SyncAV Study: Investigation of the Efficacy of the SyncAV Fusion Pacing Algorithm on Exercise

Heart failure represents a significant health problem, with the last national heart failure audit demonstrating prevalence in the United Kingdom of 900,000 patients, accounting for 5% of all acute hospital admissions. This is expected to increase with an ageing population. Despite improvements in medical therapy, prognosis remains poor, with an in- hospital mortality of 9.6%, and an estimated mortality of 30-40% at 1 year after diagnosis.

Multiple randomised controlled trials have demonstrated that cardiac resynchronization therapy pacemaker devices ("CRT") are an effective therapy for patients with poor pumping function and altered electrical conduction of the heart (left bundle branch block, "LBBB"), improving both morbidity and mortality. However, only 60-70% of patients notice a symptomatic improvement with CRT and even in patients who do respond, response can often be improved further by optimising how the device is programmed.

CRT devices improve coordination of heart pumping by pacing both ventricles of the heart. A key function of this is to coordinate the timing of contraction of the the atria and the ventricles (atrio-ventricular, or "AV", optimisation). This allows maximal filling of the left ventricle with blood. More recently, it has become important not only in standard bi-ventricular ("BiV") pacing, but to allow left ventricular pacing to be timed with intrinsic conduction to the right ventricle to provide CRT (so called 'fusion' pacing).

Multiple methods have been described to assess AV optimisation, including echocardiographic measurements. However, echo based methods are labour intensive, and their value is uncertain. Device algorithms utilising analysis of the intra-cardiac electrogram ("IEGM") have become an attractive alternative due to their rapid and automated nature, although evidence suggests that they may not have clinical benefit over using fixed AV delays. In addition, intrinsic AV conduction is known to alter with exercise, normally becoming shorter. Optimisation of AV delays in CRT on exercise has been shown to improve cardiac output. Device algorithms can therefore allow dynamic adjustment of AV delays as they change with exercise and heart rate (rate-adaptive AV delay or "RAAVD"). Indeed, use of individually tailored RAAVD in CRT patients has demonstrated an increase in exercise capacity.

Patients with heart failure and LBBB often have normal intrinsic right ventricular activation through the right bundle. Utilization of timed left ventricular ("LV") pacing to merge (or fuse) with this intrinsic conduction may confer benefits over standard BiV pacing, but requires relatively normal intrinsic AV conduction as well as correct timing of LV pacing to right ventricular ("RV") activation. Algorithms now exist which allow dynamic reassessment of intrinsic conduction and so adjustment of the optimal AV delay. They can therefore compensate for changes in the intrinsic AV delay on exercising, and so maintain adequate fusion pacing and CRT optimisation. One such software algorithm is SyncAV, developed by Abbott (Abbott Vascular, 3200 Lakeside Drive, Santa Clara, California 95054-2807). SyncAVTM assesses intrinsic AV conduction every 256 beats. It then sets a shorter programmed AV delay by subtracting a set period (known as the "delta" - adjustable but nominally set to 50ms) from the intrinsic time.

There is some evidence that fusion pacing gives benefit in terms of both acute pumping function of the heart and long term response to CRT. However, what remains unclear is whether the effect of dynamic AV optimisation and fusion pacing is maintained on exercise. Firstly, the re-analysis and adjustment intervals may be insufficient to allow effective fusion throughout exercise. It is therefore possible that with rapidly changing heart rates this coordinated timing is lost, leading to inefficient conduction of electricity through the heart. This could result in large periods of time on exercising without effective CRT and so poor exercise tolerance. Secondly, there is evidence that in patients with heart failure AV intervals do not alter with change in heart rate in a similar way to healthy controls. One study found that the degree of change is greater on exercise, whilst one demonstrated that in a CRT population only a third of patients had shorter optimal AV delay intervals on exercise, with a third being unchanged and a third longer. The use of a fixed "delta" in SyncAVTM may therefore result in incorrect adjustment of AV intervals as intrinsic conduction changes, with the effectiveness of SyncAV therefore depending on how the intrinsic interval changes.

We will use a prospective single-centre randomized single-blind crossover study to investigate the effectiveness of SyncAV on exercise, by randomising participants to either use of SyncAV or fixed AV delays, and then carrying out cardiopulmonary exercise testing ("CPET").