Approach Considerations
Second-degree atrioventricular (AV) block in the asymptomatic patient does not require any specific therapy in the prehospital setting. If the patient is symptomatic, standard advanced cardiac life support (ACLS) guidelines for bradycardia, including the use of atropine and transcutaneous pacing, are indicated. [11, 35]
No specific therapy is required in the emergency department (ED) for Mobitz I (Wenckebach) second-degree AV block, unless the patient is symptomatic. Patients with suspected myocardial ischemia should be treated with an appropriate anti-ischemic regimen and worked up. Second-degree block at the level of the atrioventricular node (AVN) may be due to digoxin, beta-blockers, or calcium channel blockers. Decreasing the dose and/or discontinuing these medications may restore normal AV conduction.
Mobitz II block is more likely to progress to complete heart block and thus requires a different approach. As with Mobitz I block, AV nodal agents should be avoided, and an anti-ischemic regimen should be instituted if ischemia is suspected.
Permanent pacing is considered in accordance with the relevant guidelines (see Pacemaker Implantation). Except for the use of atropine in selected cases of transient AV block, permanent cardiac pacing has replaced medical interventions in the treatment of patients with symptomatic, otherwise untreatable, AV block.
Atropine and Transcutaneous/Transvenous Pacing
Mobitz I block
Admit patients who have symptoms or who have concomitant acute myocardial ischemia or myocardial infarction (MI). Admission should be to a unit with telemetry monitoring, which has transcutaneous pacing capabilities.
Symptomatic patients should be treated with atropine and transcutaneous pacing. However, atropine should be administered with caution in patients with suspected myocardial ischemia, as ventricular dysrhythmias can occur in this situation.
The goal of atropine administration is to improve conduction through the AVN by reducing vagal tone via atropine-induced receptor blockade. However, this goal will only be effective if the level of the blockade is at the site of the AVN. Patients with infranodal second-degree AV block are unlikely to benefit from atropine. In addition, in patients who have denervated hearts (eg, patients who have undergone a cardiac transplant), atropine is also not likely to be effective.
Mobitz II block
Admit all patients to a unit with monitored beds, where transcutaneous and transvenous pacing capabilities are available. The admitting cardiologist should determine whether permanent pacemaker implantation is indicated.
Transcutaneous pacing pads should be applied to all patients with Mobitz II second-degree AV block, including those who are asymptomatic patients, because such patients have a propensity to progress to complete heart block. The transcutaneous pacemaker should be tested to ensure capture. If capture is not able to be achieved, then insertion of a transvenous pacemaker is indicated, even in asymptomatic patients.
Urgent cardiology consult is indicated for patients who have symptomatic type II block and for those asymptomatic patients who are unable to achieve capture with transcutaneous pacing.
Some institutions recommend insertion of a transvenous pacemaker for all new Mobitz type II blocks, although this practice varies greatly from institution to institution.
Patients who are hemodynamically unstable for whom an emergency cardiology consult is not available should undergo placement of a temporary transvenous pacing wire in the ED. A chest radiograph is required to confirm position of the wire and to exclude complications, including hemothorax or pneumothorax.
2:1 block
In cases where there is a 2:1 block and one is unable to determine if there is a Mobitz I block or Mobitz II block, the patient should be admitted and cardiology consultation should be obtained. In such cases, it is safest to assume that a Mobitz II second-degree AV block exists.
Pacemaker Implantation
Indications for permanent pacing in second-degree AV block are explained in detail in the guidelines published by the American College of Cardiology (ACC), the American Heart Association (AHA), and the North American Society for Pacing and Electrophysiology (NASPE) in 2002 [2] and by the ACC, the AHA, and the Heart Rhythm Society (HRS) in 2008. [3] The core of the recommendations for permanent pacemaker implantation is based on two factors: the presence of symptoms associated with the rhythm disorder as well as the presence of an infranodal AV block. A summary of the indications is as follows:
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Second-degree AV block associated with signs such as bradycardia, heart failure, and asystole greater than or equal to 3 seconds
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Second-degree AV block with neuromuscular diseases, such as myotonic muscular dystrophy, Erb dystrophy, and peroneal muscular atrophy, even in asymptomatic patients (progression of the block is unpredictable in these patients); in some of these patients, an implantable cardioverter defibrillator (ICD) may be appropriate
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Mobitz II second-degree AV block with wide QRS complexes
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Asymptomatic Mobitz I second-degree AV block with the block at intra- or infra-His level found on electrophysiologic testing (level II recommendation)
Second-degree AV block may occur after MI, and it may be transient or asymptomatic. In this case, pacemaker placement may not be needed. However, persistent and symptomatic second-degree AV block after MI, especially if it is associated with bundle-branch block, warrants permanent pacemaker placement. High-grade AV block after anterior MI, even if transient, may warrant permanent pacing. (AV block resulting from right coronary artery occlusion tends to resolve over a few days after revascularization versus left anterior descending artery MI, which results in permanent AV block.)
