Published online before print April 26, 2007, 10.1148/radiol.2433060993
(Radiology 2007;243:766-774.)
© RSNA, 2007
Effects of CT Irradiation on Implantable Cardiac Rhythm Management Devices1
Cynthia H. McCollough, PhD,
Jie Zhang, PhD,
Andrew N. Primak, PhD,
Wesley J. Clement, BSEE, and
John R. Buysman, PhD
1 From the Department of Radiology, Mayo Clinic College of Medicine, 200 First St SW, Rochester, MN 55905 (C.H.M., J.Z., A.N.P.); and Cardiac Rhythm Disease Management, Medtronic, Minneapolis, Minn (W.J.C., J.R.B.). Received June 8, 2006; revision requested August 10; revision received November 21; final version accepted December 6.
Address correspondence to C.H.M. (e-mail: mccollough.cynthia{at}mayo.edu).
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ABSTRACT
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Purpose: To prospectively measure the response of a variety of models of implantable cardiac rhythm management devices (ICRMDs) to the radiation delivered by computed tomography (CT), for both maximum and typical dose levels.
Materials and Methods: Twenty-one ICRMDs (13 pacemakers, eight cardioverter-defibrillators) manufactured by Medtronic (Minneapolis, Minn) were exposed to ionizing radiation from CT systems in both spiral and dynamic acquisition modes at maximum and typical dose levels. Devices were monitored during exposure to check for any operational abnormalities and were interrogated after exposure to check for any residual abnormalities. Total radiation dose and peak dose rate were measured, and the volume CT dose index was recorded.
Results: Oversensing was observed in 20 of 21 devices at maximum doses and in 17 of 20 devices at typical doses. Oversensing most often manifested as inhibition, although it occasionally manifested as tracking or safety pacing. Two devices inhibited for more than 4 seconds in spiral mode at clinical dose levels. Oversensing was transient and ceased as soon as the device stopped moving through the x-ray beam or the beam was turned off. The partial electrical reset (PER) safety feature was activated in two models, InSync 8040 and Thera DR. With the exception of PER, programming was not altered. Effects occurred only if the x-ray beam passed directly over the ICRMD.
Conclusion: CT irradiation at typical clinical doses results in oversensing of ICRMDs in the majority of devices tested, although the identified effects were predominantly transient.
© RSNA, 2007
Editor's note: In January 2006 (From the Editor), I announced a new section in Radiology—Innovations. Under this banner, we wish to publish original research that may possibly have far reaching implications. Authors interested in having their manuscripts considered for Innovations should first read the Editorial to learn of more specifics. Regarding the manuscript by McCollough et al, positive comments we received concerning its publication in Innovations included: "It is good science, innovative, important, and should be published in your Innovations section," and "Their results could be potentially very important."
—Anthony V. Proto, MD, Editor
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INTRODUCTION
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Implantable cardiac rhythm management devices (ICRMDs) are increasingly used in the care of patients with cardiac diseases. When cross-sectional imaging is required in these patients, computed tomography (CT) is currently the preferred modality because magnetic resonance imaging is presently contraindicated in patients with ICRMDs (1,2).
When x-rays strike electronic circuit components within ICRMDs, the x-rays can expel electrons, causing "free" electrons in the materials. The free electrons may allow small currents to flow within insulators or alter the control of currents by semiconductors, which in turn can cause small changes in the circuit voltages (3). Certain pacemaker and implantable cardioverter-defibrillator (ICD) circuits, such as sense amplifiers and voltage monitors, have the function of measuring small voltage changes. Hence, these circuits can sometimes detect the small voltage changes induced by radiation. At the dose levels delivered in radiation oncology, ionizing radiation can adversely affect ICRMDs (3–10). However, at the dose levels associated with diagnostic examinations that involve x-rays, radiation levels have been considered to be below the threshold where interference with ICRMDs will occur (7,9).
Recently, there have been reports of CT interference with ICRMD function (11,12). Thus, the purpose of our study was to prospectively measure the response of a variety of ICRMD models to the radiation delivered by CT, for both maximum and typical dose levels.
