The process of recording signals from the various electrode catheters is accomplished by means of a data acquisition system or physiologic recorder. This system is comprised of a junction box where the electrode pins from the catheters are connected. The signals then pass to an amplifier that filters and amplifies the signals and then to a multichannel physiologic recording system. These computerized systems are capable of selecting which pairs of electrode poles in the junction box are paired to record a bipolar signal and of displaying these signals in real time on an LCD screen as well as storing them for subsequent review. Most systems will have a real time and a review screen that allows for the physician conducting the study to analyze the information as it is being obtained (Fig. 7-1). Typically, the endocardial signals undergo amplification, high-pass (30 to 40 Hz) and low-pass (400 to 500 Hz) filtering, and analog-to-digital conversion for subsequent display on an LCD screen and storage. A notch filter is also often available to help reduce 60-cycle noise; however, its use can potentially result in some loss of information. Bipolar electrograms are recorded from two closely spaced electrode pairs approximately 2 to 5 mm apart and are highly filtered. They are used primarily to time the wave front of electrical depolarization of cardiac cells as it passes by this electrode pair. Bipolar electrograms are useful for providing information regarding the timing and locations of a signal. Unipolar electrograms are less highly filtered and are recorded from a single electrode to a widely spaced distant or “indifferent” electrode. The unipolar signal provides additional information regarding the direction that the wave front is traveling either toward (in which case a +R wave is inscribed) or away from (QS complex) the electrode that is recording it. This is of particular use in mapping the site of origin of focal tachycardias or the

insertion of accessory pathways where a QS signal is indicative of the catheter being at the site of origin of the signal.

The stimulator delivers a constant current output through one or more output channels in order to pace the heart and deliver at least four coupled extrastimuli. The stimulator can be connected directly to the junction box where pacing poles are used to connect electrode pairs for pacing, or as is more commonly done today, the stimulator output is connected to the recording system where the computer controls which electrode pairs are used for pacing. This allows for different stimulation protocols to be set up using a multitude of specific electrode sites for stimulation. Often integrally connected to the physiologic recording system, the stimulator allows for pacing stimuli to be delivered to the specific electrode pairs on the electrophysiology catheters. An input from the recorder allows the stimulator to synchronize its output with a signal sensed from another catheter. By convention, pacing stimuli are delivered at twice late diastolic threshold or 2 mA at a pulse width of 2.0 ms. Pacing at higher outputs is often used to improve capture when the electrodes are not in direct contact with the tissue, for example, within the coronary sinus; however, higher outputs can be pro-arrhythmic and can result in “far-field” capture remote from the catheter tip. This can result in inaccuracy when “pace mapping” at high outputs during ablation procedures. In general, pace mapping should be performed at the lowest current that results in continuous capture. Conversely, the inability to pace the heart at high pacing outputs (10 mA and 2.0 ms) has been used to define areas of “unexcitable scar” during substrate mapping of arrhythmias.

Electrophysiology catheters are typically made of woven Dacron, a woven copolymer, polyurethane, or plastic. The electrodes are usually silver or platinum based. Electrode pairs are usually closely spaced approximately 2 mm apart and with varying amounts of space between the pairs. Typical diagnostic catheters will have four electrodes (a proximal pair and a distal pair spaced approximately 5 mm apart) and a fixed curve at the distal end. The catheter is rotated at its proximal end by the physician as it is inserted from a vascular access site and is advanced under fluoroscopic imaging and precisely positioned within the heart so as to record the desired signal (Fig. 7-2). Quadripolar fixed curve diagnostic catheters are usually positioned at the high right atrium, just across the tricuspid valve in order to record a His bundle potential and at either the right ventricular (RV) apex or RV outflow tract (RVOT). Specially designed multipolar catheters are sometimes added in order to record signals from multiple sites from within a structure such as the coronary sinus or a pulmonary vein or adjacent to an anatomic structure such as the tricuspid annulus (Halo catheter) or the crista terminalis (Crista catheter). Steerable catheters incorporate a “pulley” mechanism that allows the operator to vary the curve at the end of the catheter. This is most useful in positioning catheters in specific locations, such as the coronary sinus from a femoral approach or for mapping and ablation. Mapping catheters can bend in one direction (unidirectional) or two directions (bidirectional) and deflect along a specific radius of curvature. Finally, the size of the distal electrode on ablation catheters through which

