Generally, site and degree are the major classifications of AV nodal blocks. This terminology helps classify and communicate the management of patients with these arrhythmias. However, more discrete descriptions may be required, and we will touch briefly on these other categories. 

2:1 Heart Block

2:1 block refers to one ventricular contraction for every two atrial contractions. It can erroneously be considered a Mobitz II block due to a seemingly stable PR interval between conducted P waves to the ventricles. However, a 2:1 block may be either Mobitz I or II 2nd-degree block. To distinguish the two possibilities, the PR interval of conducted P waves can be evaluated to determine if it is truly stable or increasing. Clues that can help distinguish the two possibilities include:

• Prolonged PR interval of conducted beat and narrow QRS suggests Mobitz type I AV block.
• Narrow QRS responding to atropine suggests Mobitz I due to increased vagal tone.
• Wide QRS will likely be unresponsive to atropine and should be managed with TCP or beta-adrenergic medications. 
• Presence of a previous bundle branch block (a wide QRS before heart block onset) may make distinguishing the two possibilities impossible.

AV dissociation 

AV dissociation refers to a disconnection between the electrical activity of the atria and the ventricles. In these cases, the atria and the ventricles have different rates that are independent of each other. As discussed previously, this can happen in two instances. The first is 3rd-degree or complete block in which the atria beat more rapidly than the independent ventricles. The second is with underlying ventricular arrhythmias (e.g., VT) or ventricular escape rhythms in which abnormal electrical activity arises outside of the normal conduction pathway. In these cases, the ventricles beat more rapidly than the independent atria.

• In significant bradycardia, such that the sinus rhythm is excessively slow, an escape pacemaker may develop a more rapid rate. Depending on the location of the escape rhythm, the QRS may be narrow (close to the AV node) or wide (distal to the AV node). These are termed accelerated junctional (or idioventricular) rhythms when they are more rapid than sinus rhythm.
• Often in VT, ventricular activity is not conducted retrograde towards the atria through the AV node. Consequently, atrial contractions are completely independent of ventricular contraction, and their rate is slower. On ECG, atrial impulses may independently “march through” the ventricular tachyarrhythmia. This finding suggests AV dissociation. 

Advanced Heart Blocks

• These are more severe types of 2nd-degree AV blocks that lead to multiple dropped beats. However, occasionally a P wave will conduct to the ventricle. This suggests a more advanced conduction abnormality and is more likely to become symptomatic and progress to complete heart block.
• When at least two P waves in succession drop, the diagnosis is advanced (high-degree) heart block. The QRS complex can provide information as to the site of the block, with the wide QRS suggesting a more distal site. Additionally, a stable versus prolonging PR interval for the P waves that are conducted can also help determine the blockage site.  
• Regardless of the site, since advanced heart block is more likely symptomatic due to the frequently dropped beats, these patients need to be monitored closely with easy access to and preparations made for pacing. 

Treating Advanced Heart Block of Unknown Site

Management

Both asymptomatic and symptomatic advanced heart block should be monitored closely and evaluated in conjunction with the patient’s history and presentation. For a wide QRS advanced block, a distal site of origin is likely, and the team should consult cardiology, obtain IV access, and consider transvenous pacing. 

In cases of advanced heart block, atropine may not be effective, and management may require transcutaneous pacing if IV access is not immediately available until transvenous pacing can begin. 

The team should follow the algorithm for adult bradycardia with a pulse. Atropine 1.0 mg is given IV at 3- to 5-minute intervals up to a cumulative dose of 3 g (if an infranodal site is likely, this step may be ineffective).

TCP is provided immediately if the patient is unstable or medications prove ineffective. The team ensures the patient is tolerant of this procedure, checks for electrical capture, and monitors for adequate mechanical function. TCP may be painful, so adequate analgesics or sedation is required. Certain sedatives can negatively affect the cardiac rhythm.

If TCP cannot be used or is ineffective, the patient can be treated with an:

• IV infusion of dopamine at 5–20 mcg/kg/minute
• IV infusion of epinephrine at 2–10 mcg/minute

During this time: 

• Cardiac consultation is obtained
• The team prepares for transvenous pacing