'GPS' for the Heart

Douglas Beinborn, RN, BSN, MA, Mayo Foundation, Rochester, Minnesota; Larry Fontaine, Medtronic, Inc., Minneapolis, Minnesota; Robert Rea, MD, Mayo Foundation, Rochester, Minnesota

A New Approach to Cardiac Navigation

Figure 1. The LOCALISA intracardiac navigation system consists of a compact workstation and a bedside stand. The workstation includes a Silicon Graphics computer and monitor running the Linux operating system and an uninterruptible power supply. The bedside stand includes input ports for tracking intracardiac electrodes and outputs for the orthogonal electric fields, as well as a completely adjustable flat-panel LCD display.

OOn May 1st, 2001, Medtronic announced the commercial release of its LOCALISA intracardiac navigation system at the 22nd annual North American Society of Pacing and Electrophysiology (NASPE) meeting. This system, named after the term 'localization,' is the first to provide three-dimensional visualization of conventional EP catheters during EP studies and ablation procedures. It allows real-time, nonfluoroscopic, 3-D navigation without requiring a special mapping catheter. It is sufficiently accurate for detailed catheter mapping and the creation of linear or complex RF lesion patterns, yet cost-effective and straightforward enough for everyday use. This system has the potential to reduce x-ray exposure and ablation procedure time significantly as compared to conventional two-dimensional imaging systems. Figure 1 shows the components of the LOCALISA system.

Figure 2. A schematic illustration of the three orthogonal planes used to define the 3-D space of the heart. Each plane consists of a voltage gradient established with a pair of skin electrodes.

A novel method of 3-D mapping.

The LOCALISA system does not extrapolate detailed electrical activation maps, but rather allows real-time imaging of catheters and marking of intracardiac points of interest. It does this by recording the voltage potentials on regular intracardiac electrodes within three electric fields that define a coordinate system. These voltage potentials are translated into a measure of distance in relation to a fixed reference catheter, giving the operator a 3-D representation of catheter location within the heart chamber. Individual locations can be saved, annotated, and revisited later in the procedure if desired. Repeatability tests have shown that an EP catheter can be returned to within 2 mm of any previously marked position with 99% confidence.

Principles of operation.

The foundation for the operation of the LOCALISA system is Ohm's law: the voltage drop (V) across a resistor (R) equals the magnitude of the current (I) times the value of that resistor, thus V = I x R. Similarly, an electric current, externally applied to the thorax, will result in a voltage gradient across internal organs such as the heart. A change in intracardiac electrode position across this voltage gradient will thus result in a different voltage. When this method is applied in three orthogonal directions (Figure 2), this principle allows measurement of the position of catheter electrodes in three dimensions during catheter mapping and ablation procedures.

Three pairs of skin electrodes are used to send three 1-mA currents through the thorax in three orthogonal directions (caudal-cranial, anterior-posterior, and left-right), using slightly different frequencies in each direction. Maps created with the system are thereby immune to patient movements, large or small, because the coordinate axes are generated on the body.

Figure 3. The reference electrode catheter used with the LOCALISA system is the unique catheter-delivered screw-in temporary pacing lead, Medtronic Model 6416.

In order to translate recorded voltage potentials into locations, the system must use a bipolar reference catheter with a known interelectrode distance. For this purpose, a 3.5 French (Fr) screw-in temporary pacing lead, delivered through a 6 Fr introducer sheath (Figure 3), is placed transvenously. This lead may substitute for a conventional quadripolar EP catheter in the high right atrial position, and its active fixation virtually eliminates reference dislodgement.

After the system is calibrated, the voltage potentials on each electrode are recorded and analyzed continuously with a digital signal processor to produce real-time location information for each of the three planes. These locations are then displayed on an X-Y-Z coordinate, which can be rotated and viewed from any perspective desired (see the accompanying case study). Because the LOCALISA system displays real-time electrode movements, catheter movements due to the cardiac and respiratory cycles are very similar to those seen with fluoroscopy. The software does have a filter to slow the sampling rate and minimize the effects of the cardiac and respiratory movements, if so desired.

In conclusion, the Medtronic LOCALISA system provides real-time, nonfluoroscopic, 3-D visualization of conventional intracardiac catheter electrodes. It is sufficiently accurate for detailed catheter mapping and the creation of linear or complex RF lesion patterns, yet cost-effective and straightforward enough for everyday use. The freedom of catheter choice, improved visualization of catheters in 3-D space, and broad clinical applicability make this system a valuable new tool for ablation procedures.

Clinical Applications for the LOCALISA System

EP studies. Engaging the CS can be difficult and time-consuming. The system can facilitate locating the CS ostium by marking strategic landmarks such as the tricuspid valve, AV node, and fossa ovalis. The CS can also be located by a process of elimination, whereby labels are placed at points where the CS ostium was not found.

Figure 4. Demonstrates the tricuspid valve annulus being mapped. Green point = the CS ostium; blue-green point = a His location; white points = the 3, 6, 9, and 12 o'clock annulus locations; red tip = the ablation catheter; yellow tip = the eight distal electrodes on a duodecapolar catheter.

