You are taking care of a young woman who has come to the emergency department stating, “my heart keeps beating real fast.” She tells you that it has been a very stressful day, and that she is functioning on little or no sleep. Off and on all day her heart will race and then suddenly stop only to start up again an hour or so later. After going through this several times, she got scared. Her mother convinced her to come to the emergency department. She is 24 years old. She has good color, her skin is warm and dry. She appears anxious. She admits to drinking “a lot of caffeine to help her stay awake.” Her BP is moderately elevated at 146/94 and her heart rate is 88, strong and regular. Her pulse ox is 98% of room air with respirations of 24. She has no significant past medical history except for normal childhood diseases.
As you are speaking with her she suddenly gets a bit ashen and says “oh no, here we go again!” You check her pulse and it is rapid but suddenly slows down. You quickly place her on the monitor and you see the following rhythm.
What is this rhythm? You note that it is a bit tachycardic and that the rhythm itself is regular except for the rather frequent premature beats. These beats occur early in relationship to the rhythm (premature). They are narrow (so they originate in the atrium), therefore they are PAC’s (premature atrial contractions). These can be normally occurring benign events, can be caused by stress, fatigue and certainly by caffeine. In the case of our patient, she has all three!
As you are watching as the following occurs to her rhythm:
What just happened? This patient has experienced what is commonly called paroxysmal supraventricular tachycardia. This is a catchall term for a number of arrhythmias. This one happens to be atrioventricular nodal reentrant tachycardia (AVNRT). This is the most common type of PSVT and tends to be self-limiting. It almost always responds to vagal maneuvers or adenosine for termination.
What exactly is AVNRT? It happens when a circuit of re-entry exists within the AV Node. Instead of just one way for electricity to go through the AV node, there are two. They are different types of circuits. The normal pathway is fast conducting and has a long refractory period. The other one is slower but has a shorter refractory period. If a PAC or PVC (premature ventricular contraction) occurs when the normal pathway is still refractory (unable to conduct electricity), it will be conducted down the faster pathway that is ready to accept it (shorter refractory period). After the current reaches the ventricles, it will conduct back through the normal pathway that is no longer refractory and a reentry circuit will be created.
To differentiate this rhythm from other similar looking rhythms, you note that:
We ask our patient to perform a Valsalva maneuver. The rhythm immediately terminates. If this was not the case, we could start an IV and administer adenosine 6mg Fast IV push followed by a bolus of saline. If unsuccessful, the adenosine can be repeated at a 12mg dose.