Device Interrogations and Utilization of Diagnostic Data
Charles J. Love
INTRODUCTION
Implantable cardiac rhythm devices have progressed from fixed function, fixed rate, nonprogrammable pacemakers, to the current generation of incredibly sophisticated, microprocessor-based, automated implantable cardioverter-defibrillators (ICDs). A good friend of mine likes to compare device management of 20 years ago to what we do today as flying a Cessna versus flying an F-16. The level of sophistication is several orders of magnitude greater for the latter, and the chance and consequences of making an error are much greater as well. On the other hand, the ability to provide phenomenal improvements in individual therapy and the opportunities to capture and manage critical information about patients and their rhythms have expanded markedly as well. More interesting is the development of functions and algorithms integrated into the devices to both provide programming guidance to the clinician, and even to allow automatic optimization of programmed parameters.
Learning how to manage and use the vast amount of data that can be retrieved from a device can be overwhelming, even for a “seasoned” device expert. Full interrogations of devices can produce dozens of sheets of paper with parameters, device information, diagnostics, and electrograms. The best way to approach this volume of data is to “divide and conquer;” that is, to deal with each part of the functionality of the device one piece at a time. It is also essential to use the graphs and histograms to provide an instant overview of the data. The author prefers to approach the interrogations in the following systematic manner:
Device functional information
Patient rhythm diagnostics
Rhythm-specific information
Appropriate programming values based on observed data
Other monitored parameters
GETTING STARTED
The first step to analyzing the data from a device is to obtain the data from the device. In order to do this, one must first identify the manufacturer so that the proper
programmer can be utilized. Unfortunately, despite many years of requests from those of us doing device work, there is no “universal programmer,” nor is there likely to be one in the foreseeable future. The manufacturer can be determined by one of several methods. First, most patients carry an identification card with them that will have the manufacturer, model, and serial number of the device. One word of caution is that if a patient has had more than one device, they may still have the card from the previous device which can lead to an erroneous identification. Should the patient not have an ID card, a chest radiograph can be inspected. Many devices have a radio-opaque identification tag that will have the company logo. If no logo is present, or getting a radiograph is not possible or convenient, each company has a toll-free phone number that can be called to obtain device registration information. One can call the number of each company until the one that has the information on the patient is reached. The phone numbers of the major device companies are listed in Table 12-1. As you can see, there has been a lot of consolidation in the industry!
programmer can be utilized. Unfortunately, despite many years of requests from those of us doing device work, there is no “universal programmer,” nor is there likely to be one in the foreseeable future. The manufacturer can be determined by one of several methods. First, most patients carry an identification card with them that will have the manufacturer, model, and serial number of the device. One word of caution is that if a patient has had more than one device, they may still have the card from the previous device which can lead to an erroneous identification. Should the patient not have an ID card, a chest radiograph can be inspected. Many devices have a radio-opaque identification tag that will have the company logo. If no logo is present, or getting a radiograph is not possible or convenient, each company has a toll-free phone number that can be called to obtain device registration information. One can call the number of each company until the one that has the information on the patient is reached. The phone numbers of the major device companies are listed in Table 12-1. As you can see, there has been a lot of consolidation in the industry!
TABLE 12-1 Phone Numbers of Major Device Companies | ||||||||||||||||||||||||||||
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Finally, if none of these options are available, one can try to interrogate a device using each programmer in turn until one “links up” with the device. This is generally frowned upon due to the (at least theoretical) possibility of an unintended effect on the implanted device. Note that in some situations, interrogation may not be possible due to the device being nonfunctional from battery depletion or electronic failure.
Once the proper programmer is obtained, the programmer is turned on and the wand is placed over the device, interrogation will either begin automatically or one simply presses the “Interrogate” (or “Find Device”) button to initiate the process. Once the interrogation process has completed, it is always a good idea to print out a number of reports, including current programmed settings (so you know where you are starting), measured data on the leads and battery, diagnostic data and episode data. Having these data is important not only for initial review, but also to have the information available should the counters become reset. Some devices reset counters
automatically after each device interrogation, which could result in the data being lost. Loss of these data may not only impact your ability to manage the patient, but your attending staff (or worse, the device clinic nurses) will be upset with you. Other than the potential for some data to be cleared, as long as the “Program” or “Transmit” button is not pushed, you will not change anything regarding device function. Now that you have access to the information, it is possible to attempt to make some sense of it.
automatically after each device interrogation, which could result in the data being lost. Loss of these data may not only impact your ability to manage the patient, but your attending staff (or worse, the device clinic nurses) will be upset with you. Other than the potential for some data to be cleared, as long as the “Program” or “Transmit” button is not pushed, you will not change anything regarding device function. Now that you have access to the information, it is possible to attempt to make some sense of it.
DEVICE FUNCTIONAL INFORMATION
First, determine what type of device you are attempting to evaluate; whether this is a pacemaker, implantable defibrillator, rhythm monitoring device, or hemodynamic monitoring device. Note that the latter two types of device provide NO therapy, and simply monitor and report on heart rhythms or hemodynamic parameters. Next, your attention should be focused on the basics of device function. What are the current settings for mode and rate? Is the battery status OK? Is the lead integrity and function within normal limits? For an ICD, is the high-voltage circuit working properly?
