Background In ambulatory patients with complete heart block and preserved sinus node activity (CHBs), dual chamber pacing confers well-established physiological benefits versus single chamber pacing. There is limited evidence as to whether these benefits extend to very frail patients, especially those over 90 years of age. Purpose In nonagenarians presenting with emergent CHBs from 2016–2019, we compared the clinical characteristics of patients selected for single versus dual chamber pacemakers (PPM), and evaluated the symptomatic and prognostic implications of these devices. Methods Baseline characteristics were discerned from electronic records, and physiological data extracted from serial PPM interrogations. Frailty was quantified according to the Rockwood clinical frailty scale (1–9). Cause of death was provided by the patients' General Practitioner. Cox proportional hazards analysis (HR, 95% CI) examined associations with all-cause mortality and death from congestive cardiac failure (CCF). Results 168 consecutive patients were included (44.3% Male, Median age: 91 (2) years) and followed-up for 26.9±14.6 months. 22 patients (13.1%) were implanted with single chamber pacemakers (all programmed VVIR); when compared with patients receiving dual chamber devices, these patients had similar median age (93 (3) versus 91 (2) years, p=0.15) and LV systolic function (LVEF: 49.2% ±9.7 versus 50.7% ±10.1, p=0.71), but were more frail (Rockwood scale: 5.2±1.8 versus 4.3±1.1, p=0.004) and more likely to have severe cognitive impairment (27.3% versus 9.2%, p=0.018). Post implant, patients who received single chamber devices had higher average respiratory rates (21.3±2.4 breaths per minute versus 17.5±2.6 breaths per minute, p=0.002), lower average heart rates (65.5±10.1 bpm versus 71.9±8.6 bpm, p=0.002), and lower daily activity levels (0.57±0.3 hours of activity versus 1.5±1.1 hours of activity, p=0.016) than those with dual chamber devices. Death from CCF was more common in patients receiving single chamber devices (40.9% versus 6.2%, log rank p<0.0001); this association persisted when adjusting for age, frailty and cognitive impairment (adjusted HR: 6.2 (2.2–17.3, p=0.0005). However, in this age group, single chamber pacing was not independently associated with all-cause mortality when compared with dual chamber pacing (adjusted HR: 1.9 (0.95–3.6, p=0.07). Conclusions In nonagenarians with CHBs, dual chamber pacing was associated with improved symptomatic outcomes and a reduced risk of death from CCF, but did not affect all-cause mortality when compared with single chamber pacing. Funding Acknowledgement Type of funding sources: None.