Pacing therapy for patients with refractory neurocardiogenic syndromes associated with severe bradycardia or asystole is controversial. Clinical studies have shown that rate-responsive pacing clinically benefits patients by restoring physiologic heart rate during exercise. 18 Another form of sinus node dysfunction is “chronotropic incompetence,” defined as an inadequate sinus rate response to stress or exercise. In turn, atrial overdrive pacing (pacing at a rate higher than the native sinus rate) may reduce episodes of obstructive sleep apnea in these patients. In patients with sleep apnea, treatment of the apnea frequently will reduce the occurrence of pauses. The clinical significance of such pauses is uncertain. In the general population, monitoring during sleep may reveal sinus pauses of variable duration.

16, 17 These findings are caused by increased vagal tone and usually do not indicate the need for pacing. During sleep, these athletes may have a heart rate as low as 30 beats per minute with associated pauses or type I second-degree AV block resulting in asystole as long as 2.8 seconds. Trained athletes often manifest physiologic sinus bradycardia with resting heart rates as low as 40 to 50 beats per minute. Many studies have documented that pacemaker therapy can reduce symptoms, improve quality of life and, in certain patient populations, improve survival. Biventricular pacing (resynchronization therapy) recently has been shown to be an effective treatment for advanced heart failure in patients with major intraventricular conduction effects, predominately left bundle branch block. They also are effective in the prevention and treatment of certain tachyarrhythmias and forms of neurocardiogenic syncope. Permanent pacemakers are implanted in adults primarily for the treatment of sinus node dysfunction, acquired atrioventricular block, and certain fascicular blocks. In general, rate-responsive devices are preferred because they more closely simulate the physiologic function of the sinus node. In older patients, devices that maintain synchrony between atria and ventricles are preferred because they maintain the increased contribution of atrial contraction to ventricular filling necessary in this age group. Current pacemaker devices treat bradyarrhythmias and tachyarrhythmias and, in some cases, are combined with implantable defibrillators. Second-degree atrioventricular block must be distinguished from other 'causes of pauses.' Nonconducted premature atrial contractions and atrial tachycardia with block are common conditions, which may mimic second-degree AV block.Each year, pacemaker therapy is prescribed to approximately 900,000 persons worldwide. In most cases of second-degree heart block, including cases of 2:1 conduction, it is possible to determine the site of the AV block (intranodal or infranodal) using information about the age of the patient, the clinical setting, and the width of the QRS complex on the surface electrocardiogram. Type II AV block is more likely to progress to complete heart block and Stokes-Adams arrest.

In Mobitz type II block the site is almost always below the AV node in Mobitz type I block the site is usually within the AV node. The distinction between type II and type I block is descriptive of greater importance to the clinician is the anatomic site of the block and the prognosis. Mobitz type II second-degree block is an old term, which refers to periodic atrioventricular block with constant PR intervals in the conducted beats. Second-degree AV block is a form of 'incomplete' heart block, in which some, but not all, atrial beats are blocked before reaching the ventricles.

Occasionally, heart block results from drug toxicity, hyperkalemia, cardiac valvular calcification, myocarditis, or infiltrative cardiomyopathy. Atrioventricular block is also a common manifestation of sclerodegenerative conduction system disease. Acute atrioventricular (AV) block occurs frequently in patients with myocardial infarction.

Mobitz type 2