AV Delay Optimization vs. Intrinsic Conduction in Pacemaker Patients With Long PR Intervals

Cardiac pacing is the only effective treatment for symptomatic sinus node dysfunction. Most
patients with preserved left ventricular function receive dual chamber pacemakers; however,
right ventricular pacing can have detrimental effects on left ventricular function due to the
abnormal electrical and mechanical activation pattern of the ventricles.

Many patients receiving dual chamber pacemakers for symptomatic bradycardia have prolonged
intrinsic AV conduction (first degree AV block), and as a result, will receive a significant
amount of ventricular pacing if programmed at physiologic AV intervals. As an alternative,
many pacemakers can be programmed to minimize ventricular pacing at the expense of allowing
longer AV delays. However, these long AV delays may not be physiologic and may also lead to
reduced cardiac output. At present the standard of care is either to program the pacemaker at
an physiologic "natural" AV delay of about 160 msec or to program the pacemaker with a long
AV delay to minimize ventricular pacing.

The main scientific questions being addressed in this study are to evaluate the acute and
chronic effects on cardiac output, functional status, sense of well-being, and cardiac
remodeling of a long AV delay allowing for intrinsic conduction as compared to an
echocardiographically optimized AV delay during dual chamber pacing.

Patients enrolled in the trial will complete a run-in period of two weeks prior to
randomization in which pacemakers will be programmed with a long-fixed AV delay to allow
intrinsic conduction and minimize ventricular pacing (standard). At two weeks, patients will
receive a baseline echocardiogram. To determine optimal AV delay, all patients will undergo
echocardiographic analysis at varying AV delays. Optimal AV delay will be defined as the AV
delay associated with the largest average aortic Doppler velocity time integral (VTI). Then,
patients will be randomized to either the short, optimized (experimental) or long, fixed
(standard) AV delay groups. To assess functional status and sense of well-being, patients
will complete a six minute walk test and Short Form-36 Medical Outcomes Study Questionnaire.
Patients return to clinic for another study visit at 6 months and repeat research procedures,
including baseline echocardiogram, questionnaire, and 6 minute walk test.

Patient population: Individuals with 1st degree atrioventricular (AV) block who have
received a dual chamber pacemaker for symptomatic bradycardia.

Inclusion Criteria:

1. Patients greater than 18 years of age

2. Patients with symptomatic sinus bradycardia

3. Patients who meet standard indications for dual chamber pacemaker implantation

4. Patients who have 1st degree AV block determined by PR interval > 200ms

Exclusion Criteria:

1. Patients with complete or high grade AV block

2. Patients who are unable to complete dual chamber pacemaker implantation for any reason

3. Patients with congestive heart failure determined by a Left Ventricular Ejection
Fraction < 45%

4. Patients with persistent atrial fibrillation

5. Sustained premature ventricular contractions (PVCs), premature atrial contractions
(PACs), atrial flutter, or other heart conditions that may interfere with
echocardiography measurements

6. Patients who are pregnant

7. Patients with Paroxysmal Atrial Fibrillation that have had an episode(s) within 30
days of consent