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MATERIALS AND METHODS
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Industry Support
Two authors (W.J.C. and J.R.B.) are employees of Medtronic (Minneapolis, Minn), whose products were evaluated in this study. Authors who were not employees of Medtronic (C.H.M., A.N.P., and J.Z.) were present during all testing and maintained a separate copy of all data used for this study. The single exception to this was the quality assurance and calibration testing performed subsequent to CT irradiation, which was performed at the Medtronic factory by nonauthor testing engineers. Analysis of the ICRMD operational and residual responses to radiation were performed in consensus by two employees of Medtronic (W.J.C. and J.R.B.) and one author who is not employed by Medtronic (C.H.M.). Medtronic supplied all of the tested devices and ICRMD programming and interrogation equipment; the devices and equipment were returned to Medtronic at the completion of data collection. An hourly equipment use fee was paid by Medtronic to the Mayo Foundation for use of the CT systems.
Models Tested
Thirteen pacemaker models and eight ICD models (Table 1) were exposed to ionizing radiation from both spiral and dynamic CT at maximum and typical dose levels (all authors). Devices were monitored during exposure to check for operational abnormalities and interrogated after exposure to check for residual abnormalities (see below). All devices were manufactured by Medtronic.
CT Measurements
Two CT systems were chosen to represent current state-of-the-art CT technology: a 16-channel system (LightSpeed 16; GE Healthcare, Waukesha, Wis) and a 64-channel system (Sensation 64; Siemens Medical Solutions, Forchheim, Germany).
Two CT scan acquisition modes were used—spiral (helical) scanning, where the x-ray tube continuously rotates around the patient while the patient is moved through the plane of the x-rays, and dynamic scanning, where the x-ray tube continuously rotates around the patient but a portion of the patient remains stationary within the plane of the x-rays. Currently, most CT examinations use spiral acquisition modes; however, dynamic scanning is used for the evaluation of blood flow through an organ (perfusion imaging) and during interventional procedures.
Measurements were made for two scenarios: maximum possible dose to the device and typical doses. For the maximum dose, the highest possible x-ray output of the systems was used with the ICRMD placed in the isocenter of the rotating x-ray beam without any attenuating materials (Fig 1a).
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Figure 1a: Experimental placement of ICRMDs within CT scanner. (a) Maximum dose was delivered when ICRMD was placed on foam pad in center of CT scan plane (isocenter), without any additional attenuating material in the beam. (b) Typical dose was delivered when ICRMD was placed on upper left anterior chest of anthropomorphic (patient-equivalent) phantom, similar to where such a device would be placed clinically. (c) Small amount of superficially attenuating material was placed over the ICRMD on the phantom to simulate overlying tissue in a clinical situation. (d) CT localizer image shows location of ICRMD relative to phantom's internal structures. (e) Purple box on CT localizer image shows craniocaudal extent of CT angiography and coronary artery calcium (CAC) scanning (left-right borders determine image reconstruction size only; all anatomic areas between upper and lower boundaries of the box are irradiated). White box shows craniocaudal extent of pulmonary embolism (PE) and routine chest scanning. In spiral scanning, irradiation extends 1–4 cm above and below the boundaries shown on the image to enable acquisition of sufficient information to reconstruct images at the boundaries of the desired scan range. Thus, irradiated tissue volume is somewhat larger than what each box shows. The exact amount of additional irradiation depends on the CT system and selected detector mode.
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Figure 1b: Experimental placement of ICRMDs within CT scanner. (a) Maximum dose was delivered when ICRMD was placed on foam pad in center of CT scan plane (isocenter), without any additional attenuating material in the beam. (b) Typical dose was delivered when ICRMD was placed on upper left anterior chest of anthropomorphic (patient-equivalent) phantom, similar to where such a device would be placed clinically. (c) Small amount of superficially attenuating material was placed over the ICRMD on the phantom to simulate overlying tissue in a clinical situation. (d) CT localizer image shows location of ICRMD relative to phantom's internal structures. (e) Purple box on CT localizer image shows craniocaudal extent of CT angiography and coronary artery calcium (CAC) scanning (left-right borders determine image reconstruction size only; all anatomic areas between upper and lower boundaries of the box are irradiated). White box shows craniocaudal extent of pulmonary embolism (PE) and routine chest scanning. In spiral scanning, irradiation extends 1–4 cm above and below the boundaries shown on the image to enable acquisition of sufficient information to reconstruct images at the boundaries of the desired scan range. Thus, irradiated tissue volume is somewhat larger than what each box shows. The exact amount of additional irradiation depends on the CT system and selected detector mode.
