Implantable medical devices, such as pacemakers or implantable cardioverter defibrillators (ICDs), are vulnerable to the powerful magnetic fields associated with magnetic resonance imaging (MRI). In particular, pulsed gradient components, if strong enough, can induce parasitic currents that may damage the device or cause parasitic pacing that may trigger an arrhythmia in the patient. The static magnetic field components of the MRI typically do not induce parasitic currents, even though they may be as strong as the pulsed gradient components. Accordingly, techniques are described herein for specifically addressing the pulsed gradient components of the MRI fields so as to reduce the risk of parasitic currents. In one example, a pacemaker switches to tri-state pacing outputs in the presence of strong pulsed gradient magnetic fields. The device continues with normal bi-state pacing outputs so long as the pulsed gradient fields are not strong, even in the presence of a strong static magnetic field. As an added safety feature, the pacemaker switches to fixed-rate ventricular pacing whenever strong static MRI fields are detected.