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Tools of the Trade - Cardiac Mapping Systems

Image Courtesy of St. Jude Medical

Conventional Data in an Unconventional Format

           Cardiac mapping is the process of collecting and displaying information gathered from cardiac electrograms. From the earliest days of documenting the signals that represent the electrical activity of the heart, there has been some method of deriving information from these recordings. A grid was originally used to represent velocity and amplitude of surface ECG’s. By knowing how the grid was set up could derive a good deal of information from the lines of the electrocardiogram.
          With the advent of computer technology, medical inventors have furthered the capabilities both the recording and display of cardiac electrograms. The systems of today have advanced these functions to the point where the entirety of electrical activation within a single chamber can be recorded and displayed in a single beat. This section of The EP Lab will look at how the cardiac mapping systems function.

          For all practical purposes, there are two primary cardiac mapping systems available today. The first system that was commercially available was the Carto Electro-Anatomical Mapping System from Biosense Webster. Carto utilizes a magnetic field to navigate to a catheter with a chip that allows the system to detect its location within the magnetic field. Approximately five years after Carto was presented to the EP community, Endocardial Solutions presented the EnSite 3D Cardiac Mapping System. The original application of the Endocardial Solutions mapping systems was the EnSite Array. The Array used non-contact unipolar mapping, an approach that has confounded many who did not comprehend how this approach functioned. Several years after the Array was introduced, Endocardial Solutions introduced its own version of sequential contact mapping in the form of NavX, an impedance based navigation system. In 2002, St. Jude Medical acquired Endocardial Solutions and added the EnSite 3D Mapping System to their product line.

          No matter which mapping system you currently have, there are certain primary assumptions with cardiac mapping that were in place long before these tools were developed. An article entitled "The Assumptions of Isochronal Mapping", wriiten by Raymond Idecker et all, provides a detailed overview of how the process of cardiac mapping should be performed. Information on how to obtain this article can be found on our EP Articles Page. In the opening paragraph of this article, Ideker sets out four "assumptions" that must be met for a cardiac mapping system to acquire and provide data in a format that allows it to be diagnosite. The following is a quote from that article;

These assumptions include the following:
1) the location of the recording electrodes is known with sufficient accuracy to determine the mechanism of an arrhythmia or to guide therapy;
2) a single, discrete activation time can be assigned to each recording electrode location;
3) the presence or absence of activation at an electrode site can be reliably ascertained, and when activation is present, the time of activation can be determined with sufficient accuracy to specify the mechanism of an arrhythmia or to guide therapy; and
4) the recording electrodes are close enough together that the activation sequence can be estimated with sufficient accuracy to determine the mechanism of an arrhythmia or to guide therapy.
 Ideker et al; The Assumptions of Isochronal Mapping.

         Every mapping system should conform to these four assumptions.  When you are learning how the different mapping systems operate, keep in mind these assumptions and try to determine how the system you are using applies these concepts.

What functions should a mapping system provide?

         Before any comparisons start, we should take some time to review some of the key functions a cardiac mapping system should provide. These are the questions that you may want to ask when considering a purchase of a mapping system.

A)  Stand alone, or does it connect to alternate systems?     
          Integration is becoming more and more important in the lab. As the complexity of EP cases continues to grow, any methods of improving work flow have greater value. System integration is a small yet helpful link in this process. Eventually, one of two things needs to happen for this process to reach the point where it will have a positive impact on the EP Lab. One piece of equipment needs to become the primary point of data entry feeding out all the patient specific information to the other EP systems in the room, or all EP equipment needs to have the capability of recieving this information from a hospital network. The current method of entering patient and case specific data is slow and cumbersome. With the technology available today, the only reason this functionality exits is that the major companies involved have not dedicated the time and resources to make this happen.

B) What method of navigation is utilized to locate catheters?
          Non-fluoroscopic navigation of catheters can be accomplished using either magnetic fields of impedance. Both of these approachs have their limitations though impedance based navigation seems to provide the more reliable and more versatile navigation. As either magnetic or impedance based navigation systems have some measure of distortion within the displayed fields it becomes important to determine if the mapping system under consideration has any mechanism to compensate for these distortions. With magnetic navigation, the distortion comes at the edge of the magnetic fields which are curved. This becomes most apparent when a segmented image is imported and displayed as an overlay on the magnetic field. A similar phenomenon occurs in impedance based systems when catheters are navigated out into the veins where the local impedance is substantially different from the cardiac chambers.

