May 15, 2023

Dear colleagues:

Good day!  As discussed in the last installment of UH EMS-I’s Pharmacy Phriday, we continue a review of medications that are used within our protocols that directly affect and treat heart arrhythmias. Our drug this week, Adenosine, is often considered a chemical reset when our patient is in a stable, regular, Supraventricular Tachycardia (SVT) that is a primary arrhythmia, not secondary from an underlying treatable cause.

Supraventricular tachycardia is a catch-all term used to describe tachycardias originating from the bundle of His or above and includes atrial tachycardia, inappropriate sinus tachycardia, atrial fibrillation, and junctional tachycardia, to name a few.  Adenosine has been the primary drug therapy used in the treatment of SVT rhythms that are stable, regular, and have a narrow-complex.  In some cases, a regular monomorphic wide-complex tachycardia can also be treated with Adenosine.  The medication can also be considered in cases of unstable SVT while other crew members are setting up for cardioversion.

Adenosine is contraindicated for some of those rhythms that technically fall into the “SVT” category.  That is why proper recognition of the type of SVT is important.  Adenosine works on the AV node, slowing the conduction of impulses.  Because of this action, Adenosine should not be used for A-fib, A-fib RVR, A-flutter, blocks, known Wolf Parkinson White (WPW- remember that delta wave from medic school?), or irregular, polymorphic ventricular tach, as it may cause an increased ventricular response.

In the case where a rhythm is misinterpreted (In various studies, the rate of paramedics correctly interpreting SVT ranged from 70.3% to 96.7%.), the administration of Adenosine may slow down the heart rate long enough to reveal that the tachycardia is due to a different narrow complex tachycardia (e.g., atrial fibrillation or atrial flutter). That is one of the reasons a three lead ECG should be recorded during administration of the drug.  12 Lead ECGs should be completed prior to and following treatment as well.

When researching Adenosine, one can find many articles and studies related to its use.  Various studies have investigated improvements in performing vagal maneuvers before the use of Adenosine, improvements in the administration of Adenosine (considering the extremely short half-life of the medication), and even dosing of the medication.

Various vagal maneuvers are typically successful about 25% of the time.  These maneuvers are performed to stimulate vagal tone and slow the heart rate.  Examples of such maneuvers include bearing down as if they are going to have a bowel movement, coughing, or blowing out through a syringe or straw.  The use of carotid massage is no longer recommended.  A modified Valsalva Maneuver has been studied and claims a much higher success rate than normal. 

Another area of treatment in SVT being reviewed relates to the administration of Adenosine and its short half-life.  Adenosine has an onset and time to peak effect of 20-30 seconds, a duration time of 30 seconds, and a half-life of 10 seconds.  For this reason, rapid administration is needed for the medication to be effective.  Typical administration recommendations include a rapid bolus followed by a 20 ml flush of normal saline (10 ml flush for the pediatric patient).  Whether using a two syringe method, stop cock method, or some other method of administration, most require planning and coordination of multiple providers.  Regardless of your preferred method, be sure to use the proximal port on the IV tubing.  It will be interesting to see if alternate methods are successful in making the procedure easier for the field provider and remain effective in converting patients out of the SVT rhythm.  Additional research is also in progress regarding the dosing of Adenosine. 

As new innovations in care present themselves, the team at the UHEMS Institute review and evaluate these studies and recommendations and seek to include those deemed appropriate within our protocols.  As promising as some of the above-mentioned changes may seem, until approved, we must be sure to follow current protocols.

Current protocol guidelines within the UH system regarding Adenosine remain unchanged at this time.  For the adult patient, those doses are 6 mg as the initial dose, followed by 12 mg if there is no ECG change within 1 minute. In the pediatric patient, the dosing is 0.1 mg/kg initial dose, followed by 0.2 mg/kg if there is no ECG change in 1-2 minutes.

Before ending this installment, it is important to review some of the other concerns with Adenosine.  The medication can cause bronchospasms and, as such, should be used with caution in asthmatic and COPD patients, especially if they are experiencing an active attack. (In such a case, the provider may rethink the potential cause for the rapid heart rate!)  In most cases of a past medical history of asthma, Adenosine can be used, and the patient is monitored closely for possible prolonged dyspnea or bronchodilation.  In cases of a severe reaction, the medic would follow the respiratory distress protocol.

A final consideration is to advise patients of possible side effects prior to administration. These include experiencing a sense of anxiety, headache, dizziness, flushing, shortness of breath, chest pain, or other temporary side effects. Though usually lasting less than 10 seconds due to the short half-life of the medication and the rate at which the medication is metabolized by the body, these effects can be quite scary to the patient experiencing them.  Reassurance and calm on the provider’s part can be crucial in these instances.

Be sure to check back in the next installment when we discuss Amiodarone.  Till then, stay safe! 

Sincerely,



The UH EMS-I Team

University Hospitals