Table 2.

Device Variables Before and After MRI^*

Variable	Patients, n†	Median Value at Baseline (IQR)	Median Difference (IQR)	Median Percentage Change Compared With Baseline (IQR)‡	P Value§
			Immediately After MRI
P-wave amplitude, mV	303	2.9 (2 to 4.3)	0 (−0.2 to 0.1)	0 (−7 to 5)	0.40
Right ventricular R-wave amplitude, mV	461	11.6 (8.2 to 14.2)	0 (−0.7 to 0)	0 (−7 to 0)	<0.001
Left ventricular R-wave amplitude, mV	20	10.4 (6.4 to 22.2)	−0.8 (−1.8 to 0.3)	−7 (−20 to 4)	0.097
Atrial lead impedance, Ω	364	489 (419 to 562)	−2 (−13 to 0)	−0.3 (−3 to 0)	<0.001
Right ventricular lead impedance, Ω	518	540 (430 to 655)	−4 (−16 to 0)	−0.9 (−3 to 0)	<0.001
Left ventricular lead impedance, Ω	51	585 (475 to 684)	−11 (−40 to 0)	−1.8 (−6 to 0)	0.002
Atrial capture threshold, V	332	0.8 (0.5 to 1)	0 (0 to 0)	0 (0 to 0)	0.53
Right ventricular capture threshold, V	531	0.9 (0.6 to 1)	0 (0 to 0)	0 (0 to 0)	0.46
Left ventricular capture threshold, V	55	1.5 (0.8 to 2)	0 (0 to 0)	0 (0 to 0)	0.55
Battery voltage, V||	472	2.78 (2.75 to 3.1)	0 (0 to 0)	0 (0 to 0)	<0.001
			Long-Term Follow-up After MRI
P-wave amplitude, mV	124	2.9 (2 to 4.3)	0 (−0.3 to 0.2)	0 (−13 to 10)	0.41
Right ventricular R-wave amplitude, mV	198	11.6 (8.2 to 14.2)	0 (−1.1 to 0.3)	0 (−12 to 3)	0.004
Left ventricular R-wave amplitude, mV	9	10.4 (6.4 to 22.2)	0 (−1.1 to 0.1)	0 (−19 to 1)	0.28
Atrial lead impedance, Ω	167	489 (419 to 562)	−3 (−23 to 19)	−0.5 (−5 to 4)	0.154
Right ventricular lead impedance, Ω	234	540 (430 to 655)	−3 (−29 to 15)	−0.5 (−5 to 3)	0.044
Left ventricular lead impedance, Ω	23	585 (475 to 684)	0 (−40 to 16)	0 (−6 to 3)	0.45
Atrial capture threshold, V	105	0.8 (0.5 to 1)	0 (0 to 0)	0 (0 to 0)	0.28
Right ventricular capture threshold, V	190	0.9 (0.6 to 1)	0 (0 to 0.2)	0 (0 to 20)	0.012
Left ventricular capture threshold, V	23	1.5 (0.8 to 2)	0 (−0.5 to 0.4)	0 (−21 to 33)	0.85
Battery voltage, V	224	2.78 (2.75 to 3.1)	−0.01 (−0.04 to 0)	−0.4 (−1.4 to 0)	<0.001

IQR = interquartile range; MRI = magnetic resonance imaging.

^*Lead impedance variations exceeding 30% (20, 21), capture threshold variations exceeding 50% (22), and sensing variations exceeding 40% (23, 24) indicate clinically significant changes in lead performance.

^†The number of comparisons for each device variable is unique, primarily owing to variability in the number of leads, the absence of intrinsic P/R waves, the presence of atrial arrhythmia, and pulse widths during measurement of capture threshold at follow-up interrogation. The proportion of missing data for each variable is discussed in the Discussion section.

^‡Based on long-term follow-up results available for 266 (61%) patients undergoing 278 MRI examinations.

^§Obtained by using the Wilcoxon signed-rank test.

^||Of the 472 comparisons for acute battery voltage, 92 (19.5%) were decreases, 22 (4.7%) were increases, and 358 (75.8%) did not change.