`GPS` for the Heart

A New Approach to Cardiac Navigation On 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. 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. 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.