Overview of Electrophysiology Study and Ablation of Atrioventricular Nodal Reentrant Tachycardia
Atrioventricular nodal reentrant tachycardia, or AVNRT, is a common type of rapid heart rhythm disorder. This condition is typically treated with a procedure called catheter ablation. The majority of people affected by AVNRT are young to middle-aged adults who are otherwise healthy, with women being more commonly affected than men.
AVNRT is caused by a loop of electrical signals in the heart near a section called the AV node. This loop causes the heart to beat very quickly, but does not involve the lower chambers of the heart, or ventricles. The exact location of this loop of signals can be tricky to find, but doctors have been able to use specific landmarks in the heart to guide treatment with catheter ablation for the past 25 years.
Sometimes it can be difficult to tell AVNRT apart from other heart rhythm disorders. To accurately diagnose AVNRT, doctors may need to use special techniques to observe the heart’s electrical activity. The diagnosis and treatment of AVNRT involves a thorough understanding of these heart electrical patterns and the use of ablation therapy.
Anatomy and Physiology of Electrophysiology Study and Ablation of Atrioventricular Nodal Reentrant Tachycardia
Moe and his team were the first to discover that there are two pathways within the atrioventricular (AV) node, a part of the heart that helps control its rhythm. They found these pathways had different times of conduction and levels of refractoriness, which is how resistant they are to stimulation. These findings were first made in animal models and then later found to also be true in humans. This deviation in the pathways can potentially lead to a certain type of abnormal heart rhythm, initially known as AV junctional tachycardia, now called AVNRT. Being able to effectively treat this specific type of abnormal heartbeat by targetting specific areas of the “fast” and “slow” pathways further confirmed this information.
Then additional studies found that the circuit, which this abnormal heart rhythm uses, actually goes beyond the AV node itself into surrounding heart tissue. This could potentially explain why there are variations of tachycardia (fast heart rate), as there may be more than one slow pathway. Some researchers proposed that the slow pathway aligns with a backward extension of the AV node, whereas the fast pathway resides within a forward nodal extension. Post-mortem studies have verified the presence of these ‘damage spots’ a few millimeters below the AV node after successfully halting AVNRT.
Further research provided evidence of rightward and leftward posterior AV nodal extensions, the possible existence of connections between the AV node and the atrial myocardium, most likely representing the fast pathway. Current models of the AV nodal circuit include these extensions and portions of the tricuspid and mitral vestibules, which form parts of the heart’s structure.
While the exact anatomy causing the development of AVNRT is not fully understood, heart doctors or electrophysiologists use a triangle of Koch as a guide for where to place the catheter during ablation procedures, which are used to destroy the areas of the heart causing the irregular heartbeat. This triangle is bound by the opening of the coronary sinus at the bottom, the septal leaflet (a part of a heart valve) at the front, and the tendon of Todaro, a fibrous structure within the heart, at the back above.
AVNRT is a sudden-onset and ending abnormal heartbeat, primarily occurring in the AV node region, not necessarily involving the heart’s upper (atria) and lower chambers (ventricle). Two forms of AVNRT currently are known: typical and atypical.
Typical AVNRT, also known as slow-fast AVNRT, is mostly identified by the absence of distinguishable P waves, heart’s electrical signals. This is caused due to the forward conduction through the slow pathway and backward conduction through the quick pathway of the circuit. The atrial signals become masked and can appear as small deflections at the start or end of the QRS complex, a series of peaks and valleys in an electrocardiogram (ECG). This forms are a pseudo-R in lead V1 or a pseudo-S in leads II, III, and aVF. The RP interval of <70 msec is a well-recognized ECG criterion for diagnosing typical AVNRT. Typical AVNRT is much more common than the atypical variant in clinical practice. Atypical AVNRT occurs in two variants. In one variant, the forward conduction limb is fast while the backward conduction limb is slow, resulting in what's known as fast-slow AVNRT. In the second variant, called slow-slow AVNRT, both limbs of the circuit are slow. In this case, one would observe P waves before the QRS complex due to backward conduction through the slow pathway, resulting in a long RP tachycardia. An extended PR interval after an atrial extrasystole, an extra heartbeat starting in the atria, indicates initial forward conduction through the slow pathway. Even though clinical and ECG criteria are important for the initial diagnostic approach to AVNRT, electrophysiology studies are the only definitive way to distinguish between various regular fast heartbeat rhythms, confirm an AVNRT diagnosis, and guide treatment using catheter ablation.