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Figure 1c: Experimental placement of ICRMDs within CT scanner. (a) Maximum dose was delivered when ICRMD was placed on foam pad in center of CT scan plane (isocenter), without any additional attenuating material in the beam. (b) Typical dose was delivered when ICRMD was placed on upper left anterior chest of anthropomorphic (patient-equivalent) phantom, similar to where such a device would be placed clinically. (c) Small amount of superficially attenuating material was placed over the ICRMD on the phantom to simulate overlying tissue in a clinical situation. (d) CT localizer image shows location of ICRMD relative to phantom's internal structures. (e) Purple box on CT localizer image shows craniocaudal extent of CT angiography and coronary artery calcium (CAC) scanning (left-right borders determine image reconstruction size only; all anatomic areas between upper and lower boundaries of the box are irradiated). White box shows craniocaudal extent of pulmonary embolism (PE) and routine chest scanning. In spiral scanning, irradiation extends 1–4 cm above and below the boundaries shown on the image to enable acquisition of sufficient information to reconstruct images at the boundaries of the desired scan range. Thus, irradiated tissue volume is somewhat larger than what each box shows. The exact amount of additional irradiation depends on the CT system and selected detector mode.
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Figure 1d: Experimental placement of ICRMDs within CT scanner. (a) Maximum dose was delivered when ICRMD was placed on foam pad in center of CT scan plane (isocenter), without any additional attenuating material in the beam. (b) Typical dose was delivered when ICRMD was placed on upper left anterior chest of anthropomorphic (patient-equivalent) phantom, similar to where such a device would be placed clinically. (c) Small amount of superficially attenuating material was placed over the ICRMD on the phantom to simulate overlying tissue in a clinical situation. (d) CT localizer image shows location of ICRMD relative to phantom's internal structures. (e) Purple box on CT localizer image shows craniocaudal extent of CT angiography and coronary artery calcium (CAC) scanning (left-right borders determine image reconstruction size only; all anatomic areas between upper and lower boundaries of the box are irradiated). White box shows craniocaudal extent of pulmonary embolism (PE) and routine chest scanning. In spiral scanning, irradiation extends 1–4 cm above and below the boundaries shown on the image to enable acquisition of sufficient information to reconstruct images at the boundaries of the desired scan range. Thus, irradiated tissue volume is somewhat larger than what each box shows. The exact amount of additional irradiation depends on the CT system and selected detector mode.
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Figure 1e: Experimental placement of ICRMDs within CT scanner. (a) Maximum dose was delivered when ICRMD was placed on foam pad in center of CT scan plane (isocenter), without any additional attenuating material in the beam. (b) Typical dose was delivered when ICRMD was placed on upper left anterior chest of anthropomorphic (patient-equivalent) phantom, similar to where such a device would be placed clinically. (c) Small amount of superficially attenuating material was placed over the ICRMD on the phantom to simulate overlying tissue in a clinical situation. (d) CT localizer image shows location of ICRMD relative to phantom's internal structures. (e) Purple box on CT localizer image shows craniocaudal extent of CT angiography and coronary artery calcium (CAC) scanning (left-right borders determine image reconstruction size only; all anatomic areas between upper and lower boundaries of the box are irradiated). White box shows craniocaudal extent of pulmonary embolism (PE) and routine chest scanning. In spiral scanning, irradiation extends 1–4 cm above and below the boundaries shown on the image to enable acquisition of sufficient information to reconstruct images at the boundaries of the desired scan range. Thus, irradiated tissue volume is somewhat larger than what each box shows. The exact amount of additional irradiation depends on the CT system and selected detector mode.
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For the typical dose, an anthropomorphic phantom (The Phantom Laboratory, Salem, NY) was placed on the patient table, and the ICRMD was positioned on the phantom chest wall (Fig 1b). Three centimeters of tissue-mimicking material (SuperFlab; Radiation Products Design, Albertville, Minn) was placed on the device to provide attenuation similar to that which would occur above a superficially implanted device (Fig 1c). Four common types of thoracic CT examinations were evaluated: coronary artery CT angiography, CT for CAC testing, CT for detection of PE, and routine chest CT. Scanner output was set to be appropriate to phantom size, and data were acquired with and without automated tube current modulation schemes (13). Tables 2 and 3 detail the scanning parameters for the maximum- and typical-dose examinations, respectively, as well as the volume CT dose index (CTDIvol) in milligrays reported for each examination on the scanner console.
