What is the best initial treatment for a hypertensive man with acute onset palpitations, irregular tachycardia (heart rate 170/min), normal cardiac and chest examinations, normal blood pressure (120/70 mmHg), mild tachypnea (respiratory rate 18/min), and mild hypoxemia (oxygen saturation 91%)?

Adenosine is the Best Initial Treatment

For this hemodynamically stable patient with acute onset irregular tachycardia at 170/min, adenosine 6 mg rapid IV push is the best initial treatment option, as this presentation is most consistent with paroxysmal supraventricular tachycardia (PSVT), for which adenosine has a 90-95% conversion rate and is recommended as first-line pharmacologic therapy by the American College of Cardiology. 1, 2

Clinical Reasoning

Why This is PSVT, Not a Hypertensive Emergency

• The patient's blood pressure of 120/70 mmHg is normal, not elevated, which immediately excludes hypertensive emergency as the primary diagnosis 3
• Hypertensive emergency requires BP >180/120 mmHg WITH acute target organ damage—neither criterion is met here 1, 3
• The acute onset palpitations with heart rate of 170/min and irregular pulse in a hemodynamically stable patient (normal BP, no altered mental status, no chest pain) is the classic presentation of PSVT 1, 2
• The mild hypoxemia (SpO2 91%) and tachypnea (RR 18/min) are secondary to the tachycardia itself, not indicators of acute target organ damage 2

Why Adenosine is First-Line

• Adenosine is the treatment of choice for hemodynamically stable patients with narrow-complex SVT, with a 93-95% success rate in terminating PSVT 1, 2
• The American College of Cardiology gives adenosine a Class I recommendation for acute treatment of AVNRT and orthodromic AVRT 1
• Adenosine works within 30 seconds with a half-life of <10 seconds, making it both effective and safe 2
• Dosing: 6 mg rapid IV push, followed by 12 mg if the first dose is unsuccessful 1, 2

Why NOT the Other Options

B. Cardioversion is reserved for:

• Hemodynamically unstable patients (hypotension, altered mental status, acute heart failure, ongoing chest pain) 1
• Patients who fail pharmacologic therapy 1
• This patient is hemodynamically stable with BP 120/70 mmHg, making cardioversion premature 2

C. Amiodarone is indicated for:

• Ventricular fibrillation and hemodynamically unstable ventricular tachycardia 4
• NOT first-line for SVT—the FDA label specifically states amiodarone is for "frequently recurring ventricular fibrillation and hemodynamically unstable ventricular tachycardia" 4
• This patient has supraventricular tachycardia, not ventricular arrhythmia 2

D. Observation is inappropriate because:

• Active treatment is indicated for symptomatic tachycardia at 170/min 1, 2
• Adenosine has minimal risk and high efficacy, making observation unnecessarily conservative 2
• The patient is symptomatic with palpitations and mild hypoxemia requiring intervention 2

Treatment Algorithm

Immediate Management Steps

1. Provide supplemental oxygen to address SpO2 91% 2
2. Establish IV access and obtain 12-lead ECG if available (don't delay treatment) 2
3. Attempt vagal maneuvers first (Valsalva maneuver or carotid sinus massage)—success rate ~28% 2
4. Administer adenosine 6 mg rapid IV push followed by 20 mL saline flush 1, 2
5. If unsuccessful after 1-2 minutes, give adenosine 12 mg rapid IV push 1, 2
6. May repeat 12 mg dose once more if needed 2

Expected Outcome

• 85-95% conversion to sinus rhythm within 30 seconds 2
• Transient side effects (flushing, chest discomfort, dyspnea) resolve spontaneously due to adenosine's ultra-short half-life 2
• Monitor for recurrence and consider beta-blocker or calcium channel blocker for ongoing management if episodes recur 2

Safety Considerations

• Contraindications: second- or third-degree AV block, sick sinus syndrome, active asthma/bronchospasm 2
• Have cardioversion equipment immediately available in case adenosine precipitates atrial fibrillation with rapid ventricular response 1
• Monitor BP closely during and after administration 2

Critical Pitfalls to Avoid

• Don't confuse this with hypertensive emergency—the BP is normal, not elevated 3
• Don't use amiodarone for SVT—it's indicated for ventricular arrhythmias, not supraventricular 4
• Don't jump to cardioversion in a stable patient—pharmacologic therapy should be attempted first 1
• Don't give adenosine as a slow push—it must be rapid IV push followed immediately by saline flush to reach the heart before metabolism 2
• Don't forget to address the underlying hypertension after acute stabilization, as hypertension increases risk of recurrent arrhythmias 5, 6