A diagnostic workup of AV block may reveal reversible causes, such as electrolyte abnormalities or drug toxicities. These causes can be immediately addressed by correction of electrolyte abnormalities or by withdrawal of the offending pharmacologic agent. In the case of digoxin toxicity, Digibind may be utilized.
Symptomatic AV block should be addressed immediately. In first-degree AV block or Mobitz Type I second-degree block, atropine may result in return to a regular rhythm. Atropine is given at 05.-2.0 mg IV.
In symptomatic AV block unresponsive to atropine, temporary pacing should be initiated, and cardiology should be consulted immediately for placement of a permanent pacemaker.
Although transcutaneous pacemakers are widely utilized, temporary pacemakers may also consist of a pulse generator and an electrode placed transvenously into the right atrium. This can be performed in the emergency department setting, or in the patient’s room, under sterile conditions, especially if the cardiologist and cardiology suite are unavailable immediately. Central venous access is obtained preferably through the subclavian or internal jugular approach. Temporary pacing is only useful for a period of a few hours.
Permanent pacemakers are surgically sutured into the epicardium, in the operating room. The indications for permanent pacing in AV block include symptomatic and irreversible block due to etiologies other than electrolyte disturbance or drug toxicity. A general increase in sympathetic tone causing AV block is not an indication for pacing.
Permanent pacemakers also consist of a pulse generator and electrode, and can be placed in the atrial or ventricular epicardium. The electrical impulses generated by the pulse generator are delivered by the electrode catheter to adjacent cells, which are then depolarized, causing depolarization of the entire chamber.
Different types of pacing can be initiated through programming of the pacemaker, and these modalities are identified by a pacemaker “code” which utilizes letters representing the chamber which is paced and the chamber in which the sensing electrode is placed. In addition to “A” and “V” in the first position, the letter “D” indicates pacemaker placement in dual chambers. The third letter represents the way the pacemaker responds to the spontaneous activity of the myocardial cells.
A pacemaker placed in the atrium, sensed in the ventricles, and which is inhibited by spontaneous cardiac depolarization is indicated by the code “AVI”, which indicates pacing in the atrium, sensing in the ventricles, with pacemaker discharge inhibited by the spontaneous activity of the heart. The letter “T” in the third position indicates that the pacemaker is triggered by a refractory period, and the letter “D” indicates that the pacemaker is inhibited by ventricular activity, or triggered by a sensed atrial discharge. There is a fourth letter, “R”, which indicates that the pacemaker responds to physiologic demand.
When setting rate limits for the pacemaker, the lower rate limit is the intrinsic cardiac rate below which the pacemaker begins to discharge. The upper limit is the highest rate at which the pacemaker can fire.
In AV block, a DDD pacemaker is most commonly utilized. This is normal dual chamber pacing. Pacemakers have switching modes that allow a change in pacing mode in response to a rapid rate in the atrium. This prevents rapid ventricular response.
A magnet applied to a pacemaker will turn off the sensor, allowing the pacemaker to function asynchronously.
Complete guidelines for pacing are listed in the 2008 guidelines written by the American College of Cardiology, the American Heart Association and the Heart Rhythm Society. A detailed review of that report is beyond the scope of this discussion, but key points to be remembered in the case of pacing indications for AV block include the key concept that first-degree and Mobitz Type I second-degree block do not require pacing unless they are symptomatic and irreversible. As mentioned early, other etiologies, such as enhanced sympathetic or parasympathetic tone, electrolyte abnormalities, and drug toxicities must be considered and treated if present.
Mobitz II second-degree AV block and third-degree AV block usually require pacing, and it is usually permanent, although a temporary pacemaker may be inserted if the arrhythmia is symptomatic and unresponsive to other therapies. However, a permanent pacemaker should be placed as soon as possible in that case.
Diffuse conduction system disease is indicated in Mobitz II block when it is associated with wide QRS complexes that indicate widespread conduction system disease. This is an indication for permanent pacing even in the absence of symptoms. The existence of transient third-degree AV block coupled with irreversible bundle branch block is also an indication for permanent pacing.
In the setting of myocardial infarction, recall that in inferior MI Mobitz Type I block may occur due to ischemia in the AV node, or due to increased vagal tone. It usually resolves, and unless symptomatic, requires no treatment. First line of treatment in this setting with symptomatology is atropine, which may be effective. If not, temporary pacing with a transcutaneous or transvenous pacemaker may be required.
Mobitz Type II block is associated more often with anterior myocardial infarction. The more distal lesions in the conduction system become increasingly problematic, and in Mobitz II, permanent pacing is usually indicated. When coupled with conduction delays within the ventricles, this indicates an extensive area of injury to the heart, and pacing becomes inevitable.
With the development of complete heart block in the setting of an acute MI, temporary pacing is indicated, particularly in the case of an anterior MI, or with involvement of the His-Purkinje system.
Prognosis is poor for patients with Mobitz Type II block after myocardial conduction, as well as for those with complete heart block and intraventricular conduction delays, despite placement of temporary pacemakers. Again, this is related to the size of the infarct, rather than to the damage to the conduction system itself.