Total dose (in milligrays) and peak radiation dose rate (in milligrays per second) to the device were measured (C.H.M., A.N.P., J.Z.) by using a calibrated solid-state CT dose probe (CT-SD16; RTI Electronics, Goteborgsvagen, Sweden). The probe was placed at the location of the ICRMD, either at the isocenter (maximum dose, Fig 2a) or on the phantom (typical dose, Fig 2b).
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Figure 2a: Experimental placement of solid-state CT dosimetry probe within CT scanner. (a) Maximum dose and dose rate were measured in center of CT scan plane (isocenter), without any additional attenuating material in the beam. (b) Typical dose was measured with probe placed on upper left anterior chest of anthropomorphic (patient-equivalent) phantom, similar to where an ICRMD would be placed clinically. A small amount of superficially attenuating material (shown folded back) was placed over the probe on the phantom to simulate overlying tissue in a clinical situation.
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Figure 2b: Experimental placement of solid-state CT dosimetry probe within CT scanner. (a) Maximum dose and dose rate were measured in center of CT scan plane (isocenter), without any additional attenuating material in the beam. (b) Typical dose was measured with probe placed on upper left anterior chest of anthropomorphic (patient-equivalent) phantom, similar to where an ICRMD would be placed clinically. A small amount of superficially attenuating material (shown folded back) was placed over the probe on the phantom to simulate overlying tissue in a clinical situation.
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Before each CT examination (in spiral or dynamic mode), a low-dose CT localization image was acquired to prescribe scanning over the ICRMD. Observed effects from the CT irradiation were compared with the location of the x-ray beam with respect to specific components within the devices (all authors). The majority of models were tested with both the 16-channel (17 of 21 models) and 64-channel (19 of 21 models) scanners at maximum doses. Typical doses were tested only with the 64-channel system for 20 of 21 models.
Data Collected from ICRMD Models Tested
Each ICRMD was connected to a 500-
resistive load to simulate in vivo conditions and was programmed to maximum sensitivity for the maximum dose testing and to nominal sensitivity for the typical dose settings (the majority of devices implanted use nominal settings). A digital oscilloscope (TDS5104; Tektronix, Beaverton, Ore) was used to record pacing outputs and timings. A photodiode placed on the gantry housing, in the x-ray plane, was used to set the oscilloscope trigger according to x-ray tube position and to record x-ray output.
The presence of operational abnormalities (extraneous or missed pulses, early or late pulses) was determined by two individuals with extensive experience in ICRMD technology (W.J.C., J.R.B.) and one novice observer (C.H.M.) from the oscilloscope traces (Fig 3). The specific effects evaluated included oversensing, which is any sensed event other than those that represent intrinsic cardiac activity; inhibition, which is when depolarization is sensed during normal pacing operations and the sensed event resets the timing cycle in that chamber and "inhibits" pacemaker output; and tracking, which is, for devices programmed to a P-synchronous pacing mode, when sensed events on the atrial sensing channel trigger ventricular pacing. After completion of device irradiation, the manufacturer programming device was used to interrogate the device, and two experts in ICRMD technology (W.J.C., J.R.B.) determined whether the device programming had been altered. Additionally, quality assurance and calibration testing were performed on each device at the manufacturer's factory by nonauthor test engineers.
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Figure 3a: Oscilloscope traces for InSync 8040 ICRMD during (a) spiral irradiation (ie, with device moving into and out of x-ray beam plane) and (b) dynamic irradiation (ie, with device remaining stationary in x-ray beam plane). Time scale for both traces is 20 seconds from left to right (2 seconds per large division along the horizontal axis). Yellow trace is radiation level at position in gantry closest to the ICRMD and gives an indication of position of x-ray tube. In a, x-ray dose increases and decreases as patient table is moved through the scan plane. Oscillating pattern superimposed on this curve represents variation in dose owing to phantom attenuation as x-ray tube moves around the phantom. In b, oscillations with tube position within scan plane remain, but because the table is not moved during irradiation, the dose at the ICRMD is the same for each rotation of the x-ray tube. Blue trace is atrial pacing pulse. Purple and green traces are right and left ventricle pacing pulses of the ICRMD, respectively; these pulses occur at identical times, so the purple trace overlies the green trace. In normal operation, all three pacing pulses should occur at regular intervals, approximately 1 second apart. Atrial oversensing is seen during irradiation in a when the blue atrial paces disappear owing to radiation-caused oversensing. Purple and green ventricular traces, in the absence of an atrial pulse, represent ventricular tracking. The device goes into complete inhibition when there are neither atrial nor ventricular pulses. In b, there is first an atrial oversense and ventricular tracking for one beat, followed by a delayed atrial and ventricular pace, and then complete atrial inhibition (no blue pulses) and ventricular tracking for the remainder of the 10-second dynamic scan. Continuous tracking, as illustrated here, is unusual because, more commonly, ventricular oversensing will occur and inhibit pacing, as occurred in a.
