Nearly everyone reading this column has used a joystick remote control to play a video game. Weaving a player through a dangerous maze of obstacles and around corners seeking to damage or destroy the enemy in an effort to win the game is a great form of entertainment, but for some of us, a similar technique may one day save our lives.
Nearly 90,000 Americans are diagnosed with supraventricular tachycardia (SVT) per year. This heart rhythm can drive the heart to beat up to 200 beats per minute and cause shortness of breath, chest pain, and lightheadedness. There are several subclasses of SVT, with the most common being atrioventricular nodal reentry tachycardia (AVNRT) and atrioventricular reciprocating tachycardia (AVRT). Each of these has an abrupt onset and endpoint and when accompanied by unhealthy coronary arteries can cause chest pain and, potentially, an acute cardiac event.
SVT is about twice as common in women as it is in men and the condition is generally thought to arise because of a person’s anatomy, that is, there is more than one electrical pathway carrying the electrical impulse from the atrium to the ventricle. If a beat occurs at an incorrect time, the two different pathways cause a circular transmission of the impulse that in turn causes the rapid heart beat.
SVT is treated with medications such as calcium channel blockers and beta-blockers as well as antiarrythmic medications. For severe cases, electrical cardioversion is used. When persistent, the abnormal electrical pathway can be destroyed or ablated using a catheter approach. There are two different destructive methods used in the United States — radiofrequency (think microwave) and freeze destruction (cryotherapy) — both of which have specific advantages and disadvantages that are beyond the scope of this article.
The ablation catheter is inserted — in a manner similar to the more commonly done coronary catheter studies — into a major blood vessel. The long flexible catheter can be turned or bent using an internal wire that is manipulated to allow the tip to move up and down in two different directions. To move in other directions, the catheter is twisted or rotated. In addition, the catheter is pushed or pulled to advance or retract.
The catheter is threaded into the heart through the atrium, where sensitive electronic monitors assist in detecting and selecting the spot where the abnormal circuit is located, similar to a stud finder being used to detect a hidden 2x4 behind drywall. The cardiologist uses fluoroscopy to “see” the catheter.
The use of the fluoroscope is a limiting factor in this procedure as it exposes the patient and, to a lesser extent, the cardiologist to radiation. This is an iterative process and after the proper position is located, often after several trials, the area is ablated using the energy sources described.
The ideal catheter would have directional movement that closely mimicked that of your shoulder where the tip could move in any direction rather than just back and forth. Using the somewhat limited “wire pull” approach, this omni-directional motion is just not possible.
However, using a different technology developed by Stereotaxis, a new catheter named the Helios II accomplishes this 360 degree motion.
The Helios catheter uses a magnetized tip that moves in response to a strong magnetic field created by a device that might remind the reader of an MRI machine. The Stereotaxis Magnetic Navigation System (MNS) surrounds the patient and consists of two permanent magnets located on opposite sides of the patient, which are controlled by a computer and several motors as well as a device that advances and retracts the catheter through the skin incision.
As the permanent magnets move and rotate, the catheter tip can be made to rotate to match the architecture of the blood vessels through which the catheter is passed. At the same time the catheter can be advanced or retracted using the second device.
The advantage of this system is that other than introducing the device into the body, the cardiologist can be in a control booth that is completely shielded from the radiation created by the fluoroscope. Also, as the mapping proceeds and the best position is chosen, the computer can reproduce the exact location from memory, reducing the need for extra fluoroscope time for the patient.
Once positioned, the catheter can be connected to a variety of FDA-approved radio frequency generators with the appropriate interface cable to destroy the accessory pathway. The catheter has an integrated temperature monitor to ensure that the critical temperature is reached to destroy the affected tissue.
The Helios II Ablation Catheter was approved based upon two clinical studies, the Arrhythmia Treatment with a Thermocouple Radiofrequency Ablation Catheter (ATTRAC I) that consisted of 210 patients and ATTRAC II (89 patients). Both included endpoints of elimination of the AVRT or AVNRT or, in patients with atrial fibrillation with difficult-to-control ventricular response, to create a complete AV nodal block.
Obviously the studies could not be blinded to either the patient or physician because the process differs so much from a normal ablation. Patients were at least 18 years old and had experienced at least one episode of tachyarrhythmia within 12 months of their participation in the trial. ATTRAC I had an acute success rate of 96.2 percent, of which 98.6 percent were still meeting the end point six months later and 87.1 were able to stop taking medication. ATTRAC II had an acute success rate of 94.7 percent, of which 94.4 percent still reached the end point six months later and 88.9 percent were able to stop taking medication. Nearly 9 out of 10 patients were able to discontinue the use of antiarrythmic drugs after completion of the two studies (87.1 percent and 88.9 percent, respectively). These numbers are considered appropriate for this particular procedure.
There are limitations for the use of this catheter. It is only approved in one specific area of the vasculature — the coronary sinus. It is not approved to be used in the presence of a thrombis or myxoma of the left atrium.
Specific cardiac surgical patients also cannot be treated with this catheter: those who have had an intra-atrial surgical patch placement for a “hole in the heart,” and those patients who have had an aortic valve replaced. Finally, since it uses strong magnets, it is contraindicated in patients with iron-based metal fragments located in places that could be dangerous if the fragments shifted.
Remote controlled magnetic guidance is now a fact of life. In the United States, 66 sites will soon have access to this million-dollar medical “video game.” Future research is already under way and this technology will without a doubt be introduced into other areas where precise, repeated use of a catheter is needed. It demonstrates how virtually any leading edge technology can be harnessed for Tomorrow’s Medicine!