INDUCTION OF MICROCURRENTS IN CRITICALLY ILL PATIENTS IN MAGNETIC-RESONANCE SYSTEMS

Objective: To determine whether electric current can be induced in int racardiac catheters, thermistor wires and pacing electrodes in patient s during magnetic resonance imaging (MRI). Design: Prospective laborat ory study. Setting: Postgraduate medical school hospital. Subjects: A sheep heart model. Interventions: None. Measurements and Main Results: Voltage generated by saline 0.9% flowing through a magnetic field and distribution of current from a catheter tip within a sheep heart mode l were measured in a 0.15 Tesla MRI system. Resistance of loops formed by pacing wires, a pacing electrode, and a thermistor wire were measu red in saline 0.9%. Effects of rapidly changing magnetic fields and th e movement of the beating heart on epicardial pacing wires were calcul ated theoretically. A flow of 200 mL/min of saline 0.9% induced a curr ent of 0.1 microampere (mu A) (at 0.15 Tesla). From magnetic resonance images we derived a current density of similar to 0.004 mu A/mm(2) (a t 0.15 Tesla). Internal resistance of pacing catheters and thermistor wires was >1 megaohm (M Omega). The maximum currents calculated (for a higher field strength of 1.5 Tesla) in a circuit formed by epicardial pacing wires were 80 mu A, induced by the beating heart moving the wi res through the magnetic field and 46 mu A, induced by the rapidly cha nging magnetic fields. Conclusions: Current generated by flow of condu cting fluid should be safe. Pacing catheters and thermistor wires shou ld be safe if well insulated and disconnected from external electric c onnections. However, current induced in epicardial pacing wires may be a hazard, and precautions should be taken. External wire tips must be separated, insulated, and coiled to lie along the axis of the magneti c field. Electrocardiography is required, and defibrillation equipment should be available.