• All cause mortality after a mean follow-up of 5.5 year.
  

• Cardiovascular death
  
• Chronic atrial fibrillation
  
• Paroxysmal atrial fibrillation
  
• Arterial thromboembolism
  
• Congestive heart failure
  
• Need for pacemaker re-operations
  
• Quality of life
  
• Health economics
  

Background In patients with isolated SSS, who need pacemaker treatment, any pacemaker can be used to treat the symptomatic bradycardia: a single chamber atrial (AAI) pacemaker, a single chamber ventricular (VVI) pacemaker, or a dual chamber (DDD) pacemaker. In the USA and in most European countries, DDD pacing is used in most cases. It is now known from the Danish AAI/VVI trial, that AAI pacing is superior to VVI pacing, since VVI pacing is associated with a higher mortality and a higher incidence of atrial fibrillation, thromboembolic complications and heart failure. This confirms previous findings in observational studies. Therefore, VVI pacing should no longer be used in patients with SSS.

The main argument for using DDD pacing is the concern, that the patients will develop symptomatic atrioventricular (AV) block. In the Danish AAI/VVI trial, the risk of AV block was approximately 0.6% per year, which is equivalent to the risk found in a larger meta analysis. This is only a little higher that the risk of atrioventricular block in the age-matched non-paced population. Implantation of a DDD pacemaker in all patients will effectively prevent development of symptomatic bradycardia in the minority of patients who develops AV block. However, the most important disadvantage during DDD pacing is the stimulation (pacing) of the ventricles by the pacemaker a large part of the time, also in patients without AV block. Pacing the right ventricle causes an asynchronous electrical activation and mechanical contraction of the ventricles as compared with the normal physiological contraction.

At present time, a randomised comparison of AAI and DDD pacing in patients with SSS has never been conducted, and to our knowledge, such a trial is not planned anywhere else.

Since several of the patients with SSS suffer from chronotropic incompetence, pacemakers with rate adaptive function are chosen for all patients included in the present trial.

All patients, that fulfils the inclusion criteria and none of the exclusion criteria and who give written informed consent, are included into the study. For all other patients undergoing primary pacemaker implantation in the study period, an exclusion data sheet is filled in stating the reason for exclusion. A total of 1,900 patients are included into the study.

Prior to the pacemaker implantation patients are randomised by lot (envelope) to either AAIR or DDDR pacing. The randomisation is performed after written informed consent has been obtained from the patient. Randomisation will ensure that all centres will randomise an equal number of patients into each treatment group.

Patients randomised to AAIR pacing will have a bipolar lead implanted in their right atrium connected to a single chamber pacemaker with rate adaptive function. Patients randomised to DDDR pacing will have two leads (one bipolar lead in their right atrium and a uni- or bipolar lead in their right ventricle) connected to a DDDR pacemaker.

Out of hospital follow-up The patients must attend for out of hospital follow-up after 3 months and 12 months and then once every year.

Criteria for closing the study

The DANPACE study is stopped and results are analysed when all of the following three criteria are fulfilled:

1. 1,900 patients have been randomised.
2. The last randomised patient has been followed for at least 1 year.
3. The mean follow-up for the whole study population is at least 5.5 years.