The present prospective pacemaker registry is one of the most detailed studies regarding the influence of RV pacing on various echocardiographic and spiroergometric parameters. As in numerous previous pacemaker studies, this trial could not reveal any relevant short term changes of several echocardiographic and spiroergometric parameter regardless to the burden of RV pacing in patients with preserved ejection fraction. Several clinical studies have reported that chronic right ventricular (RV) apical pacing causes adverse cardiovascular outcomes, including negative cardiac remodeling, atrial fibrillation (AF), congestive heart failure (HF), and mortality.3, 11, 12 Moreover, it was shown that adverse left ventricular remodelling caused by conventional right ventricular apical pacing, could be prevented by biventricular pacing.12 Based on the present detailed echocardiographic data, it is difficult to explain the deteriorating clinical outcome of RV pacing in patients with preserved left ventricular ejection fraction. A large cohort study of pacemaker recipients with predominantly normal LVEF, was also unable to demonstrate a clinically relevant decrease in LVEF due to RV pacing even after 44 months of follow-up.4 While this study focused solely on the systolic left ventricular function, the current trial also examined the diastolic left ventricular function over a period of 12 month as well as the right ventricular function after 24 month of follow-up. But also the analysis of these echocardiographic parameters consistently showed the same result: The burden of RV apical pacing has no influence on the left and right systolic and diastolic function in patients with preserved LVEF.
These echocardiographic results were also reflected in the primary clinical short term outcome of the current trial, which was determined by spiroergometry. Accordingly to the echo data no relevant changes of peak exercise capacity, ventilatory anaerobic threshold and maximal oxygen consumption could be revealed independently of the RV pacing burden in the short term follow-up. As this is the first pacemaker study, which defines the clinical outcome in spiroergometric parameters, no direct comparison with previous trials, determining clinical outcome as congestive heart failure, atrial fibrillation or mortality, is possible.
However, few data exist, showing that isn’t the RV pacing burden itself but the programmed AV interval which influence the exercise capacity.13 Therefore, if a patient’s exercise capacity deteriorate under RV pacing, as primary noninvasive approach an echocardiographic optimisation of AV interval should be considered to improve the exercise capacity.
A very important aspect regarding the adverse potential of RV pacing seems to be the initial LV function. Thus in the DAVID trial patients with standard indications for ICD therapy and an LVEF of 40% or less, RV pacing increased the combined end point of death or hospitalization for heart failure.3 Furthermore, the Mode selection trial (MOST) could reveal an increased risk of heart failure and atrial fibrillation over a period of 6 years in patients with a cumulative RV pacing > 40% compared to patients with a low RV pacing burden.2 Of note, in the MOST trial the clinical endpoints were not analysed with respect of the LV function. Also, the BLOCK-HF study suggests that patients with LVEF < 50% and AV block may benefit from biventricular pacing in terms to heart failure compared to RV pacing.14
In recent years, due to the ongoing discussion about the adverse potential of RV stimulation, various pacemaker programming algorithms, switching from AAI(R) to DDD(R), a have been developed to reduce unnecessary RV stimulation as much as possible. However, the subsequent MVP trial (Managed Ventricular pacing vs.VVI 40 Pacing) failed to demonstrate non-inferiority of this algorithm.15 Also a previous meta–analysis could demonstrate that the reduction of unnecessary ventricular pacing fails to affect hard clinical outcomes in patients with preserved left ventricular function.16 In addition, it could be shown, that the burden of RV pacing does not influence the quality of life in pace maker patients.17
Inspired by an excellent editorial, written by Dr. Israel about his „top 10 excuses“ with regard to unexpected results in prospective pacemaker studies, this study has several limitations as well.18 First, the current study subjects were not randomised into the high and low RV pacing group. Furthermore, the study population was too small to make valid conclusions about clinical endpoints like heart failure or mortality. Additionally, the current study cannot rule out long-term negative effects of RV stimulation, as the follow-up period did not exceed 24 months.
In summary of all clinical studies as well as existing and current echocardiography/spiroergometry studies, the mechanism of worsened pacemaker-induced clinical outcome remains unclear, especially in patients with preserved left ventricular function. Therefore, further studies are needed to reveal which patients with preserved left ventricular function benefit clinically from HIS bundle or left ventricular pacing.