radiofrequency (RF) energy is delivered can be varied. Most catheters have 4-mm tips; however, larger tips of 5, 8, and 10 mm have also been produced. Larger sized electrodes will produce larger lesion sizes during ablation and have been shown to reduce recurrence rates for arrhythmias such as atrial flutter where creation of a long contiguous line of scar is desired, as compared with conventional 4-mm-tip catheters. This increase in lesion formation with an increase in electrode size comes at the expense of decreased electrogram signal resolution. Thus, smaller electrode sizes are preferable for detailed mapping procedures.

Irrigated saline tip catheters provide continuous cooling of the distal electrode during RF energy delivery. “Closed” loop systems circulate saline within the catheter to the tip, whereas “open” irrigation catheter rinse the outside of the catheter tip with a controlled flow rate of saline allowing it to remain cool while large amounts of RF energy are delivered to the adjacent tissue. Irrigated catheters also produce larger and deeper lesions and potentially more durable results.

Contact force between the catheter tip and the myocardial tissue has been correlated with lesion size and ablation efficacy. Ablation catheters now have the ability to measure contact force at the distal end of the catheter. The Carto Thermocool Smart Touch catheter incorporates a distal electrode that is mounted to a spring and a transmitter coil along with a sensor attached to the proximal end of the coil. Deformation of the spring is then displayed in grams on the mapping system in real time. The Tacticath catheter uses a deformable element placed between more proximal electrodes with micro-fibers placed along the element. Differences in the wavelengths of laser light passed through these fibers are correlated with contact force.

Cardiac electrophysiology studies are usually performed either in modified cardiac catheterization suites or dedicated electrophysiology laboratories. Fluoroscopy is essential for positioning electrophysiology catheters within the heart. While basic electrophysiology studies and simple device implantation can be performed using portable fluoroscopy equipment, advanced mapping and ablation as well as accurate positioning of pacing leads in the coronary sinus call for the use of high-quality fixed-image intensifiers. Biplane fluoroscopy further adds to the efficiency of complex procedures allowing the heart to be sequentially imaged in orthogonal views. In our lab, we use biplane fluoroscopy routinely for trans-septal catheterization, for mapping of accessory pathways, and for pulmonary vein angiography during ablation of atrial fibrillation.

Mapping and ablation procedures involve considerable doses of radiation to the patient and scatter radiation to the electrophysiologist. The use of “pulse” fluoroscopy combined with proper shielding and good radiation techniques can minimize this risk to both the patient and the lab personnel. Increased awareness of the risks that radiation exposure imparts to both the patient and the operator has led to both improvements in fluoroscopy equipment and to methods that allow catheters to be localized and moved using alternative imaging modalities. In pediatric electrophysiology, catheters are moved predominantly within the “shell” created by 3D mapping systems and entirely fluoroless ablation of arrhythmias have been reported. Intracardiac ultrasound can be used for catheter navigation aiding in advancing catheters from the groin into the heart or moving a catheter from one pulmonary vein to another in the left atrium or between coronary cusps in the aortic root. Finally, several mapping systems incorporate software (CARTO UniView; Stereotaxis) that allows for the mapping catheter
to be displayed in real time over a fixed image, obtained at the start of the study in several fluoroscopic views in order to eliminate any additional use of X-rays. In our lab, combinations of these techniques have resulted in major reductions in radiation exposure. Typical ablation procedures for SVT, VT, or atrial flutter are performed with 3 to 8 min of fluoroscopy exposing the patient to well under 100 mGy. Pulmonary vein isolation that used to require 30 to 50 min of X-ray is now accomplished routinely with 5 to 12 min of fluoroscopy time.