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Figure 3b: Oscilloscope traces for InSync 8040 ICRMD during (a) spiral irradiation (ie, with device moving into and out of x-ray beam plane) and (b) dynamic irradiation (ie, with device remaining stationary in x-ray beam plane). Time scale for both traces is 20 seconds from left to right (2 seconds per large division along the horizontal axis). Yellow trace is radiation level at position in gantry closest to the ICRMD and gives an indication of position of x-ray tube. In a, x-ray dose increases and decreases as patient table is moved through the scan plane. Oscillating pattern superimposed on this curve represents variation in dose owing to phantom attenuation as x-ray tube moves around the phantom. In b, oscillations with tube position within scan plane remain, but because the table is not moved during irradiation, the dose at the ICRMD is the same for each rotation of the x-ray tube. Blue trace is atrial pacing pulse. Purple and green traces are right and left ventricle pacing pulses of the ICRMD, respectively; these pulses occur at identical times, so the purple trace overlies the green trace. In normal operation, all three pacing pulses should occur at regular intervals, approximately 1 second apart. Atrial oversensing is seen during irradiation in a when the blue atrial paces disappear owing to radiation-caused oversensing. Purple and green ventricular traces, in the absence of an atrial pulse, represent ventricular tracking. The device goes into complete inhibition when there are neither atrial nor ventricular pulses. In b, there is first an atrial oversense and ventricular tracking for one beat, followed by a delayed atrial and ventricular pace, and then complete atrial inhibition (no blue pulses) and ventricular tracking for the remainder of the 10-second dynamic scan. Continuous tracking, as illustrated here, is unusual because, more commonly, ventricular oversensing will occur and inhibit pacing, as occurred in a.
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Each ICRMD was irradiated up to three times for the maximum and typical dose conditions according to the following protocol: If any (first, second, or third) irradiation resulted in an effect, the device was considered sensitive and its testing complete. If no effect was recorded, the irradiation was repeated. If an effect did not result after three irradiations, the ICRMD was considered to be not sensitive to CT irradiation. Prior to irradiation, we verified that device interaction was not the result of non–x-ray electromagnetic interferences by shielding the device with lead during irradiation.
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RESULTS
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Total Dose and Dose Rate
In the typical dose scenario, the total dose to the ICRMD was considerably less than in the maximum dose scenario, because the phantom and superficial tissue layer attenuated a considerable portion of x-rays. However, because the device was closer to the x-ray source when it was placed on the phantom, the dose rate was transiently higher whenever the tube passed closest to the device and transiently much lower (owing to phantom attenuation and an increased distance from the x-ray source) whenever the tube passed under the phantom, producing a time-varying exposure (Fig 4).
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Figure 4: Graph shows time-varying intensity pattern recorded with dosimetry probe located on phantom at location of the ICRMD. Peak radiation intensity occurred when the x-ray beam plane was at the position of the ICRMD. Periodic oscillations demonstrate the rotation of the x-ray tube around the supine anthropomorphic phantom, with minimal radiation intensities when the x-rays were entering the phantom posteriorly and maximum radiation intensities when the x-rays were entering the phantom anteriorly.
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Effects of CT Irradiation at Maximum Doses
Operational abnormalities during irradiation due to oversensing occurred at least once in 20 of 21 models tested (Table 4). For some devices, oversensing produced varying degrees of inhibition, lasting only while the width of the x-ray beam directly irradiated the device's electronics, a time that was generally from 1 to 5 seconds in duration (Fig 3). In some devices, there was brief tracking, for about 1 second, until inhibition superseded, and, in some cases, ventricular safety pacing occurred for one cycle. The AT500 model was not found to be susceptible to oversensing in any tests.