Why do People Need Electrophysiology Study and Ablation of Atrioventricular Nodal Reentrant Tachycardia
AVNRT, or Atrioventricular nodal reentrant tachycardia, is a type of abnormal heart rhythm that can significantly affect a person’s quality of life. It can also result in high medical costs because it often causes frequent hospital visits and stays. Studies suggest that a procedure known as catheter ablation (CA) can be a helpful treatment. This procedure can increase quality of life and reduce medical costs for patients with AVNRT, especially when compared to drugs that control abnormal heart rhythms.
If a patient has recurring symptoms of AVNRT, CA is often the best option for long-term management of this heart condition. Recent guidelines from professionals in the healthcare field consider this procedure as a top recommendation for treating AVNRT.
When a Person Should Avoid Electrophysiology Study and Ablation of Atrioventricular Nodal Reentrant Tachycardia
There are no definite reasons why an electrophysiology study and CA, a cardiac procedure, can’t be carried out in people suffering from AVNRT (Atrioventricular Nodal Reentrant Tachycardia), a type of abnormal heart rhythm. This type of treatment seems safe to use for heart rhythm issues, even in older people. However, if someone already has PR prolongation – a slowed heart conduction where electrical signals take too long to travel from the upper heart chambers to the lower ones – they may be more likely to develop a severe form of AV block, a condition where electrical signals in your heart are delayed or blocked, during or after this procedure.
Equipment used for Electrophysiology Study and Ablation of Atrioventricular Nodal Reentrant Tachycardia
If the heart’s electrical activity is being tested in a special lab, also known as an electrophysiology laboratory, it must have certain tools and equipment. It could be a dedicated lab, or it could be used for other heart-related procedures, like cardiac catheterization which is a procedure used to diagnose and treat conditions. Here is a list of necessary equipment:
A fluoroscopic system – This is a device used to obtain real-time moving images of the internal structures of a patient’s body using a special type of X-ray. This could be a single or a bi-plane system, and comes with the C-arm (a device for capturing those blurry, black and white images), a radiographic table, and an image intensifier (a tool to enhance the images).
Data acquisition system for electrophysiology – This system includes a computer workstation with software to process data, a 12-lead ECG or electrocardiogram (a tool that measures the electrical activity of your heart), and at least 24 channels for collecting heart recording inside the body, with monitors to display these recordings.
A programmable electrical stimulator – This equipment is used for delivering burst pacing and premature stimuli. Essentially, this device helps control the heart’s rhythm during the test.
Radiofrequency ablation system and cryo-console – These are used for cryoablation, a procedure to destroy abnormal areas in the heart that are causing a heart rhythm problem.
Diagnostic and ablation catheters – These long, thin, flexible tubes are used for both, diagnosing and fixing a heart rhythm problem.
A monitoring system for hemodynamics – This is used to constantly check on the blood circulation and pressure during the procedure.
Emergency trays – These are needed in case of emergencies where fluid around the heart (pericaridiocentesis) or in the chest cavity (thoracentesis) needs to be removed.
Transthoracic and transesophageal echocardiography – These are types of ultrasound tests that use sound waves to create pictures of your heart, and can help rule out any issues with fluid potentially building up around the heart, or assist during procedures dealing with the left atrium of the heart.
There are also additional tools that are useful to have:
A 3-dimensional electroanatomic mapping system – This is useful to accurately place the catheter while minimizing exposure to radiation.
