Abstract Body (Do not enter title and authors here): Introduction
Patients with transvenous cardiac implantable electrical devices (CIEDs) including pacemakers are typically excluded from magnetic resonance imaging (MRI) examinations for at least 6-weeks following implant. However, many MRI scans are required on urgent or emergency basis, potentially resulting in suboptimal management for patients following a recent CIED implant. The absolute risks of adverse lead/generator events with early MRI have not been quantified. Specific transvenous pacing systems from one manufacturer (Abbott) do not mandate a routine 6-week post-implant restriction for MRI conditionality, although real-world human data investigating lead parameter stability with MRI early (<7 days) post-implant are lacking.

Research Question
Can MRI be performed early (<7 days) after pacemaker implantation without safety events or significant changes to lead parameters?

Methods
Patients undergoing implantation of Abbott Assurity™ MRI pacemakers with active-fixation MRI conditional leads (Abbott Tendril™ 52cm/58cm or UltiPace™ 65cm) with no contraindication to MRI underwent cardiac MRI within 7 days of implant. Patients were scanned at 1.5 or 3T in an asynchronous pacing mode. Device interrogation was performed before and after MRI. All patients provided written consent.

Pre- and post-MRI lead parameter measurements were compared using the two one-sided t-Tests (TOST) equivalence test. Equivalence margins were prespecified: lead impedance ±30Ω, lead sensing ±0.5mV, lead threshold ±0.25V.

Results
20 patients (median age 77 (IQR 72-80) years, 75% male) were recruited. 65% received right ventricular pacing and 35% conduction system pacing.

Median time from implant to MRI was 3 [IQR 1-5] days. All lead parameters were statistically equivalent pre- and post-MRI:

Atrial lead impedance mean difference 5 Ω (90% CI -3 to +12), TOST p<0.001
Ventricular lead impedance -1 Ω (90% CI -9.2 to +7.2), TOST p<0.001
Sensed P wave 0.04 mV (90% CI -0.13 to +0.22), TOST p<0.001
Sensed R wave -0.06 mV (90% CI -0.28 to +0.17), TOST p=0.001
Atrial capture threshold -0.01 mV (90% CI -0.04 to +0.01), TOST p<0.001
Ventricular capture threshold -0.01 mV (90% CI -0.05 to +0.03), TOST p<0.001

Conclusion
MRI within 7 days of pacemaker implantation was safe, with no adverse events and no significant change in lead parameters. These findings provide real-world data to complement manufacturer guidance and support access to early MRI when indicated for patients with conditional pacemaker systems.