The only programming (or residual) error found after the irradiation was the PER, which was activated in only two models, InSync 8040 and Thera DR. The PER is an automatic safety feature that resets key pacing parameters to specific default settings. There were no spurious changes in the programmed settings in any device, except as intended secondary to PER. No device failed manufacturer quality assurance and calibration testing on return to the manufacturer. No effects were detected for any device for irradiations in which the device was shielded with lead.
Effects of CT Irradiation at Typical Doses
Oversensing was observed in 17 of 20 devices (Table 5). Effects occurred only when the x-ray beam was directly over the ICRMD electronics module. Two devices demonstrated oversensing for 4 or more seconds. No models exhibited PER, altered programming, or damage. No effects were noted during acquisition of the low-dose CT localizer image.
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Table 5. Effects of CT Irradiation on ICRMDs at Typical Doses for Ranges of Total Dose and Peak Dose Rate Evaluated
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DISCUSSION
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This study has revealed that ionizing radiation from CT examinations can indeed influence ICRMD operation. The effects included oversensing (inhibition, tracking, and safety pacing) and PER. The effects observed were associated only with direct irradiation of the pulse generator (electronics module). No device was permanently damaged.
Oversensing was observed in most devices at both dose levels. Oversensing can be benign or potentially problematic. For pacemakers that operate in a P-synchronous mode, oversensing on the atrial sense amplifier may trigger nonphysiologic tracking, resulting in inappropriately rapidly paced ventricular rates. Because pacing rate is limited by an upper rate limit feature, this is typically benign. Oversensing occurring on the ventricular sense amplifier may cause inhibition of pacing. This may be clinically important for pacemaker-dependent patients if inhibition is persistent (ie, during dynamic scanning directly over the device). (Published guidelines for pacing [14] consider pauses > 3 seconds to be potentially clinically important.)
For pacemakers with atrial antitachycardia features, oversensing introduces extra senses that may simulate an atrial arrhythmia and cause a false detection and delivery of an unnecessary atrial antitachycardia pacing therapy. This is typically benign. This effect, however, was not observed in the AT500, which was the only model with antitachycardia features tested.
In ICDs, the tachycardia detection algorithm continuously monitors the sequence of sensed events to detect ventricular tachycardia or ventricular fibrillation. If ventricular tachycardia occurs, the algorithm may prescribe ventricular antitachycardia pacing or a high-energy shock; if ventricular fibrillation occurs, it will prescribe a high-energy shock. Antitachycardia pacing can be initiated immediately, but to deliver a shock, the device must first charge its high-energy capacitor. Most contemporary ICDs reconfirm the tachyarrhythmia after charging but before the shock, and in most CT examinations, oversensing will end before the confirmation check is made. For a typical chest CT examination with modern spiral CT equipment, the patient is moved through the x-ray beam, which is 1–4 cm wide, at a speed of 3–12 cm/sec. Thus, the dwell time of the radiation over the approximately 1-cm electronics module will be less than 1 second. (The dwell time over the device can be calculated with the following relationship: x-ray beam width plus ICRMD electronics module z-axis dimension divided by table speed.) As a result, for most CT examinations the probability of a shock is extremely low. The alternative is when the ICD is functioning in a "committed" mode such that once the capacitor is charged, the shock is delivered regardless of rhythm.
In patients for whom possible inappropriate therapies are a concern, such as when dynamic scanning (involving no table movement) is performed directly over the device, therapies can be averted by temporarily programming an ICD to "monitor only." The clinical staff must ensure that the original program is restored before the patient is dismissed from the hospital or imaging department.
During inhibition, the heart reverts to its underlying rhythm. In pacemaker-dependent patients, asystole can occur during inhibition. The duration of inhibition depends on the duration of irradiation over the electronic circuit module. For helical scanning, this is approximately a few seconds. For dynamic and interventional scanning (ie, when there is no table movement), this would be the entire duration of the examination. Generally, patients do not notice single pauses up to 3 seconds. Pauses of more than 3 seconds may result in varying degrees of symptoms (14). Inhibition can be averted by programming the pacemaker to an asynchronous mode. The clinical staff must ensure that the patient cannot leave the hospital or imaging department until the original program is restored. A very small fraction of patients are at risk for pace-induced tachyarrhythmia during asynchronous pacing; thus, consideration of the patient's cardiac condition is required.