Intracardiac echocardiography – This test helps move the catheter around inside the heart, which can make it possible to perform the procedure without using radiation.
Vascular ultrasonography – This tool, used to get access to the femoral vein that’s located in your thigh, can lead to fewer vascular complications. This refers to problems related to the body’s network of blood vessels.
Who is needed to perform Electrophysiology Study and Ablation of Atrioventricular Nodal Reentrant Tachycardia?
The necessary team for performing a catheter ablation, a procedure to correct heart rhythm problems, includes: a certified electrophysiologist (a doctor who specializes in heart’s electrical activity), a first assistant, a cardiac nurse (a nurse who specializes in heart care), and an electrophysiology technician or specially trained electrophysiology nurse. There will also be extra personnel on standby who can help in case of emergencies, this includes individuals from anesthesiology (the doctors who make you sleep during a procedure), interventional cardiology (specialists in non-surgical heart procedures), and cardiac surgery (doctors who perform surgery on the heart).
Preparing for Electrophysiology Study and Ablation of Atrioventricular Nodal Reentrant Tachycardia
Before undergoing a procedure known as Cardiac Ablation (CA) for a heart condition known as Atrioventricular Nodal Reentry Tachycardia (AVNRT), there are a few important things to remember. The patient must not eat or drink for at least six hours before the procedure. Additionally, any medications that treat abnormal heart rhythms must be stopped for at least five half-lives of the drug, which is the time it takes for half of the medication to leave the body. Adjustments might also be needed for any medications treating low blood sugar levels.
On the day of the procedure, several things will happen when the patient arrives at the electrophysiology lab. The patient will be connected to several devices that monitor heart activity, blood pressure, oxygen levels, and other important readings. For a more detailed picture of the heart, some doctors might use a special 3-dimensional mapping system; if that’s the case, additional electrodes will need to be placed on the body.
The next step is to prepare for the procedure by numbing a small area in the leg. This allows the doctor to insert catheters (thin, flexible tubes) into a vein found in the thigh area known as the femoral vein. These catheters are placed in different parts of the heart to help the doctor locate the area causing the rapid heart rhythm.
A special catheter called an ablation catheter is then used. Depending on the situation, this might be a radio frequency (RF) ablation catheter, which uses heat, or a cryoablation catheter, which uses extreme cold. Either type of catheter can effectively disrupt the area causing the fast heart rate.
Lastly, the matter of patient sedation depends on the doctor and personal preference. While some doctors might avoid sedation to make the fast heart rate easier to trigger and treat, some patients might receive conscious sedation, which makes a patient relaxed and sleepy but not fully unconscious. This is especially common in children.
How is Electrophysiology Study and Ablation of Atrioventricular Nodal Reentrant Tachycardia performed
Distinguishing between different types of fast heart rates, especially a less common type called a-typical AVNRT, can be challenging. There are similar types of fast heart rates, such as atrial tachycardia (AT), and orthodromic atrioventricular tachycardia (AVRT), that also need to be considered. When diagnosing these, doctors look for certain signals during a normal heart rhythm and when the heart is beating fast (tachycardia).
In some cases, the presence of an accessory pathway (a pathway in the heart that isn’t normally there) may make it less likely for the heart condition to be AVNRT. Still, these pathways might not be involved in the abnormal heart rhythm itself.
If the heart signals move from the inside to the outside of the heart after pacing (a technique used to speed up the heart rate), this could indicate that the accessory pathway is not located on the left side of the heart. In addition, if the pathway between the lower and upper chambers of the heart (ventriculoatrial pathway or VA) is blocked, any form of accessory pathway can be ruled out. This would in turn rule out the diagnosis of AVRT. But remember, this does not rule out that an accessory pathway could still exist in other areas of the heart.