C) How many catheters can be displayed?
         One big difference between the two primary systems available today is the number of catheters they can display. In the EP labs of today, being limited in the number or type of catheters that can be used is a serious limitation. Proprietary hardware was phased out years ago with devices and leads. Any lab that would allow themselves to be roped into buying a system that restricts utilization of catheters have placed themselves into a position where they are dependant upon that company. In todays market, this can be a costly mistake.

D) How anatomically detailed is the representation of the cardiac surface?
          Three dimensional mapping systems use constructed models to represent the cardiac chambers. Here consideration of the various system is in the detail that each provide when these models are created. Anatomic accuracy has been one of the driving forces behind the push to use segmented CT or MRI images as a representation of the cardiac chamber, especially in the left atrium where a wide variety of anatomic varients may be present. Before you purchase any mapping system, take a close look at how detailed the structures created by each of them are.

E) How many electrodes can be utilized to collect electrical information?
          The speed with which data can be collected is another strong consideration that should be taken when looking at a cardiac mapping system. The old point by point approach is rapidly fading towards obsolesance. When both static and dynamic maps are available with a few clicks of the mouse, it seems ludicrous to spend hours moving a catheter to different sections of the cardiac chamber. If simultaneous mapping is not to the users liking, then ask each vendor how many data points can be collected at one time.

F) What resolution is provided in the map?
          Resolution of the map provided is the next consideration. The resolution of a map can be dependant upon the number of points collected, it can also be dependant upon how the points are displayed. Consider that no matter what type of model is constructed, that model is created out of simple geometric shapes. These shapes are held together by a grid that acts as the anchors for both anatomic and electrical data points. If the grid surrounds larger spaces, the potential anchor points will be further dispersed. A smaller grid offers better resolution. Ask each vendor how many of the basic shapes (usually triangles) go into the construction of the model. Then ask what is the maximum number of electrical data points that can be applied to the model. These two pieces of information together will tell you which system has the better resolution.
           Non-contact mapping offers the best resolution of all the different approaches. The model is constructed using 36,000 triangles and the resolution range of the maps provided by the EnSite Array is 2048 to 3300 data points. This information can be collected in a matter of seconds. Many people shy away from this technology because it uses unipolar electrograms. I hate to burst the bubble, but non-contact unipolar mapping has been the gold standard in EP labs since before Dr. Ben Scherlag recorded the first HIS signal in 1965. You have to all the way back to Einthoven to find the creater of the first non-contact (cardiac contact) unipolar mapping system, for that is exactly what the surface ECG does. Mapping with non-contact unipolar electrograms uses the exact same skills.

G) What are the costs associated with a mapping procedure?
          Another serious consideration should be the cost associated with each procedure. How much money you spend and how much your facility can be reimbursed should be a big factor in the decision making process. Most of the companies will provide information on reimbursement and cost of the items used in each procedure. Keep in mind that if you have a proprietary system that the cost of catheters required for use with that system must also be taken into account. Compare this with the cost of using non-proprietary catheters. I would also recommend getting this information from all vendors under consideration and then comparing the numbers they gave you. It may also be worthwhile to have a billing specialist review the information.

H) What types of rhythms can be mapped?
          The function of a mapping system is to provide a tool that allows the user to achieve results that can not otherwise be achieved using conventional techniques. When considering a system, ask the vendor about system limitations. Can the system map non-sustained arrhythmias? How about hemodynamically unstable rhythms? Can it help with the really tough cases? After all, that is what this technology is supposed to do. Ask the vendor these questions and then ask others who have used the different systems.

I) How accurate is the navigation?
          When a system like this is validated with the FDA, there are studies that are done to verify that the system functions as intended. Ask to see the validation papers on the various systems under consideration. Reviewing this information can be helpful in the decision making process.

          When it comes right down to it, you have to ask yourself one primary question; Will the mapping system increase successful outcomes? After all, the main reason these systems exist is to help you bring successful outcomes to your patients. If it was your family member on the table, what capabilities would you want in the hands of the physician?

The EP Lab would like to display images of various equipment used in EP Labs. If you have an image you would like to see in this section, please contact us by clicking on Submit Information .

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