At maximum dose levels, PER was observed in both the InSync 8040 and Thera DR models. At clinical dose levels, PER was not observed in any model; this finding is consistent with that of previous investigators (11,12), who were unable to reproduce a PER for a routine chest CT examination in clinical conditions. The manufacturer has recently received anecdotal reports of PER in which the physicians involved believed that the PER occurred at the time of a CT acquisition. The exact doses of the CT examinations and a true causal relationship between the PER and CT have not been able to be determined. In the tested models, PER resets the pacing rate to 65 beats per minute and the output to 5 V at 0.4 msec. A PER message is stored that will display when the device is interrogated. In rare circumstances, the reset parameters could adversely affect a patient who has a very high pacing threshold exceeding 5 V or 0.4 msec.
The results of this study support the general conclusion that it is safe for patients with ICRMDs to undergo CT examinations in which the device moves briefly (approximately < 3 seconds) through the x-ray beam (as in sequential or spiral scanning), as does the fact that many of the ICRMD models investigated in this study have been in use for more than 10 years, yet only recently has an effect been reported to manufacturers (11,12). Effects likely have not previously been noticed because many CT examinations did not irradiate the ICRMD, and, for those that did, any effects were likely brief, benign, and with no lasting consequence.
We observed oversensing typically during the first rotation in which x-rays directly irradiated the ICRMD. Hence, we believe that peak dose rate has the more important influence on ICRMD operational response (ie, during irradiation) compared with residual effects (ie, postirradiation), which are more commonly associated with the total dose to the device. We believe that the similar incidences of oversensing observed, independent of total dose to the device, was because all protocols tested delivered a peak dose rate high enough to cause an effect. This will be true for most relatively new CT scanners, such as those used in our study, because of the continuing adoption of shorter gantry rotation times and higher x-ray power levels, which together deliver higher peak dose rates.
A limitation of our study was that the devices were all from the same manufacturer. Different manufacturers may apply different integrated circuit technologies in the design of their products, and alternate designs might have a different incidence of interference. In addition, although we included various implantable devices, the device mode was most often dual-chamber pacing, dual-chamber sensing, dual-sensing actions enabled. Other device modes might have different incidences of interference.
The first consideration for a patient with an ICRMD who presents for a CT examination is whether the electronics module of the device will enter the x-ray beam; if not, there is no concern. This can readily be determined from the low-dose CT localizer image acquired at the beginning of a CT examination to plan the beginning and ending locations of the CT scan. CT operators should receive training regarding the appearance of this part of the device and in how to avoid direct irradiation, if deemed appropriate. ICRMD dependency and the duration that the electronics module will be in the x-ray beam should be considered. Defibrillators may be temporarily programmed to "monitor only" for the duration of scanning.
The historical absence of adverse reports regarding patients with ICRMDs undergoing CT implies a general level of safety in these applications. However, the findings in this study demonstrate that the influence of radiation during CT must be considered. Although the identified effects are generally benign, it is appropriate to consider situations in which these effects may produce clinically important risk and methods to mitigate these risks.
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IMPLICATION FOR PATIENT CARE
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- CT irradiation directly over an implantable cardiac rhythm management device (ICRMD) can cause transient interruptions in device function. Although generally benign, these effects merit consideration in ICRMD-dependent patients presenting for CT.
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FOOTNOTES
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Abbreviations: AEC = automated exposure control • CAC = coronary artery calcium • CTDIvol = volume CT dose index • ICD = implantable cardioverter-defibrillator • ICRMD = implantable cardiac rhythm management device • PE = pulmonary embolism • PER = partial electrical reset
See Materials and Methods for pertinent disclosures.
Author contributions: Guarantor of integrity of entire study, C.H.M.; study concepts/study design or data acquisition or data analysis/interpretation, all authors; manuscript drafting or manuscript revision for important intellectual content, all authors; manuscript final version approval, all authors; literature research, C.H.M., J.Z., W.J.C., J.R.B.; experimental studies, all authors; and manuscript editing, all authors
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References
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ABSTRACT
INTRODUCTION