BASIC PROGRAMMED DATA
In order to evaluate what a device is doing, and whether it is doing it correctly, there are many parameters that need to be evaluated (Fig. 12-1). However, for the novice, we will keep it simple and look at the two key parameters for pacing; pacing mode and pacing base rate. Obviously, the rate seen on an ECG should not be lower than
the rate programmed into the device. There are some exceptions to this (such as rate hysteresis, sleep rate, and rate drop response), but we will stick to the most common methods of device setup. The mode will be expressed as a series of three or four letters denoting which chambers are paced, which are sensed, and the response to a sensed event. A fourth letter “R” may be present if rate modulation is activated (e.g., activity sensor and/or minute ventilation sensor). For ICDs, one will also need to evaluate whether the device is active or inactive/monitoring for detection and response to a ventricular tachyarrhythmia. This may be displayed in a number of ways as noted in Figure 12-2. One can also evaluate the rates at which ventricular tachycardia (VT) and ventricular fibrillation (VF) will be detected and treated. There may be occasion when the device will need to be disabled due to unnecessary shocks, or when the VT rate will need to be lowered due to a slow VT falling below the detection rate of the device.
the rate programmed into the device. There are some exceptions to this (such as rate hysteresis, sleep rate, and rate drop response), but we will stick to the most common methods of device setup. The mode will be expressed as a series of three or four letters denoting which chambers are paced, which are sensed, and the response to a sensed event. A fourth letter “R” may be present if rate modulation is activated (e.g., activity sensor and/or minute ventilation sensor). For ICDs, one will also need to evaluate whether the device is active or inactive/monitoring for detection and response to a ventricular tachyarrhythmia. This may be displayed in a number of ways as noted in Figure 12-2. One can also evaluate the rates at which ventricular tachycardia (VT) and ventricular fibrillation (VF) will be detected and treated. There may be occasion when the device will need to be disabled due to unnecessary shocks, or when the VT rate will need to be lowered due to a slow VT falling below the detection rate of the device.
 Figure 12-1 A printout of basic device parameters from a pacemaker. The parameters are divided into groups representing basic mode, timing outputs and other therapy features. |
BATTERY STATUS
Most devices now give an estimated longevity or a “gas gauge” to let one know the status of the battery. Examples of battery status indicators are shown in Figure 12-3. Some devices display a graph of battery voltage and/or remaining longevity, which is also useful to determine the presence of adequate power for the device.
HIGH VOLTAGE CHARGE CIRCUIT STATUS
ICDs have the ability to take a 3 V battery, charge a capacitor in about 10 seconds, and deliver an 800 V (40 J) discharge (this will be somewhat longer for the subcutaneous ICD as it delivers about twice the shock energy relative to transvenous devices). In order to provide this level, one or more capacitors are required to charge in a matter of seconds to the energy required to defibrillate the heart (Fig. 12-4). An excessive charge time can indicate a problem with the capacitor(s), or a depleted battery. Charge times to the full output is device-specific, but any charge time in excess of 20 seconds is clearly abnormal and should be addressed.
LEAD STATUS
There are three major components to the status of a pacing lead. Note that for most ICD systems, the shock lead is a multi-component lead, and has the ability to sense, pace and deliver the high energy shock.
Impedance (Resistance)
This is a core measure of the electrical integrity of the lead and connection to the device. For pacing leads (or the pacing component of an ICD lead), the normal pacing impedance can generally be anywhere from 300 to 1,000 Ohms (there are some exceptions for leads designed for high pacing impedance). For the shock component, the impedance is much lower, usually from 30 to 100 Ohms. The steadiness of the number over time is more important that the absolute value obtained. Many devices will store the values and display them as a graph upon interrogation (Fig. 12-5). A high impedance is associated with a fracture of the conductor coil or a poor connection to the device. A low impedance is associated with a short circuit, most commonly due
to a failure of the insulation. Be aware that with changes in pacing polarity (bipolar, unipolar, “extended” bipolar [also known as tip to shock-coil]) will affect the measured impedance due to differences in conductor surface area and contact area with the myocardium. Also note that most modern cardiac resynchronization therapy (CRT) devices have left ventricular pacing leads with up to four conductors. When one includes the options of pacing between pairs of these conductors as well as pacing from one of them to the device can, shock coil or RV pacing anode, there are many impedance values that can be measured.
to a failure of the insulation. Be aware that with changes in pacing polarity (bipolar, unipolar, “extended” bipolar [also known as tip to shock-coil]) will affect the measured impedance due to differences in conductor surface area and contact area with the myocardium. Also note that most modern cardiac resynchronization therapy (CRT) devices have left ventricular pacing leads with up to four conductors. When one includes the options of pacing between pairs of these conductors as well as pacing from one of them to the device can, shock coil or RV pacing anode, there are many impedance values that can be measured.
 Figure 12-2 Three different programmer printouts (A, B, and C) showing the status of tachycardia detection and therapies from different manufacturers. These all appear different, and one must be aware that careful inspection may be needed to determine if the device is active or not. |
 Figure 12-3 (A) “Gas Gauge” from a device showing a device nearing the replacement time. This format does not estimate time remaining or give information about battery voltage or impedance. (B) Another newer version of the “Gas Gauge,” now showing a full battery reserve, and in this case an estimation of the longevity. (C) Traditional form of data presentation showing battery voltage, battery impedance and estimated longevity. Lead measurements are also shown. |
 Figure 12-4 This data printout shows both the ICD status and lead data. The battery voltage is shown with the elective replacement indicator (ERI) referenced. Both the last full charge time and last capacitor reformation time (a test charge of the device) are shown. These may be the same. Generally, charge times to full output are in the range of 7 to 12 seconds. Times beyond 20 seconds are abnormal and should be addressed.
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