Abstract Body (Do not enter title and authors here): Background:
Long term right ventricular apex pacing has been reported to affect myocardial metabolism and perfusion to lead to impaired left ventricular function. However, there is little information available on the effect on fatty acid metabolism, myocardial perfusion and function in RV septal or His bundle pacing.

Methods and Results:
We studied 94 patients (mean age 77±11 years, male 40%, AV block 71%) admitted for pacemaker implantation in our prospective cohort study, excluding patients with coronary artery disease who had past histories of coronary arterial revascularization and severe stenosis in coronary CT or coronary angiography. Forty-five patients had right ventricular apex pacing (RVA; n=10) or right ventricular septal pacing (RVS; n=35), 17 had selective or non-selective His bundle pacing (HIS) and, as a control, 32 had right atrial pacing (RA). The patients underwent 123I-β-methyl-P-iodophenyl-pentadecanoic acid (BMIPP) SPECT imaging and Tc-99m- methoxyisobutylisonitrile (MIBI) SPECT imaging between 3 months and 1 year after implantation. The uptakes of both tracers were displayed on a polar map, which was divided into 17 segments. Regional tracer activity at each segment was quantified as %uptake. Left ventricular ejection fraction (LVEF) was also obtained in MIBI imaging.
There were no significant differences in baseline characteristics such as age, gender, or underlying heart disease between the four groups. LVEF in MIBI rest imaging in RVA group was significantly lower than RA group (58.3±2.1% vs 64.9±1.2%, p=0.013), while there was no significant difference in LVEF among RVS, HIS and RA groups, with non-significantly (p=0.25) lower LVEF in RVS (62.5±1.1%) than HIS (65.3±1.6%) groups. The average of BMIPP %uptake in 17 segments significantly correlated with LVEF (r=-0.259, p=0.01). BMIPP and MIBI (stress and rest) %uptake were significantly lower in RVA group and RVS group than RA group in the septal or anterolateral area. Especially, BMIPP %uptake in RVA group was significantly lower at the broad area. On the other hand, BMIPP and MIBI (stress and rest) %uptake in HIS group were almost similar to those in RA group.

Conclusion:
His bundle pacing, but not RV septal pacing, maintains myocardial metabolism, perfusion and function.