Changes in cardiac excitability and vulnerability in NMR fields

Alterations in ventricular excitability and vulnerability were assessed in nine isolated perfused rabbit hearts in and out of static external magnetic fields (4.7 tesla) associated with radiofrequency pulsing (5 gauss). Ventricular refractoriness was assessed with the strength interval relationship in and out of the NMR magnet. Strength interval curves were measured at threshold, at the midpoint of the strength interval relationship, and at 10 mA. The refractory period measured at threshold was 193 +/- 24 mS outside the magnet and 195 +/- 24 mS inside the magnet (P = ns). Ventricular refractoriness measured at the midpoint of the strength interval curve was 169 +/- 16 mS outside and 167 +/- 17 mS in the magnet (P = ns). At 10 mA the refractory period outside of the magnet was 162 +/- 16 mS and 161 +/- 17 mS in the magnet (P = ns). To assess ventricular vulnerability the repetitive response threshold and the ventricular fibrillation threshold were also determined in and out of the NMR magnet. The repetitive response threshold was 61 +/- 16 mA out of the magnet and 75 +/- 24 mA inside the magnet. This was significant at the P = 0.04 level. The ventricular fibrillation threshold was 71 +/- 14 mA out of the magnet and 81 +/- 20 mA in the magnet (P = ns). In summary, static magnetic fields associated with radiofrequency pulsing have no measureable effect on the strength interval relationship. There is no increase in ventricular vulnerability as assessed by the repetitive response threshold and the ventricular fibrillation threshold.