Figure 5. Note the pink dot, which shows the concealed entrainment of the atrial flutter circuit demonstrated at approximately the 6 o'clock position. This view looks down from above, as indicated by the degrees of rotation at the bottom center of the screen.

Figure 6. Linear ablation was planned from the tricuspid annulus to the IVC; note the red points demonstrating the "practice line."

Figure 7. In this figure, the IVC-RA junction is marked in purple.

Figure 8. Shows the block that was confirmed through mapping.

Figure 9. A second line, marked in orange, was completed medial to the first, from the tricuspid annulus to the CS os.

Figure 10. A third, more lateral line (yellow) from the tricuspid annulus to the IVC and a fourth line (lavender) from the CS os to the IVC completed this ablation.

Atrioventricular nodal reentrant tachycardia. The system can mark structures such as the CS ostium, tricuspid valve, and AV node. Distances from the proximal HIS recording can be measured to assess the risk associated with ablating a point of interest (the slow pathway, for example).

Atrioventricular reentrant tachycardia. Allows the marking of mapped points and the manual recording of activation time at those points. This eases finding the site of earliest activation. 'Good' spots don't need to be abandoned in hopes of finding a 'better' (earlier) site.

Typical atrial flutter. This is covered further in the case study. The system can mark anatomic points of interest such as the CS os, tricuspid valve, and IVC. A line of ablation lesions can be visualized and revisited to assess the completeness of conduction block.

Focal atrial fibrillation. The fossa ovalis can be marked for safer access during the transseptal puncture. Once on the left side the pulmonary veins and mitral valve annulus can be labeled. RF lesions can be placed in relation to a circular multipolar catheter, which can be tracked in real-time around a PV ostium. The transseptal puncture site can be marked and recrossed with a steerable catheter if access is desired after the transseptal sheath is removed.

Incisional atrial tachycardia. Complex arrhythmias that originate from previous surgical intervention can be mapped, including marking incisional scars.

Fascicular left ventricular tachycardia. Anatomic areas of interest within the left ventricle such as the aortic valve, outflow tract, septum, and mitral valve can be marked. Electrophysiologic markers can be identified as well, such as the His, fascicular potentials during sinus rhythm, and mapped early potentials during VT.

Right ventricular outflow tract tachycardia. Anatomic landmarks such as the tricuspid valve, pulmonary valve, and the anterior, septal, and posterior areas of the outflow tract can be outlined, as well as early activation points during tachycardia and ablation lesions.

Ischemic ventricular tachycardia. Anatomic points of interest like the aortic valve, mitral valve, and the walls of the left ventricle can be outlined. Markers can be placed on areas of low amplitude, fractionated signals, and areas of scar. Labels can also be placed at points of early activation and RF lesions.

Potential clinical applications. Many other uses of the LOCALISA system can be considered. For example, it could be used to help choose an optimal pacing site within the right atrium. Each potential site could be labeled with the pacing thresholds and sensing levels recorded at that point. It could also help in navigating the coronary venous system, marking the location of side branches and storing threshold data at potential biventricular pacing sites. A pacing lead could then be guided directly to the optimal location. The 'open' nature of the system will undoubtedly lead to many other valuable uses.

Case Description

We describe a case where 3-D navigation eased a difficult atrial flutter ablation. The EP physician on this case was Robert Rea, MD, from the Mayo Foundation. The patient was a 70-year-old male with typical clockwise atrial flutter.

First, the tricuspid valve annulus was mapped with a steerable quadripolar electrode catheter to document the route of the circuit. In Figure 4, note the green point marking the CS ostium, blue-green point marking a His location, and the white points marking the 3, 6, 9, and 12 o'clock annulus locations. The ablation catheter, depicted with a red tip, and the eight distal electrodes on a duodecapolar catheter, depicted with a yellow tip, were tracked in real time throughout the procedure.

Next, concealed entrainment of the atrial flutter circuit was demonstrated at approximately the 6 o'clock position, marked with a pink dot. This view in Figure 5 is looking down from above, as indicated by the degrees of rotation at the bottom center of the screen.

Linear ablation was planned from the tricuspid annulus to the IVC, using the Medtronic ablation workstation. A "practice" line was completed, shown in Figure 6 as red points. The map was rotated and viewed from different angles to help foresee difficult anatomic areas. These points were hidden prior to the start of the first ablation.

A line of blocked was created along the cavotricuspid isthmus. The IVC-RA junction was marked in purple (Figure 7). The image was rotated and reviewed for potential gaps, which were interrogated by mapping.

Gaps in the line appear to be filled in. Block was confirmed through mapping. This is shown in Figure 8.

With atrial flutter persisting, a second line (marked in orange) was completed medial to the first, from the tricuspid annulus to the CS os (Figure 9). Biplane fluoroscopy was used sparingly to verify catheter positions during the procedure. Excellent correlation was noted between the catheter positions on the LOCALISA display and the catheter positions on fluoroscopy.

A third, more lateral line (yellow) from the tricuspid annulus to the IVC and a fourth line (lavender) from the CS os to the IVC completed this ablation (Figure 10). Bidirectional block was observed, and the patient left the lab in normal sinus rhythm. He has not experienced any further atrial flutter since the procedure.