Third Degree AV Block

Third-Degree Heart Block aka Complete Block 
A Third-Degree Heart Block occurs when none of the electrical impulses can reach the ventricles. When the ventricles, (lower chambers of the heart), do not receive electrical impulses from the atria (upper chambers of the heart), they may generate some impulses on their own called junctional or ventricular escape beats. Ventricular escape beats, natural backup signals, usually are very slow, however, and cannot generate the signals needed to maintain full functioning of the heart muscle. 
Summary of ECG criteria:
QRS - Generally normal looking. When block occurs at the AV node or bundle of His, the QRS complex will appear normal. When block occurs at bundle branch level, the QRS complex will be widened (greater than 0.12 seconds if pacemaker is ventricular; less than 0.12 if pacemaker is junctional)
P waves - Normal, but has no correlation to the QRS complex
Rate - The atrial rate will be unaffected by third-degree AV block. The ventricular rate will be slower than the atrial rate. With intranodal third-degree AV block, the ventricular rate is usually 40 to 60 beats/min; with infranodal third-degree AV block, the ventricular rate is usually less than 40 beats/min.
Rhythm - The atrial rhythm is usually regular, although sinus arrhythmia may be present. The ventricular rhythm will be regular.
PR interval - Since the atria and ventricles are depolarized from different pacemakers, they are independent of each other, and the PR interval will vary.
Pacemaker site - SA node and AV junction or ventricle
Clinical Significance:
Cardiac output can be severely compromised due to a decreased heart rate and loss of coordinated atrial kick
Definitive treatment is pacemaker insertion. In the field, stabilize the patient with medications if need be. Administer 0.5mg of Atropine IV if heart rate falls and patient is symptomatic. Repeat every 3-5 minutes as needed, until a satisfactory rate is obtained or until 3mg is reached. Use transcutaneous cardiac pacing (TCP) if available.


Use Atropine with caution on patients that have high-degree blocks because it may accelerate the atrial rate, but may worsen the AV nodal block. Never use Lidocaine to treat a third-degree block with ventricular escape beats.
Group members: 
Abbie Gloyd
Joe Krcmar
Dustin Moody
Brady Paramedic Care Volume 3
American Heart Association
Heart Rhythm Society
eMedicine from WebMD
RnCeus Interactive