AV nodal reentrant tachycardia (AVNRT) or atrioventricular nodal reentrant tachycardia is a type of supraventricular tachycardia and one of the most common ones. This means it is a type of fast heart rhythm originating from above the ventricles. This is felt as a series of fast, regular,palpitations which start and stops abruptly. Women experience this arrythmia more often than men. Though it can occur in childhood people in their twenties or thirties are more frequently seen with this form of tachycardia.
In people with AVNRT,instead of having one AV node, they have two pathways: a fast one and a slow one. Thereby the electrical impulses travelling from atria to ventricles get confused or more correctly they move in circles like a dog chasing its own tail. The heart beat rises fro 120 to 250 per minute. The beats are not erratic eventhough tachycardia is seen. But the heart's pumping gets affected. In most patients with AVNRT an underlying heart disease is not seen.
Fast, regular, palpitations which start and stop suddenly are the main symptoms.Anxiety,caffeine,alcohol,etc., may be the causes in most persons, but it need not be so in all. Occasionly patients may feel a slight dizzy feeling, chest tightness, or breathlessness. After the short episode of palpitations are over, the patients may feel a need to urinate or they might find themselves a bit lethargic. The syncope or dizzy feeling associated with this form of tachycardia may be due to a lowering of blood pressure. If the heart rate is very fast and the patient suffers from coronary artery disease, then he may develop chest pains similiar to angina. From the chest the pain radiates to the left arm and angle of the left jaw.
There are different types of AVNRT and they can be divided into three:
1.Common AVNRT - In this form the anterograde conduction is via the slow pathway and the retrograde conduction via the fast limb of the circuit. Hence this form is also referred as the "Slow-fast" AVNRT.
2.Uncommon AVNRT - Here as opposite to common AVNRT,it uses the fast pathway for antegrade conduction and the slow pathway for its retrograde conduction.So it's also called the "Fast-slow" AVNRT.
3.Atypical AVNRT - In this the anterograde conduction is through the slow route and the retrograde conduit is through the left atrial fibres that approach the AV node from the left side of the inter-atrial septum.
An ECG taken during the time of palpitations will show the typical changes. But if the symptoms are irregular,the doctor may order a Holster monitor (a 24 hour portable ECG) and a recorder will show the changes.Sometimes infrequent episodes may require the attachment of a microchip under the skin which will record the irreegular cardiac activity which can be read throught the skin. These ECG's also help to distinguish between AVNRT and other tachycardias and tachyarrythmias,for eg.Wolf-Parkinson - White syndrome. Some blood tests are also done in people with palpitations.They are:
- TFT(Thyroid Function Test) - Hyperthyoidism increases the risk of AVNRT.
- Electrolytes - Abnormal levels of potassium, calcium, and magnesium may predispose to AVNRT.
- Cardiac markers - this is done only if AVNRT is suspected to be a cause for myocardial infarction.
Blocking the AV node is the means to overcome supraventricular tachycardia caused by AVNRT. It can be done by a variety of methods.
1.Activating the vagus nerve.
2.Medication which slow the AV nodal conduction like beta blockers,calcium channel blockers or adenosine are commonly used.But in asthmatic patients, beta blockers and adenosine should be used with caution as they cause tightening of the airways.Diditaalis drugs are also used as therapeutic.
3.Cardioversion(electrical restoration of the normal herat rhythm) is done only if all other means fail or if the patient has a chance of heart failure.
4.Electrophysiology study-This is usually done to confirm the diagnosis of AVNRT. Radiofrequency catheter ablation of the slow pathway if carried out can cure the patient of AVNRT. This is carried out if the patient has frequent episodes of AVNRT, or if the drug therapy fails, or the patient doesn't want drug therapy, or if the patient cannot tolerate drug therapy.
Alcohol should be avoided as it can be a trigger. Caffeine, theophylline(tea), and theobromin(chocolate) must be used with caution in patients where the episodes are triggered by these alkaloids. Rest in a supine position during the period of attack. Patient should be educated on the risks and complications of the arrythmic disorder.
Complications are rare, but there are chances for myocardial infarction,tachycardia-induced cardiomyopathy, syncope, myocardial ischemia, congestive heart failure, and hemodynamic compromise.
In the absence of any structural disease, patients respond well to medication, or radiofrequency ablation which is 95% curative with low complications.