In Depth: Narrow Complex Tachycardia
There is a considerable number of narrow QRS complex tachycardias. In general they are split between those that are caused by atrial tissue and those that originate in the atrioventricular junction. The approach to many of these narrow complex tachycardias is the same, but careful differential diagnosis may be necessary for definitive treatment.
|Atria Tissue||Atrioventricular Junction|
|Sinus tachycardia||AV nodal reentrant tachycardia (AVNRT)|
|Atrial fibrillation||Atrioventricular reentrant tachycardia (AVRT)|
|Atrial flutter||Junctional tachycardia|
|Inappropriate sinus tachycardia|
|Sinoatrial nodal reentrant tachycardia (SANRT)|
|Intraatrial reentrant tachycardia (IART)|
|Multifocal atrial tachycardia|
Approach to narrow complex tachycardia
The first step in managing narrow complex tachycardia is to determine if the patient is hemodynamically stable. Indicators of hemodynamic instability are low blood pressure, shortness of breath, a decrease in consciousness, or chest pain (usually pressure). If the patient is hemodynamically stable, there is more time to evaluate the patient’s arrhythmia and provide therapy.
If the patient is hemodynamically unstable than rapid intervention is indicated:
The next question to ask is whether the rhythm is sinus or not:
- If the rhythm is not sinus in a hemodynamically unstable patient, then the first intervention is cardioversion as quickly as possible. This is because it is likely that the cardiac arrhythmia is causing the hemodynamic issue.
- If the patient is hemodynamically unstable but has sinus tachycardia (of any kind), then the first intervention is to support cardiovascular function and to search for and correct the cause.
Slowing the heart rate
In a non-emergent situation, efforts to slow the heart rate can be both diagnostic and therapeutic. If the patient is cooperative and able you may try a Valsalva maneuver. This can be accomplished in several ways, but one of the most common is to have the patient forcefully exhale against a closed nose and mouth. This increases intrathoracic pressures and intra-abdominal pressures, which can stimulate the vagal nerve and slow the heart rate.
Another way to slow the heart rate is the carotid massage. The clinician can feel for the carotid artery pulse and massage the area in the neck that corresponds to the carotid body. The stimulation also affects the vagus nerve and ideally causes the heart rate to slow. Carotid massage should not be performed in people with carotid artery disease either documented or suspected. Needless to say, only one side of the carotid artery should be massaged at any one time.
If the Valsalva maneuver or the carotid massage are effective, even partially, it may make it easier to determine whether the rhythm is sinus or not. In some cases these vagal maneuvers might be enough to bring the patient back into normal sinus rhythm, but this is uncommon. If vagal maneuvers are not fully therapeutic then it is appropriate to administer intravenous adenosine.
In adults the recommended first dose of intravenous adenosine is 6 mg. The dose is administered rapidly and then followed by a saline flush. Adenosine is only present in the circulation for about 5 seconds, so it is an excellent drug for diagnosis and treatment. If adenosine is to be successful, it will affect heart rate and rhythm within moments of administration. Likewise, most untoward effects from the drug usually only last momentarily. These may include flushing, shortness of breath, palpitations, lightheadedness, and chest pain. Since these symptoms overlap many narrow complex tachycardias, most patients do not even recognize that adenosine was administered. In the minority of patients who have an accessory pathway in their cardiac conduction system, adenosine can have serious consequences, namely the provocation of ventricular fibrillation. Therefore, adenosine should only be administered when emergency resuscitation equipment and those trained to use it are standing by.
If there was a response to adenosine but it was not long lasting, an additional dose of 12 mg of adenosine intravenously can be attempted. Doses greater than 12 mg are not recommended.
Definitive treatment for narrow complex tachycardia
The differential diagnosis for narrow complex tachycardia is extremely broad. In most cases, a cardiac electrophysiologist will need to distinguish between the various causes, usually through an electrophysiology study. In general, patients are treated with a drug to control rate, a drug to control rhythm, or with an ablation procedure.