Double-syringe vs Single-syringe Technique of Adenosine for Termination of Regular Narrow Complex Tachycardia

Supraventricular tachycardia (SVT) is the most common tachyarrhythmias in young adults, children, and infants. The most common type in adults is the atrioventricular nodal reentrant tachycardia (AVNRT) and the incidence is 35 per 10,000 person-years or 2.29 per 1000 persons.

The treatment approach of SVT is based on hemodynamical stability. The synchronized cardioversion is recommended for hemodynamically unstable patients, on the other hand, the vagal maneuvers and medications are considered for patients with stable hemodynamic.

To date, the 2019 ESC guideline for the management of supraventricular tachycardia stated that adenosine is still the first medication for acute therapy in patients with SVT if the vagal maneuver fails or is contraindicated. A systematic review in 2021 showed that adenosine is equally efficacious for treating SVT, but it is safer than intravenous calcium channel blockers due to a shorter half-life.

Few studies reported that the effectiveness of conventional vagal maneuvers, namely, carotid sinus massage in SVT termination was around 19 to 54% when properly done. The enhanced method of vagal maneuver or named the modified syringe technique was performed by blowing into a 10 mL syringe to move the plunger, the success conversion rate of SVT was a little better than the convention technique (43% vs 17%). Therefore, adenosine is still crucial in the acute therapy of SVT in an emergency setting.

Adenosine has been discovered since 1929 and used in the acute treatment of arrhythmias. It was used as a therapeutic diagnosis in patients with regular narrow QRS complex tachyarrhythmias and it is also helpful in treating some type of regular wide QRS complex tachyarrhythmias (differentiate between SVT with aberrancy/bundle branch block and ventricular tachycardia).

Adenosine, an endogenous purine nucleoside, is the first drug of choice to terminate SVT. The pharmacological response of adenosine is mediated through cardiac adenosine type 1 (A1) receptors and has a dose-response relationship. It prolongs atrioventricular (AV) conduction and results in transient AV block that is responsible for tachyarrhythmia termination. The dosages for the peripheral intravascular (IV) route are 6 mg and follow by 12 mg if the first dose is failed.2 To achieve rhythm conversion, the injection should be performed as a rapid bolus with immediate saline flush through large veins (e.g. antecubital) in 1-2 seconds, because adenosine is rapidly metabolized by enzymatic deamination to inactive inosine within seconds, and achieve end-organ effects within 20-30 seconds.2 This conventional method of adenosine administration is well-known as the double syringe technique (DST). The repeat dose is safe when administered at least 1 minute after the first dose. The success rate of adenosine in terminating SVT was greater than 90% approximately.

The potential side effects of adenosine are transient AV block, flushing, chest pain, or dyspnea, AF can be initiated or worsened in case of preexcitation existing, premature ventricular tachycardias (PVCs)/ventricular tachycardia (VT), bronchospasm (rare), or coronary steal.5 In addition, Adenosine should be excluded or used with caution in patients with these conditions (e.g. AV block greater than first degree or sinoatrial (SA) node dysfunction, reactive airway disease, concomitant use of verapamil or digoxin, and Wolff-Parkinson-White (WPW) syndrome).5

The DST requires a two-way stopcock which may not stock in some workplaces e.g. emergency medical services (EMS) and need the nursing staff to coordinate their actions administering the drug and flush simultaneously. In the real world, small hospitals had a transient shortage of medical equipment supply e.g. syringes. The convenient method which combined adenosine and 15 mL of normal saline in one syringe was firstly proposed in the non-blind, randomized prospective study between 1999 to 2001 and published the full original article in the Korean language since 2003 by Choi SC et al. This trial demonstrated the success rate of the convenient method was higher than the standard method (85.7 vs 80%), but it's not statistically significant, in addition, the average of total administering doses, and the complication of the convenient and standard method was not statistically different.

Nearly two decades later, Marc McDowell et al. conducted a single-center, prospective, observational study with setting a 20% non-inferiority margin demonstrated that the successful conversion of SVT in the first dose (73.1 vs 40.7%) and up to three doses (100 vs 70.4%) in the single syringe technique (SST) and DST respectively. And they didn't found any serious adverse effects e.g. dyspnea, bradycardia, or asystole. Unfortunately, superiority testing was not performed due to the limiting sample size.

Therefore, we hypothesized that the SST is non-inferior to the DST for terminating stable SVT, by comparison, a rate of successful conversion of SVT at the first 6 mg-dose of adenosine and the total average dosage of adenosine administered. And we also aimed to compare the rate of side effects between both techniques of intravenous adenosine in those diagnosed with stable SVT.