Doctors also observe a phenomenon called dual AV nodal physiology, which is a sudden change in signal timing in the heart. This may be indicative of AVNRT, and can help in diagnosing the condition. However, this does not mean that AVNRT is the only arrhythmia present. This is because dual AV nodal physiology can be present in people who have never shown signs of this type of fast heart rate.
The use of certain maneuvers during the diagnostic process can reveal this dual AV nodal physiology. One approach is the cross-over maneuver and another involves using sudden extra heartbeats, both can help uncover the presence of a slow pathway in the heart.
Typically, AVNRT can be triggered by using a pacing technique that gradually slows down the heart’s rhythm. However, it can sometimes be difficult to trigger AVNRT despite extensive efforts. In these cases, thinking about the benefits and risks of performing an ablation (a therapy that can potentially cure certain types of heart rhythm disorders) can be quite difficult.
Studies done during episodes of fast heart rate can provide more insights to aid in making a diagnosis. When the heart activation seems to be moving in an unusual manner during the fast heart rate, this could indicate a condition called orthodromic AVRT. However, AVNRT can still be a potential diagnosis. Another measurement used involves looking at the timing of signals from the atria (top chambers of the heart) to the His bundle (a part of the heart’s conduction system). This can help differentiate between the different types of fast heart rates.
Termination of the fast heart-rate with a block in the heart’s electrical pathway is a strong clue towards AVNRT. The spontaneous starting of the fast heart-rate, along with a sudden prolongation of signal timing is also highly indicative of typical AVNRT. Interestingly, an increase in the time interval between the electrical signals in the top and bottom chambers of the heart with the development of a functional block in the heart’s electrical flow can confirm the diagnosis.
Possible Complications of Electrophysiology Study and Ablation of Atrioventricular Nodal Reentrant Tachycardia
Although the procedure to correct a type of fast heart rate known as AVNRT (Atrioventricular Nodal Reentry Tachycardia) is safe, it does carry a slim chance of causing an issue where the signals in the heart that control the heartbeat (known as AV block) don’t work as they should. This might even require the implantation of a pacemaker, either immediately after the procedure or at a later stage. This can affect roughly 1 to 2.3% of patients who undergo a specific part of the procedure called a ‘slow pathway ablation’.
There is also an element of the procedure called ‘fast pathway ablation’, which is used for people showing certain signs of poor heart signal conduction. This carries a greater risk of causing an AV block.
There’s an array of other complications that can occur, but these are infrequent. These include problems accessing the blood vessels, puncturing the heart, leakage of fluid into the heart sack causing compression (tamponade), bleeding, or the formation of a dangerous clot (thromboembolic events).
A recent study showed that the overall risk of death from corrections made to treat fast heart rates is incredibly low (0.1%), while the rate of other negative events happening is rated at 2.9%.
What Else Should I Know About Electrophysiology Study and Ablation of Atrioventricular Nodal Reentrant Tachycardia?
Non-inducibility of arrhythmia means stopping abnormal heart rhythms from happening, which is considered the best outcome for a type of treatment known as cardiac ablation. This procedure is used to treat a heart condition called AVNRT (Atrioventricular Nodal Reentry Tachycardia). One way of achieving this is by completely eliminating the slow pathway, which is a route the electrical signals take in the heart. However, some research suggests that completely eliminating the slow pathway might not always be necessary for the treatment to work.
Some patients are still able to produce a single echo beat (a reflection of a sound wave back to its source) during tests after ablation treatment. This means that the slow pathway in these patients has not been completely destroyed but has been modified – this is known as slow pathway modification.
But, it’s important to note that slow pathway modification might not be suitable for patients who have undergone a specific type of treatment called cryoablation (use of extreme cold to destroy diseased tissue). Having a single echo with an AH jump (a type of signal in the heart) has been linked to the condition returning in these patients. For this reason, stopping abnormal heart rhythms remains the most reliable marker of better outcomes after AVNRT ablation.
The success rate of this treatment over a long time is high. There’s a very low chance (as little as 1.5%) of the condition coming back, as shown by several studies.
