CARTO

System Operation

The CARTO navigation system calculates the position and orientation of the catheter tip, using three known magnetic sources as references. The CARTO system uses static magnetic fields that are calibrated and computer controlled. Due to the nature of magnetic fields, the orientation may also be calculated while the tip is stationary. By calculating the strength and orientation of the magnetic fields at a given location, the x,y,z position may be calculated along with the roll, pitch, yaw orientation.

The system is typically described anecdotally as similar to the GPS unit in a car, and it operates on several of the same principles. The GPS satellite network operates by transmitting the location and reference time on each satellite via a radio signal, allowing the user's GPS device to triangulate its position from multiple satellites. An inherent limitation is that neither system can determine the orientation unless the user is moving.

System Summary

The CARTO system may be described as four "black box" devices:

1. The reference device constellation is a ring with three fixed, computer-controlled magnets that is positioned beneath the patient. The magnets are calibrated to be of different strengths and at fixed positions. As the strength and position of each of the magnets is both known and fixed, this provides a suitable point of origin references for the magnetic fields in the surrounding space.
2. The positioning sensor detects the characteristics of the magnetic fields at a given point in space. One position sensor is located in the catheter tip and six more are in six patches affixed to the patient's chest and back. By having the second set of position sensors as a reference, the relative position of the catheter tip may be calculated. This offers improved accuracy within the magnetic fields along with detection of any patient movement. The relative position is important as it allows the location data to be referenced to patient anatomy.
3. A decoder unit converts and calculates the signals received from the positioning sensors in order to provide comprehensible information. The position, orientation, temperature and ECG values are determined for the catheter tip. These steps require specialised processing hardware.
4. A workstation interprets the data from the decoder unit. The user selects calculated points that are then projected onto a pre-acquired computer tomography (CT) and a map is extrapolated. An electrophysiologist may then see the displayed position and orientation as well as ECG information. This assists in targeting specific locations, monitoring catheter tip behaviour and reacquiring previous points for additional ablation.

Generated coronary map

The CARTO workstation is typically used to display two generated windows with different points-of-view. This shows the operating physician an x, y, z and pitch, roll, yaw position of the catheter.

Integration to CT scan

Points within the cardiac structures are logged and used to generate a 3D map. These points may be correlated to a CT scan. Once a suitable fit has been established, the CT can afford a great degree of detail with regard to the cardiac structures. The real-time projection of the catheter onto the CT image is useful during AF ablation procedures.

Display of conduction velocity

As the catheter tip has an ECG pickup, recordings may be taken and correlated to a 12-lead ECG, pacing wire, a Biosense-Webster Lasso catheter and/or pentarray catheter, which is an ECG catheter featuring 4 leads on each of the 5 armatures. Through tracking and plotting the local ECG characteristics against the cardiac cycle, a map of the conduction velocity can be generated as a colour-code on the CT or point-generated CARTO map. This information is useful in identifying the location of high-velocity pathways, micro-circuits or errant pacing nodes.

Operator use

This technology allows the operator to determine catheter placement relative to previously observed physical features. This is particularly useful when ablating sites along the ridges surrounding pulmonary veins, which are otherwise difficult to acquire. This device also allows for continuous catheter monitoring without exposing the patient and staff to additional radiation.

User interface

The CARTO user interface is controlled by a technician, providing two separate points-of-view images of the catheter to the surgeon. Usability of the visual information is dependent on technician skill and communication with the physician. The closeness of fit of the previously acquired CT image is manipulated by a series of points established by the physician along cardiac structures. This is also determined by the physician-technician relationship as well as the anatomical knowledge of the technician. While basic usability of the CARTO system may be performed with minimal training, a good result requires time, knowledge and training.

Accuracy

The CARTO system is capable of accuracy and specificity down to <1mm. This is entirely dependent on fitting the registered endocardial points to the previously acquired CT. Anatomical knowledge and skill on the part of the technician, along with a good working relationship and communication with the physician or surgeon is key to the usability of this system. Due to movement during the cardiac cycle, the probe position is gated against the cardiac cycle via the ECG trace.

The contraction characteristics of the heart change during arrhythmia. This can result in a shift of cardiac features by 10-20mm within the chest. Operator skill is required to disregard this movement, along with judging shift during the respiratory cycle.

Modeling

As the technician acquires points on the inside of the heart, a theoretical model is constructed. A point on this model is then flagged and correlated with a similar point on the CT image. The software automatically adjusts the CT to a best fit with the registered points and may then be updated with future points. A false sense of faith in the technology may result in misleading information being extrapolated and delivered to the technician and physician. Improved physician training and skill will more quickly identify an incorrect fit, resulting in acquiring new points to generate an accurate fit.