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Cardiovascular Anesthesiology: Technical Communication

Magnetic Interference of Cardiac Pacemakers from a Surgical Magnetic Drape

Zaphiratos, Valerie MSc, MD, FRCPC*; Donati, Francois PhD, MD, FRCPC*; Drolet, Pierre MD, FRCPC*; Bianchi, Andrea PhD; Benzaquen, Bruno MD, FRCPC; Lapointe, Jacques MD; Fortier, Louis-Philippe MSc, MD, FRCPC*

Author Information
doi: 10.1213/ANE.0b013e31827ab470
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Abstract

Approximately 3 million pacemakers and 1 million implantable cardioverter defibrillators (ICDs) were implanted in the United States between 1993 and 2006, and the implantation of cardiovascular implantable electronic devices (CIEDs) is increasing.1 Placing a magnet on a cardiac pacemaker usually enables an asynchronous mode in which the heart is paced at a predetermined frequency.2–5 However, this magnet-activated asynchronous mode may have undesirable consequences, such as battery depletion, undesirable hemodynamic effects caused by rapid pacing in some pacemakers, and rarely, ventricular dysrhythmias if pacemaker stimulation occurs in the vulnerable part of the cardiac cycle in patients with an intrinsic cardiac rhythm.2,6–9 In patients with an ICD, application of a magnet does not enable asynchronous pacing, but rather may suppress the detection of arrhythmias and prevent the device from delivering the appropriate therapy.2–4,9

Sterile magnetic drapes are commonly used during surgery to hold metal instruments in the sterile field. The drape is often placed on the patient’s thorax, the usual location for a CIED. The placement of a magnetic drape over a pacemaker has been reported to result in unintended tachycardia and cardiac arrest.10 Therefore, the purpose of this study was to determine in patients with a cardiac pacemaker whether asynchronous mode behavior develops when a magnetic drape is placed over the thorax. We further sought to determine the number of magnets in the drape necessary to produce asynchronous pacing, and at what distance from the pacemaker that this behavior ceases. Our hypothesis is that placement of the magnetic drape may result in asynchronous pacing in pacemakers and that this pacing will cease at a certain distance from the pacemaker.

METHODS

After approval of the research ethics committee of the Maisonneuve-Rosemont Hospital affiliated with the University of Montreal, patients with an implanted cardiac pacemaker (Medtronic, Minneapolis, MN, and Boston Scientific, Natick, MA) were recruited to this study from September 2009 until November 2009 during regular device follow-up visits at the outpatient pacemaker clinic. After the initial scheduled device interrogation by the cardiologist, the protocol was explained to all patients, and those who agreed to participate in the study, signed a written consent form.

Patient data including age, sex, height, weight, body mass index (BMI) were recorded. The protocol was performed with the patient supine, wearing light clothing. With continuous 3-lead electrocardiographic (ECG) monitoring in place, a strip of the patient’s baseline rhythm was obtained. A Medtronic round magnet (no.174105-2, Minneapolis, MN) was then placed on the pacemaker, and a copy of the ECG to confirm the magnet mode behavior as specified by the manufacturer was obtained. Patients with ICDs and patients in whom the magnet rate cardiac rhythm was indistinguishable from their baseline rhythm were excluded. The round magnet was removed and a 29.5 cm × 37.5 cm surgical magnetic drape containing 70 magnets (116 Reusable Drape no. 31140588, Devon by Covidien, Mansfield, MA) was placed on the patient with its approximate center over the pacemaker. Magnetic interference was identified if the cardiac rhythm was asynchronous and identical to that produced by the round magnet rhythm. If there was no change in rhythm with the full magnetic drape, the drape was folded in 2 and placed over the pacemaker. Once asynchronous rhythm occurred, either with the unfolded or the folded drape, the drape was pulled caudally in 3-cm increments until the pacemaker interference ceased.

Next, if asynchronous cardiac pacing occurred with the unfolded magnetic drape, the magnets were sequentially removed (Fig. 1) until the pacemaker’s magnet pacing mode ceased and the patient returned to his or her baseline cardiac rhythm. The 70 magnets were always removed starting at the edges and finishing in the center of the drape with the goal of determining how many magnets were necessary to maintain asynchronous pacing. Once the protocol was completed, the patient was referred to the cardiologist who, if the situation permitted, discharged the patient.

Figure 1
Figure 1:
The magnetic drape with 70 removable magnets.

Finally, the magnetic field strength of the magnets in the magnetic drape was measured at the Department of Physics at the University of Montreal using a Hall sensor. Data were obtained at the surface of the magnet and at different distances from it.11,12 Information regarding the type of magnetic material was obtained by contacting Covidien, the company that produces the magnetic drape.

