Medication Review: Adenosine
Patients with a heart rate over 100 are tachycardic. Those with a narrow complex tachycardia at a rate of 150 or more are said to be in supraventricular tachycardia or SVT. Patients in SVT with: hypotension, signs of shock, acute CHF, or ischemic chest pain are considered unstable and should be cardioverted with a synchronized shock. For stable patients in SVT, Adenosine (Adenocard) is the first IV medication given following vagal maneuvers and is today’s Pass ACLS tip topic.
For patients with sustained tachycardia, we should attempt to identify the underlying cause and address it. Hypoxia, hypovolemia, sepsis, shock, anxiety, fever, and a slew of other conditions can cause tachycardia. When the heart rate becomes too fast, generally over 150 beats/minute, cardiac output can be affected due to insufficient time for the heart’s chambers to fill – decreasing the pre-load. Patients with a sustained rate over 150 may be stable for now but won’t likely stay that way, so intervention is indicated.
After assessing vital signs, SaO2, and obtaining a 12 lead, we should start an IV of normal saline and administer oxygen if the pulse ox is less than 90%. For patients with suspected shock or hypovolemia, a fluid bolus of 250-500 ml may be helpful and should be attempted if there aren’t any contraindications such as CHF with pulmonary edema, renal failure, or anasarca.
If fluids and oxygen don’t help, vagal maneuvers should be attempted next. Common vagal maneuvers are covered in this Pass ACLS tip of the day episode. If vagal stimulation is unsuccessful, Adenosine (Adenocard) is our next intervention.
Indications
Adenosine is indicated for stable patients with sustained SVT refractory to vagal maneuvers.
Adenosine may also be considered for wide-complex, monomorphic tachycardias but isn’t a front-line intervention.
Mechanism of Action
Adenosine is a naturally occurring nucleoside that has effects throughout the body. In the heart, it temporally stops the SA node and blocks conduction through the AV node. Some liken the effects of Adenosine to that of a chemical cardioversion. Adenosine doesn’t stop atrial activity when atrial stimulation isn’t from the SA node; like in A-Fib and A-Flutter.
Contraindications & Precautions
Because Adenosine stops conduction of the SA and AV node, its use is contraindicated in patients with second or third degree heart blocks.
Adenosine has been known to cause bronchospasm so it should be used with caution in patients with a history of Asthma or COPD and is contraindicated for patients experiencing active bronchospasm.
Administration
The initial dose of Adenosine is 6 mg given rapid IV push and immediately followed with a 20 ml flush. Adenosine must be given rapidly. If SVT resumes, a second dose of 12 mg Adenosine can be given in 1-2 minutes.
A smaller starting dose of 3 mg may be effective in patients with a transplanted heart while a dose larger than 12 mg may be required if the patient has caffeine or theophylline on board.
After administration of Adenosine it’s common for patients to experience a period of chest discomfort as well as a short episode of asystole on the monitor. As the medication breaks down, we should see heart activity resume. A common practice is to start an ECG strip before administration of Adenosine and run it until heart activity resumes.
A-Fib/Flutter
In cases where the heart rate is very fast, it can be difficult to distinguish P waves on the ECG. Administering Adenosine may allow us to see underlying fib or flutter waves for a few seconds as the medication wears off. In this case, Adenosine has a diagnostic attribute. In cases of A-Fib or flutter with RVR, a second dose of Adenosine isn’t likely to be effective so we can skip the second dose and switch to a calcium channel blocker or beta blocker. These medications work by a different mechanism and are more effective for the treatment of atrial rhythms with RVR.
