(Summary based on an article by Frantz R. Melio, MD, William K. Mallon, MD and Edward Newton, MD originally published in Annals of Emergency Medicine, April 1993, pages 709-713.)

"Unstable" paroxysmal supraventricular tachycardia (PSVT) is defined as PSVT accompanied by systolic blood pressure
<90 mm Hg, chest pain, pulmonary edema, or altered mental status. Per advanced cardiac life support guidelines, patients presenting with this condition should be treated with immediate synchronized electrical cardioversion; however, this treatment is not without risks (complications may occur in as many as 14.5% of cases, although most are minor) and is generally disliked by the patient. Investigators in this study analyzed the use of adenosine in 12 cases of unstable PSVT and found it to be an effective first-line treatment.

Standard 12-lead ECGs were reviewed to confirm a diagnosis of PSVT based on the criteria of narrow complex regular tachycardia at a rate of = or >140. Adenosine was administered by rapid 6 mg IV bolus, then followed by a flush of 10 mL of normal saline to clear the IV line. A second rapid adenosine bolus of 12 mg was administered two minutes later (also followed by a saline flush) if the tachycardia was not terminated by the first dose.

Of the 12 patients identified over two years, 75% (9/12) presented with hypotension, 83% (10/12) with chest pain and 50% (6/12) with both hypotension and chest pain. No patients presented with pulmonary edema or altered mental status. The mean rate of PSVT was 187.

Conversion to normal sinus rhythm occurred in 100% of patients, 67% (8/12) following the first adenosine injection bolus, and the remaining 33% (4/12) following the second adenosine bolus. Chest pain and hypotension resolved within minutes of adenosine treatment in 100% of patients. No clinically significant adverse effects were noted other than brief ECG instability following adenosine administration (seen in all patients). Following conversion, all patients were monitored in the emergency department for a mean of 1.8 hours and there were no recurrences.

According to the investigators, standard medical means of treating PSVT in the past were not sufficient compared to emergency cardioversion because the treatments were either too slow in onset (e.g., digoxin) or could have deleterious hemodynamic effects (e.g., verapamil). With the advent of adenosine, a naturally occurring nucleotide that acts through membrane-bound adenosine receptors, the potential dangers of treating unstable PSVT patients with electrical cardioversion or verapamil can be avoided. Adenosine is characterized by a rapid onset of action, a half-life of approximately ten seconds and a brief duration of adverse effects. These results indicate that for unstable PSVT, adenosine is a safe and effective alternative to immediate electrical cardioversion.