376 M. Zhou et al.: Radioprotection 2025, 60( 4), 373 – 381
Fig. 2. Comparison of measured and simulated induced voltage versus voltage injection for unipolar and bipolar configurations.
the exposure in a real-case scenario, VIS assessment may be performed on the device under test.
3.2 Specific assessment process
In this paper, we intend to interpret the application of VIS assessment in the specific assessment process on four CIEDs( two PMs and two ICDs) from Medtronic TM( Minnesota, U. S.). We assume that the CIED-employees bearing these devices may be exposed higher than 5 kV / m. In consequence, additional investigations are required. Considering the various features of natural cardiac signals that different patients may possess and different clinical situations, each cardiac implant was inspected with three sensitivities: the nominal sensitivity( Nom.), which is the default setting and most commonly used; the maximum sensitivity( Max.) of the device, representing patients with very low-amplitude cardiac signals or high dependency on pacing; and a third sensitivity( 3rd Sen.), selected randomly between the nominal and the minimum sensitivity at 1.5 to 2 times the nominal setting, representing patients with high-amplitude cardiac signals and stable heart rhythms, or a clinical chose to prevent EMI. To conduct nonclinical investigations in these cases, the employers may ensure CIED-employees’ safety by following processes in the EF exposed workplace: a. Survey on implanted cardiac device and its setting Multiple factors may affect the severity of EMI, such as the sensing lead, sensing configuration, and sensitivity setting of the device, etc. We chose the following settings:
* Implantation method: right pectoral implantation and vertical sensing lead end( Tip-Ring pair) for all devices( worst-case scenario among pectoral implantation( Gustrau et al., 2002));
* Sensing method: unipolar and bipolar sensing for pacemakers, bipolar sensing for ICDs( unipolar sensing is not applicable for ICDs);
* Device information( Tab. 1). b. Survey on field source at workplace General information on the measurement of the fields is provided in IEC 61786-2. Field levels at the workplace can be determined either by measurement or by modeling. Here we assume that the four employees are exposed to exposure levels at the limits specified in the Directive and standards.
* Exposure field type: EF;
* Characteristics of exposure: power frequency( 50 Hz), homogeneous;
* Direction: Vertical( from exposed object’ s head to bottom);
* Values( Tab. 2). c. VIS assessment Possessing the information collected from the cardiac devices and the field sources, we carried out the VIS assessment by applying the corresponding voltage injection doses in the immunity test for the four devices under test with three sensitivities( Max., Nom, 3rd Sen.) in unipolar and bipolar sensing mode( if applicable). In an assessment, a certain amount of voltage is applied to the phantom to generate expected exposure for the cardiac implant under test while cardiac signals are continuously sent to this device. This cardiac implant is examined by its programmer after exposure of 1 minute. Missed cardiac signal detection, erroneous cardiac signal detection, and incorrect ventricular fibrillation( VF) detection are considered as device dysfunctions, which indicate that the cardiac implant is subjected to significant interference and not able to function properly. This procedure takes time but it was always performed three times to guarantee reproducibility. According to the lookup table in Figure 3, an injection of 1.19 mV was applied for unipolar setting and 0.91 mV for bipolar setting in the assessment of Source-I( 5 kV / m); an injection of 2.37 mV was applied for unipolar setting and 1.82 mV for bipolar setting in the assessment of Source-II( 10 kV / m); An injection of 4.74 mV was applied for unipolar setting and 3.64 mV for bipolar setting in the assessment of Source-III( 20 kV / m).