Advanced technology has been developed that involves the stimulation of the heart by means of a fiber-optic lead that replaces the standard metallic lead of a cardiac pacemaker. The fiber-optic cardiac pacing lead incorporates specially-designed components that incorporate a low power, semi-conductor laser to regulate the patient's heartbeat (Biophan Technologies, Inc., Rochester, NY). This innovation essentially eliminates possible dangers associated with having a conductive pacing wire in a patient undergoing an MRI examination.
The fiber-optic cardiac pacing lead (Biophan Technologies, Inc., Rochester, NY) is intended for use via connection to the Temporary Photonic Pulse Generator (Model X-801, Biophan Technologies, Inc., Rochester, NY). The cardiac pacing lead is made from 200-um fiber-optic cable. The distal end of the lead has two electrodes designed to stimulate the heart. Within the ring and tip portions of the lead are a power converter, resistor, and capacitor. Inside the ring electrode is a power converter that changes light energy into electrical energy for heart stimulation. The pulse generator for this device (Temporary Photonic Pulse Generator, Model X-801) produces a 1-millisecond pulse (variable from 0.1 to 30-msec), which drives a 150-milliwatt gallium-arsenide laser. The light pulse is connected to the distal end of the fiber-optic lead where it illuminates a band of six gallium-arsenide photo diodes. The diodes are electrically connected in series to produce a voltage pulse of 4 volts, which drive the "tip" and "ring" electrodes to stimulate the heart. Accordingly, this device is capable of generating sufficient current to pace the heart.
The results of MRI tests conducted for the fiber-optic cardiac pacing lead indicated there were only minor magnetic field interactions associated with exposure to a 1.5-Tesla MR system. In addition, there was essentially no MRI-related heating for the fiber-optic cardiac pacing lead in association with MRI conducted at a whole-body-averaged SAR of 1.5 W/kg.
Based on this information, it appears this newly developed device will not present an additional hazard or risk to patients undergoing MRI procedures under the conditions used for this evaluation.
By comparison, in an in vitro evaluation of 44 commercially-available pacemaker leads, Sommer et al. reported the maximum temperature change measured at the lead tip was 23.5 degrees C above baseline in association with MR imaging performed at 0.5-Tesla and a whole body averaged SAR of 1.3 W/kg for 10-min. Additionally, Achenbach et al. reported a peak temperature change of 63.1 degrees C measured for a temporary pacing electrode that occurred within 90-sec. of MR imaging (the SAR was not reported).
Furthermore, MR imaging at 1.5-T and an SAR of 3.0 W/kg has been shown to cause severe necrosis in the mucous membranes of dogs with transesophageal cardiac pacing leads in situ.
Previous attempts to develop an MRI-safe cardiac pacing lead have been reported by Jerzewski et al. and Hofman et al. Jerzewski et al. modified a commercially available pacing catheter by leaving out the stainless steel catheter shaft braiding and using 90% platinum/10% iridium electrodes at the tip. The electrical wiring was made of nearly pure copper of reduced conductance. Thus, this pacing device still involved the use of conductive metallic materials. MRI was performed on rabbits with this temporary pacing lead at 1.5-Tesla. These laboratory animals were primarily monitored for extrasystolic cardiac contractions. Notably, Jerzewski et al. did not assess magnetic field interactions or heating for the cardiac pacing lead but rather evaluated imaging artifacts, which is not an MR safety issue. Obviously, before it can be claimed that cardiac pacing during MRI is safe, the issue of heating of tissue around this pacing catheter should be addressed."
In an investigation performed in laboratory dogs, Hofman et al. studied the feasibility of transesophageal cardiac pacing during MRI at 1.5-Tesla and concluded that tissue around the catheter tip may become heated. As previously stated, severe necrosis in the mucous membranes of dogs with transesophageal pacing leads in situ has been found during MRI in combination with high levels of RF energy. Therefore, while it may be possible to perform transesophageal atrial pacing during MRI, it requires relatively low levels of RF energy, which is likely to be impractical for most anticipated clinical uses of this technology.
In view of the previously published reports on cardiac pacemakers and, more specifically, pacing leads, the information for the fiber-optic cardiac pacing lead is particularly compelling from an MRI safety viewpoint. Notably, this unique technology may be applied to other devices that require leads but are known to present potential hazards to patients undergoing MRI procedures (e.g., neurostimulation systems).
One final consideration for the fiber-optic cardiac pacing lead is the issue of electromagnetic interference (e.g., inappropriate or rapid pacing due to pulsed gradient magnetic fields and/or pulsed RF from the operating MR system, with the pacing lead acting as an antenna). From a theoretical consideration, because of the "fiber-optic nature" of this specially-designed pacing lead, there should be no problems related to this device acting like an antenna during MRI procedures since it should be immune from such problems.
Other devices (pulse oximeters, cutaneous blood flow monitors, electrocardiagraphic systems, thermometry systems, etc.) have likewise incorporated fiber-optic interfaces to the patients to successfully prevent EMI-related problems from occurring in the MRI environment. However, additional investigation directed at addressing the EMI aspects of the fiber-optic cardiac pacing lead in the MRI setting is warranted.
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