Interference between surgical magnetic drapes and pacemakers: an observational study comparing commercially available devices and a new magnetically isolated drape

Following approval of the research ethics committee of the Maisonneuve-Rosemont Hospital affiliated to the University of Montreal, patients with an implanted cardiac pacemaker (Medtronic, Minneapolis, Minnesota, and Boston Scientific, Natick, MA, USA) were recruited for this study from October 2011 until November 2011 during regular device follow-up visits at the outpatient pacemaker clinic. After the cardiologist performed the initial scheduled device interrogation, 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 a hospital gown. Continuous 3-lead electrocardiographic (ECG) monitoring was placed on the patient and the ECG was analysed by a medical Doctor (MD), an anesthesiologist or an anesthesiology senior resident, as well as a registered Nurse (RN) throughout the study. First, a strip of the patient’s baseline rhythm was obtained. Then, a Medtronic round magnet (#174105-2, Minneapolis, MN, USA) was 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 then removed.

Part 1 of the study was to assess two different magnetic drapes. A member of the personnel not involved in the study concealed the two magnetic drapes in an opaque plastic bag. The first drape (“CVD”) measured 29.5 cm × 37.5 cm and contained 70 ceramic ferrite magnets (116 Reusable Drape #31140588, Devon by Covidien, Mansfield, MA, USA) (Fig. 1). In a previous study [14], the magnets in the CVD drape were found to be inserted in a random fashion, regardless of polarity. In a single drape, the polarity of each magnet can impact the overall vector of magnetism of that drape. Thus, at the beginning of each data collection day, a different CVD drape was used in order to reduce bias caused by the potential magnetic signature of each commercial drape. The second magnetic drape was a prototype made with a silicone shell similar to the CVD drape, but instead contained 70 isolated ferrite magnets (Fig. 2). The isolation material consisted of a steel-based metal cup that diminished the magnetic remanence on the patient side of the drape and only encapsulated the underside of each ferrite magnet in order to create a unidirectional magnetic field. The overall magnetic field on the patient facing side was less than 10 Gauss (Fig. 3) (Additional file 1).

CVD surgical magnetic drape containing 70 ceramic ferrite magnets

Each magnetic drape was concealed and centered over the patient’s pacemaker, with the help of a measuring tape, one after the other. The investigator performing the study and the data collector were both blinded to the type of magnetic drape. 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 magnetic drape, the drape was displaced in 1–2 cm increments over the pacemaker in an effort to elicit an asynchronous rhythm. Once the protocol was completed, the patient was referred to the cardiologist who, if the situation permitted, discharged the patient.

Part 2 of the study consisted of evaluating and comparing 4 different commercially available magnetic drapes, including the CVD, with the prototype magnetic drape. All four of these commercially available magnetic drapes underwent the same protocol as the magnetic drapes in part 1 of the study. Due to the different sizes and shapes of the four drapes, it was not possible to create a blinded evaluation with an opaque plastic bag. The magnetic drapes evaluated were the CVD magnetic drape, the “JCM” magnetic drape (Reusable Magnetic Pad, Jac-Cell Medic, Town of Mount-Royal, Quebec, Canada), the “LDR” magnetic drape (Magnetic Instrument Pad #25-002, size 20? × 16?, DeRoyal Industries Inc., Powell, TN, USA), and the “SDR” magnetic drape (Magnetic Instrument Pad #25-001, size 10? × 16?, DeRoyal Industries Inc., Powell, TN, USA).

The sample size calculation for the primary outcome (first part of the study) was based on the direct comparison between the proportion of patients experiencing pacemaker interference associated with the CVD drape in one group and the new prototype in the other. A previous study having shown that using the CVD drape caused pacemaker interference in 70 % of participants [14], we elected to look for a 50 % reduction (from 70 to 35 %) in interference with the prototype drape. It was then calculated that 30 patients per group were needed to show such a difference with alpha and beta errors of 0.05 and 0.8 respectively.

The sample size calculation for part 2 was to be derived from the results obtained during the first part of the study, using the proportion of patients experiencing interference with the prototype as a base for calculation. Since no case of interference was observed, it was arbitrarily decided to recruit twenty participants to provide an exploratory overview of the pacemaker interference associated with the various commercially available devices.

Fisher’s exact test was used for comparisons in the first part of the study. Only descriptive statistics were used for the second part. Calculations and analyses were performed with Prism 5.0 statistical package (GraphPad Software Inc, La Jolla, CA, USA). Unless stated otherwise, data are presented as mean ± SD and a p value 0.05 was deemed significant.