Short Atrioventricular Delay Pacing

Inclusion Criteria: * Chronically (i.e. implanted for at least six months) dual-chamber pacemaker * Stable cardiac status (i.e. no hospitalizations or cardiac medication changes within 3 months) * Ability to provide informed consent Exclusion Criteria: * Age \<18 years or \>80 years * Pregnancy (if pregnancy is suspected, a urine pregnancy test will be performed) * Recent (within 12 months) left ventricular ejection fraction \<50% * Significant valvular heart disease (mitral regurgitation of greater than mild severity, any degree of mitral stenosis, aortic stenosis or insufficiency of greater than mild severity) * Prior coronary artery bypass graft or valve surgery, recent (i.e. within 3 months) acute coronary syndrome or percutaneous coronary intervention, or chronic stable angina * History of clinically important atrial or ventricular tachyarrhythmias, defined as complex ventricular ectopy (i.e. couplets, triplets, or ventricular tachycardia), AF, atrial tachycardia, or other supraventricular tachycardias not cured with prior ablative therapy). * Significant chronic systemic inflammatory or neoplastic disease * Chronic renal or hepatic insufficiency. Renal insufficiency will be defined as a creatinine clearance of less than 20 cc/min. Evidence of hepatic insufficiency will consist of abnormal synthetic function (INR \>1.4 without oral anticoagulant use, albumin \< 3.0 mg/dL) or abnormal clearance function (total bilirubin \>2.0). * Uncorrected thyroid abnormalities