Some MRI scans, including many cardiac and spinal scans, exceed 2 W/kg whole body SAR. We utilized the ISO/IEC JWG 10974 Tier 3 (ED2) approach to evaluate heating of pacemaker systems under normal (2 W/kg) and 1st level controlled mode (4 W/kg). Electric fields were simulated using five virtual human models with various transvenous pathways in MRI RF body coils. Clinically relevant lead states of various levels of fluid ingress were studied, and the validated lead transfer function (TF) with the highest heating was selected. It was then integrated with the extracted electric fields along lead pathways inside human models to estimate the temperature rises without blood flow (in vitro). A validated thermal model scaled the in vitro temperature estimates to in-vivo results. Uncertainties from measurements, TF, thermal model and in vivo simulations were incorporated with the Monte Carlo (MC) method. Safety was assessed based upon the accepted 43°C standard for cardiac tissue interfacing with the lead helix electrode and lead MRI filter inductor.