BACKGROUND: Recent work has shown no difference in change of LVEF between RV apical (RVA) pacing and non-RVA pacing in patients with normal LV function. We hypothesised that a more sensitive marker (global longitudinal strain, GLS) could identify a detrimental effect of RVA and that assessment of deformation could identify whether dyssynchrony, contraction inefficiency and regional LV impairment were responsible for functional changes. METHODS: In this substudy of Protect-PACE (The Protection of Left Ventricular Function During Right Ventricular Pacing. Does Right Ventricular High-septal Pacing Improve Outcome Compared With Right Ventricular Apical Pacing?), a randomised controlled trial of RVA and non-RVA pacing in pacemaker-dependent patients with preserved EF, 145 patients (76 with RVA) with echocardiograms of sufficient quality underwent measurement of LV longitudinal strain (GLS) from speckle tracking just after implantation and at 2 years. LV dyssynchrony, discoordination and regional apical longitudinal strain were also measured. RESULTS: Pacing was associated with reduced GLS after 2 years, although 2-year GLS was lower in RVA (-13.9 ± 4.1 vs -15.5 ± 4.6, p = 0.02). RVA was an independent correlate of ΔGLS, although there was a minor difference in ΔGLS between the RVA and non-RVA groups (-1.8 ± 3.6 vs -0.8 ± 3.4%, p= 0.07), reflecting impairment of GLS at baseline in RVA. Apical strain was significantly lower in RVA than those in non-RVA at baseline and 2 years (both p < 0.01). Dyssynchrony and discoordination parameters at 2 years also showed significant deterioration in RVA. Apical strain, dyssynchrony and discoordination parameters at 2 years were significantly associated with ΔGLS. CONCLUSIONS: Inefficient dyssynchronous contraction and the decrease in apical strain appear to be associated with global LV impairment in RVA. TRIAL REGISTRATION NUMBER: PROTECT-PACE ClinicalTrials.gov number NCT00461734.