Second-degree AV block after cardiac surgery may be persistent and necessitate pacemaker placement.
Second-degree AV block in patients with drug toxicity, Lyme disease, or hypoxia in sleep apnea is expected to resolve. In any situation where second-degree AV is expected to resolve as a result of correction of the underlying pathology, permanent pacemaker placement is not indicated.
AV block after transcatheter aortic valve implantation may occur. This is a relatively new technology, and there is not enough adequate evidence to guide the patient's therapies in this situation. In some cases, depending on the type of the implanted valve, baseline electrocardiographic (ECG) features, degree and location of the aortic valve calcification, and the patient's comorbidities, implanting a permanent pacemaker outside of conventional criteria may be a reasonable and safe approach.
An estimated 40% of patients with AV block undergo cardiac pacing because of syncope; pacing also appears to very successful in preventing syncopal recurrences in those with AV block. [36] Aste et al examined retrospective data (2009-2013) in 94 patients who received a permanent pacemaker for AV block and syncope as well as 138 patients who received a permanent pacemaker for AV block without syncope. Of those with both AV block and syncope, 73 had documented third-degree or Mobitz II second-degree AV block and 21 had suspected AV block; all had bundle-branch block. At 5-year follow-up, Aste et al reported a 1% actuarial syncope recurrence rate in the group with AV block and syncope compared to 3% in the group without syncope, as well as 14% in the group with undocumented AV block plus syncope. All syncopal episodes occurred in patients without overt structural heart disease. [36]
Consultations
For symptomatic patients with Mobitz I atrioventricular (AV) block, a cardiology consultation is indicated. Asymptomatic patients with a Mobitz I block can be referred to a cardiologist on an outpatient basis.
For any patient with a new Mobitz II AV block, cardiology consultation is indicated, regardless of symptoms.
Long-Term Monitoring
Patients who are discharged from the emergency department with a Mobitz II atrioventricular (AV) block should have prompt follow-up arranged with a cardiologist.
If high-grade AV block and, possibly, symptoms due to AV block are present, consider monitoring the patient and then performing Holter monitoring or treadmill testing in the future. If the AV block occurs at night, consider sleep apnea as the cause.
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Second-Degree Atrioventricular Block. Typical Mobitz I atrioventricular block with progressive prolongation of PR interval before blocked P wave. Pauses are always less than sum of 2 preceding beats because PR interval after pause always shortens.
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Second-Degree Atrioventricular Block. Mobitz II atrioventricular (AV) block with intermittent periods of 2:1 AV block. If only 2:1 block was seen in beginning of strip, site of block could not be localized with certainty; however, single dropped QRS complex at end of strip with constant PR interval indicates that this block is localized in one of the bundle branches.
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Second-Degree Atrioventricular Block. Variable-ratio Mobitz I atrioventricular block. Note marked PR-interval prolongation in first beat of each cycle. Maximum prolongation of PR interval takes place in second beat of cycle, with much smaller increments in subsequent beats. Also, notice that R-R interval actually shortens with each beat—paradox of shortening R-R interval when PR interval increases by diminishing increments.
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Second-Degree Atrioventricular Block. Sinus rhythm with Mobitz I second-degree 3:2 infranodal atrioventricular (AV) block and bifascicular block. Note that AH interval (indicative of AV nodal conduction) remains constant. HV interval (indicative of His-Purkinje conduction) increases from 65 msec (after first P wave) to 185 msec (after second P wave). Third P wave is followed a His bundle deflection (H) but no QRS complex. AV block occurs in His-Purkinje system below site of recording of His bundle potential. Note shorter PR interval after nonconducted P wave, typical of Mobitz I AV block. HRA = high right atrial electrogram; A = atrial deflection; HB = His bundle electrogram, proximal and distal; H = His bundle deflection; RV = right ventricular electrogram; T = time line, 50 msec.
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Second-Degree Atrioventricular Block. Representative 12-lead electrocardiogram in asymptomatic 78-year-old woman during recent noncardiac surgery. Patient was referred for implantation of permanent pacemaker with diagnosis of sinus tachycardia with 2:1 atrioventricular (AV) block and narrow QRS complex. As sinus rate slowed, 1:1 AV conduction resumed. Intracardiac recordings confirmed diagnosis of infra-Hisian 2:1 AV block.
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Second-Degree Atrioventricular Block. Electrocardiogram of patient with Mobitz I (Wenckebach) second-degree atrioventricular block.
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Second-Degree Atrioventricular Block. Electrocardiogram of patient with Mobitz II second-degree atrioventricular block.