To our knowledge, no prior study has reported on the potential effect of magnetic drapes on pacemakers, there were no data on which to base sample size. Thus, the number of participants was based on a convenience sample that could be recruited within the specified time frame. The primary outcome was the percentage of patients experiencing interference when the magnetic drape was applied. Secondary outcomes were the distance at which asynchronous pacing ceased and the number of magnets needed to maintain asynchronous pacing. Specific characteristics of the patients who experienced interference were analyzed.

Simple descriptive statistics were calculated. Student t test and Fisher exact test were used for demographic comparisons. A Kaplan-Meier curve was plotted to display the percentage of pacemakers that ceased having magnetic interference with increasing caudal distance. Pearson coefficient was used for correlations between demographic variables, such as weight and BMI, and distance or number of magnets needed for interference. Calculations and analyses were performed with Prism 5.0 statistical package (GraphPad Software Inc., La Jolla, CA). Unless stated otherwise, data are presented as mean ± SD, and a P < 0.05 was deemed significant.

RESULTS

Characteristics of the 50 patients studied are listed in Table 1. Thirty-five patients had a Boston Scientific pacemaker, and 15 patients had a Medtronic pacemaker. Thirty-five pacemakers (70%) displayed asynchronous cardiac pacing when the unfolded magnetic drape was placed on the pacemaker. In 12 pacemakers (24%), asynchronous pacing occurred with a folded magnetic drape. In 3 pacemakers (6%), no magnet mode rhythm was observed even with the drape folded.

Table 1
Table 1:
Demographic Data

In pacemakers with asynchronous cardiac pacing with magnetic drape placement, the drape had to be moved (mean ± SD) 6.3 ± 4.3 cm (range, 3–15 cm) caudally for magnetic pacemaker interference to stop (6.7 ± 4.7 cm and 5.3 ± 2.9 cm for the unfolded and folded drapes, respectively). Among the 35 pacemakers that had interference with an unfolded drape, interference ceased in 54% of cases when the drape was pulled 3 cm caudally. At a caudal distance of 15 cm, 100% of these patients returned to their baseline rhythm (Fig. 2). In pacemakers that responded to the simple unfolded magnetic drape, 29 ± 27 magnets (range, 1–70) were required to maintain the magnet mode rhythm. In 5 pacemakers, 1 magnet in the drape was sufficient to maintain the asynchronous pacing mode.

Figure 2
Figure 2:
Percentage of patients (n = 35) with unfolded drape whose pacemakers ceased to have magnetic interference as the drape was moved caudally.

Interference with the unfolded drape was more common in pacemakers of patients with lower weight (Table 2). There were no other differences in patient characteristics when comparing patients with and without pacemaker interference from the magnetic drape. One patient felt palpitations during the protocol; otherwise, there were no other adverse consequences observed during the protocol. At the beginning of our study, certain Medtronic pacemakers were reinterrogated by the cardiologist before discharge to verify the programming after the protocol. No changes were found and thus the pacemakers were not systematically reinterrogated at the end of the protocol.

Table 2
Table 2:
Comparison of Demographics Between Subjects Who Experienced Pacemaker Interference with the Unfolded Magnetic Drape Compared with Subjects Who Did Not Experience Pacemaker Interference with the Unfolded Drape

The magnets used in the drape are composed of ceramic ferrite. At the surface of the magnet, remanence was found to be 500 G. The magnetic field strength decreased rapidly with distance, reaching 10 G at 3.4 cm (Fig. 3). There was little variation in the remanence among the magnets in the drape.

DISCUSSION

The findings in this study suggest that magnetic drapes placed over the pacemaker during surgery are likely to activate the pacemaker’s magnetic switch and cause asynchronous pacing. Magnetic interference decreases markedly with increasing caudal distance from the pacemaker. At 15 cm, there was no magnetic interference observed.

Few studies have described the risk of interference between magnets and CIEDs, and none have specifically examined magnetic surgical drapes. Ryf et al.13 did demonstrate in an in vitro study that different neodymium magnets found in everyday life, such as in toys and jewelry can all activate asynchronous pacing modes in pacemakers when placed at various distances from the device. Magnetic interference with different neodymium magnets occurred when placed up to 3 cm to the device.14 Prolonged exposure to similar magnets was implicated in deactivation of an ICD resulting in a fatal consequence.15 In a clinical study by Hiller et al.,16 3 of 12 pacemakers had interference by small dental mini magnets made of samarium cobalt. The effect disappeared after the magnets were pulled away 1 cm.

According to manufacturer’s specifications, 5 to 10 G is required to enable the magnet mode in the pacemakers we tested,13 a level reached when a magnet from the Covidien drape is 3.4 cm away from the generator. We found that low body weight was associated with pacemaker magnetic interference. In other words, the 30% of patients who experienced no pacemaker magnetic interference with the unfolded drape were heavier than the patients who experienced magnetic interference. This may indicate that in heavier patients the deeper location of the pacemaker might provide some protection from the magnets. In fact, the 3 patients who experienced no pacemaker interference, even with the folded drape, had a mean BMI of 35 kg/m2 and an average weight of 101 kg.

