcium channel blockers. 1

• Metoprolol: 2.5-5 mg IV bolus over 2 minutes, can repeat every 10 minutes up to 3 doses 1
• Esmolol: 500 mcg/kg IV bolus over 1 minute, followed by infusion at 50-300 mcg/kg/min 1
• Propranolol: 1 mg IV over 1 minute, can repeat at 2-minute intervals up to 3 doses 1
• Avoid in heart failure, bronchospasm 1

Post-Conversion Management

• Monitor for recurrence for at least 1-2 hours—common due to adenosine's short half-life 2, 3
• Watch for premature complexes that can trigger recurrent SVT 3
• If SVT recurs, repeat adenosine or consider longer-acting AV nodal blocker (diltiazem or beta blocker) 2, 3
• Recurrent episodes may require prophylactic antiarrhythmic therapy or cardiology referral 2

Synchronized Cardioversion

• Indicated if pharmacologic therapy fails in stable patients 1
• Perform after adequate sedation/anesthesia 1
• Highly effective for terminating AVRT and AVNRT 1

Common Pitfalls to Avoid

• Do not use calcium channel blockers or beta blockers if pre-excitation (WPW) or VT suspected—can be fatal 1
• Do not give adenosine slowly—must be rapid push or it will be metabolized before reaching the heart 1, 2
• Do not use calcium channel blockers in infants/young children—risk of severe hypotension and cardiac arrest 2
• Do not assume adenosine failure means wrong diagnosis—may need higher doses or alternative agents 5, 6