Atrial tachyarrhythmias (AT) constitute the vast majority of all tachyarrhythmias. Atrial flutter (AFl) and atrial fibrillation (AF) form a significant subset of all AT making up more than 70% of all cases. AFl and in particular typical AFl is caused by the development of a reentrant electrical pathway around the tricuspid valve. This tachycardia is very commonly treated with an ablation procedure where the end point is the production of block in the isthmus of tissue formed by the inferior vena cava (IVC) and the tricuspid annulus (TA). The recurrence of AFl following ablation (9%-12%) has been correlated with the presence of conduction gaps along the RF lesion line2.
Atrial fibrillation (AF) is the most common sustained cardiac arrhythmia, present in about 0.4% of the population. The prevalence and incidence of AF increases progressively with age for persons great er than 50 years old, affecting approximately 10% of those 80 to 89 years old3. The high prevalence of AF, the aging of the population, and the difficulty in maintaining sinus rhythm make the arrhythmia a very expensive healthcare problem. The result has been an ex plosion in the amount of research directed toward understanding the mechanisms of AF and developing new strategies for its treatment.
Ablation for Atrial Tachyarrhythmias
Ablation therapy for many superventricular tachycardias is curativ e. For instance, in Wolff-Parkinson White Syndrome (WPW), accessory AV pathways are destroyed and in most cases the syndrome is eliminated completely and permanently4. Similarly with AV nodal reentrant tachycardia (AVNRT), ablation therapy is highly successful technique for curing the underlying problem of a very small reentrant circuit within the node5. Typical Atrial Flutter can also be ablated by destroying a portion of the reentrant circuit located in the isthmus of tissue between the tricuspid valve and the inferior vena cava. However, this tissue is much wider and thicker than the tissue in AVNRT or WPW and typically requires repeated burns to achieve a total block of this pathway. Recurrence of typical flutter due to electrical leakage caused by incomplete lesions is relatively common (10%-20%)2.
Atrial fibrillation ablation is becoming increasingly common with thousands of procedures performed each year. A review of the recent AF literature by Steven et al estimated that 1/3 of all AF patients are candidates for ablation therapy6. Most procedures attempt to electrically isolate the pulmonary veins of the left atria and limit large re-entrant pathways. The most commonly performed procedure is now circumferential pulmonary-vein ablation6-9. Figure 1 shows an electro-anatomical map demonstrating the procedure (ref). The green portions of the image are the left atria, the red are the pulmonary veins and the deep red circles are the locations of ablation lesions. It can be seen that the goal of the procedure is to electrically isolate the pulmonary veins to prevent aberrant electrical excitation from reaching the body of the atria. Each ablation lesion is made by moving the tip of the catheter to the points and applying RF energy for approximately one minute.
These and other similar AF ablation procedures are lengthy requiring an average of more than 200 minut es per case. Between 20% and 30% of patients report no change in symptoms after 6 months. This could be for several reasons, one of which is incomplete isolation. In one study of 187 patients, 52 reported no reduction in symptoms after 6 months. Fifteen of these patients were re-studied and 42 of 51 total pulmonary veins demonstrated electrical reconnection1. The patients were re-ablated and at 15 months all but 1 had improved symptoms. This indicates that incomplet e isolation was a likely cause for the poor outcome in these patients. This study suggests that if there were a better way to assess lesions, patient outcomes would be improved.
Biophysics of Ablation
Ablation creates a non-disruptive coagulation necrosis of cardiac tissue. It kills the muscle cells preventing generation or conduction of an action potential, yet maintains the structural integrity of the tissue. Endocardial ablation is performed by steering a catheter tip to the target tissue and delivering RF energy (nominally 500k Hz, 20 Watts) for approximately 60 seconds. The RF current resistively heats the tissue in the immediate vicinity of the tip (1-2 mm)10. The heat from this tissue then conducts into deeper tissue and back into the catheter tip, a significant amount of heat is lost to the flowing blood flowing past the endocardial surface and around the catheter. The result is a lesion approximately 1 cm in depth and 1 cm in diameter.
Tissue overheating causes evolution of gas and subsequent explosion. Popping sounds can be heard frequently during ablation procedures. Popping can be associated with unwanted effects like blood boiling, endocardial rupture, catheter dislocation, and impedance rise11.
Our group and others have shown that a host off actors affect lesion size including: duration of a burn12, catheter tip size13, tip orientation13, tip contact14, ground electrode location15 and local blood velocity16, 17. Modern RF generators monitor the temperature in the tip of the catheter and provide feedback to maintain the tip at a consistent temperature value. Thermal monitoring of the tip temperature has provided better control of overheating but lesion size is still highly variable18. With so many factors affecting lesion size, it is unreliable to predict if a lesion will be transmural based solely on parameter selection. Lesion size confirmation by imaging would significantly enhance ablation procedures by limiting unnecessary lesions, reducing the risk of steam explosions, preventing damage of collateral tissue.
Lines of electrical block in the atria are created by making a series of connected lesions between anatomical structures that are naturally non-conducting. Geometry based systems for monitoring the position of the ablation catheter tip are used to locate the lesions in space and assist the clinician in forming a contiguous line19-21. These systems use electrical (LocaLisa™), magnetic (CARTO™)19, 20 or ultrasound22 energy to measure the location of the tip. In a typical procedure with these systems, the catheter is moved to 20-80 different locations within the chamber to outline its extent. A cartoon of the chamber is created by connecting the points to form triangulated surfaces and electrical signals recorded from the tip used to indicate activation time relative to a temporal reference. A color coded map of electrical conduction is then created on the cartoon image. The combination of geometric location and electrical measurements (electro-anat omical mapping) creates a very powerful and useful tool for mapping electrical conduction in the cardiac chambers.
The clinician also uses the electro-anatomical mapping system to position the catheter within the cartoon and create a lesion at an appropriate location. The lesion location is then marked on the cartoon, the catheter repositioned, and another lesion created. A typical cartoon from the CARTO™ system is shown in Figure 2. Once electrical conduction has been modified by the lesion, a new electrical map must be creat ed to demonstrate how the lesion has affected the activation pattern. This requires the reacquisition of conduction times at another 20 or so locations within the chamber. Remapping of electrical conduction is the only method currently used to test the continuity of a lesion line.
The ablation process may depress conduction in the vicinity of a lesion, so 30 minut es is usually allowed to elapse between creation of the last lesion and the last test of continuity. Isoprot erenol infusion and rapid pacing are also used during this time to test lesion electrical integrity4, 5, 23. Ablations to repair gaps in the line necessitate another round of waiting and mapping.
There are also potential difficulties that can be caused by creating a lesion with excess energy delivery. As mentioned above there is the potential for overheating and steam micro-explosions but there is also the possibility to damage adjacent tissues. A potentially fatal sequela of left atrial pulmonary vein isolation procedures is atrial esophageal fistula24-28. This occurs when ablation near the left pulmonary veins causes damage to the esophagus weakening the wall. The rate of this complication is difficult to measure from the reported case studies.