Limitations

Our study was limited to the Covidien magnetic drape and Boston Scientific and Medtronic pacemakers because these are the vast majority of drapes and pacemakers used in our hospital. Thus, the results of our study may not apply to all magnetic drapes and pacemakers. We felt it was appropriate to test our protocol with the folded drape because at times our surgeons fold the Covidien drape in 2 to enlarge their surgical field. Although we realize this limitation can be overcome with customizable drapes, we do not carry these in our hospital. Moreover, subclinical changes cannot be excluded after exposure to the magnetic drape because there was no systematic evaluation of the devices after exposure. In addition, for >20 years, pacemakers have been placed subcutaneously, just over the pectoralis muscle, not submuscularly. Although we did not record pacemaker location in our study, this variable may also influence the pacemaker–magnet distance. We did not test movement of the magnetic drape in other directions other than caudally from the pacemaker generator. Finally, we did not test ICDs in our study; however, magnet switch activation of most ICDs will likely disable high-voltage therapy, which is often undetected.15

Each ceramic ferrite magnet in the drape has a magnetic north and south pole. The magnetic field generated is additive if the polarity of adjacent magnets point in the same direction, but is decreased if the polarities alternate. The Covidien drape was found to have a random insertion of magnets regardless of polarity. There could be less interference if magnets were inserted so that adjacent magnets had different polarities. In 5 pacemakers, only 1 magnet was necessary to maintain an asynchronous mode, suggesting that a single, well-positioned magnet can induce asynchronous rhythm.

CONCLUSION

Magnetic drapes used in the operating room may unintentionally activate the magnetic sensor, resulting in undesirable cardiac pacemaker behavior, especially if the center of the drape is over the pacemaker. With a distance of 15 cm between the center of the magnetic drape and the pacemaker, none of the pacemakers in our study had magnetic interference. Patients with lower BMI and weight are more susceptible to having their pacemaker switch to the asynchronous mode when near the magnetic drape. Careful monitoring of the heart rate and ECG for asynchronous pacing activity should be considered when magnetic drapes are used in patients with a CIED.

Figure 3
Figure 3:
Remanence (B in gauss) of the magnets as a function of distance from the magnet (x in centimeters). At a distance of 3.4 cm, the magnet generates a remanence of 10 G. The solid line is a fit to B = M/2t (cosb2 − cosb1), where b1 is defined as b1 = arctan (r/[x + t/2]), and b2 = arctan (r/[x + t]), with a radius r of 1 cm and a thickness t of 0.396 cm for the magnets in the Covidien drape.

DISCLOSURES

Name: Valerie Zaphiratos, MSc, MD, FRCPC.

Contribution: This author helped design and conduct the study, analyze the data, and write the manuscript.

Attestation: Valerie Zaphiratos approved the final manuscript.

Conflicts of Interest: Valerie Zaphiratos and Louis-Philippe Fortier share a patent for the creation of a magnetic drape that is safer for pacemaker patients.

Name: Pierre Drolet, MD, FRCPC.

Contribution: This author helped analyze the data and write the manuscript.

Attestation: Pierre Drolet approved the final manuscript.

Conflicts of Interest: The author has no conflicts of interest to declare.

Name: Francois Donati, PhD, MD, FRCPC.

Contribution: This author helped analyze the data and write the manuscript.

Attestation: Francois Donati approved the final manuscript.

Conflicts of Interest: The author has no conflicts of interest to declare.

Name: Andrea Bianchi, PhD.

Contribution: This author helped analyze the data and write the manuscript.

Attestation: Andrea Bianchi approved the final manuscript.

Conflicts of Interest: The author has no conflicts of interest to declare.

Name: Bruno Benzaquen, MD, FRCPC.

Contribution: This author helped conduct the study.

Attestation: Bruno Benzaquen approved the final manuscript.

Conflicts of Interest: The author has no conflicts of interest to declare.

Name: Jacques Lapointe, MD.

Contribution: This author helped conduct the study.

Attestation: Jacques Lapointe approved the final manuscript.

Conflicts of Interest: The author has no conflicts of interest to declare.

Name: Louis-Philippe Fortier, MSc, MD, FRCPC.

Contribution: This author helped design the study, analyze the data, and write the manuscript.

Attestation: Louis-Philippe Fortier approved the final manuscript.

Conflicts of Interest: Valerie Zaphiratos and Louis-Philippe Fortier share a patent for the creation of a magnetic drape that is safer for pacemaker patients.

This manuscript was handled by: Charles W. Hogue, Jr., MD.

ACKNOWLEDGMENTS

The authors acknowledge the hard work and dedication of our anesthesia research nurse, Nadia Godin, and the help of the technicians and personnel of the HMR pacemaker clinic in conducting this study.

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