Case summary:

An ALS crew was called to a 60s man who was on the floor and needed help up.  The patient denied having any pain but endorsed having a recent hip replacement.  He denied being on blood thinners and denied dizziness.  The patient was refusing transport to the hospital so the crew obtained a set of screening vitals which is required per our protocol for all non-transports:

Vitals: BP 109/75,  HR 166,  RR 18,  Sat 96%

The crew was concerned about the patients elevated heart rate so they obtained a 12-lead EKG which is shown here:

The crew noted that this was a narrow complex regular tachycardia, and they did not see any obvious P waves in any lead.  Based on this, they diagnosed the patient with an atrioventricular reentrant tachycardia (AVRT), which is also commonly referred to as supraventricular tachycardia (SVT).  Given that the patient was stable, they elected to treat the patient with adenosine.  The following rhythm strip was obtained during the 6mg adenosine push:

The crew noted that the HR returned to the 160s shortly after it dropped transiently but they did not notice the underlying rhythm seen during the AV pause from the adenosine.  Given that they still suspected AVRT, they gave a 12mg dose of adenosine with the following seen on the monitor just after it was pushed:

It was at this point that the crew noted the underlying flutter waves and realized that the patient had atrial flutter with 2:1 conduction which was the cause of this patients elevated heart rate.  The patient continued to remain asymptomatic and have not drop in his blood pressure.  The patient became willing to be transported so the crew transported the patient to the hospital for ongoing care.

Highlights of the case:

Differential diagnosis for regular narrow complex tachycardia

The patient in this case was found to be in a regular narrow complex tachycardia.  There is only three things that this patient can have: sinus tachycardia, SVT or atrial flutter with 2:1 conduction.  Determining which of the three is the diagnosis requires looking for P waves and flutter waves.  As the rate gets faster, it can be hard to decern since the flutter waves and P waves can get buried in the T waves.  It is important to look at all 12 leads to look for any evidence of buried flutter waves or P waves.  If any lead shows these, it rules out SVT and gives you the proper diagnosis.   Looking at the original EKG this crew obtained, it would be reasonable to call this SVT since I agree there are no clear P waves. 

Treatment of regular narrow complex tachycardia

The first thing we need to decide is if the patient is stable or not.  If the patient is unstable, the treatment of choice is immediate electrical cardioversion.  This is true for all unstable tachyarrhythmias.  The one exception is if this patient has sinus tachycardia since the treatment for that is to correct the underlying cause.  However, if you shock a patient with sinus tachycardia, it is unlikely to make them worse and will correct all other causes of the rhythm.  For this reason, it is reasonable to have a low threshold to attempt electrical cardioversion for unstable tachycardias that are not obviously sinus tachycardia.  Adenosine is an option only for STABLE patients experiencing a tachyarrhythmia that is suspected to be SVT.  Given that this patient was experiencing a stable, narrow complex tachycardia that was not clearly atrial flutter or sinus tachycardia, it was very reasonable for this crew to administer adenosine. 

Look at the rhythm strip while giving adenosine

Adenosine treats SVT by blocking the AV node.  SVT is due to a reentrant circuit that goes through the AV node.  If you block the AV node, you abort the cycle and can therefore treat the SVT.   While blocking the AV will treat SVT, it can also help show what the underlying atrial rhythm is in cases where the patient did not have SVT.  In the case of this patient, the cause of the tachycardia was atrial flutter with 2:1 conduction.  When the crew gave 6mg of adenosine, the successfully caused a sinus and revealed the underlying atrial rhythm which was atrial flutter (saw toothed waves at a rate of ~300).  Had this crew noticed this, they would have known this was not SVT and not given the second dose of adenosine.  The crew was able to see this after the 12mg dose which helped them make the diagnosis of atrial flutter.  While adenosine is the treatment for SVT, it can also be used to confirm the diagnosis of other atrial rhythms so always watch the rhythm strip when giving this medication.  The treatment of atrial flutter is transport to the hospital since this is treated the same as atrial fibrillation.  These patients need rate or rhythm control as well as likely will require anticoagulation given the risk of stroke.  Atrial flutter is one of the most misdiagnosed rhythms given it can look like SVT and sinus tachycardia.  Anytime you have a HR between 130 and 170, we should consider this diagnosis.