So now that we got a brief review of the cardiac conduction system and the ECG, we can now talk about the different atrioventricular (AV) blocks. ECG’s are not to scale and not typical, and often the rate is shown faster than it would actually be to show detail.
We will start out with the first degree AV block. This a fairly simple and subtle AV block caused by the delay of the impulse at the diseased AV node. It is reflected in the PR interval, which the is part of the waveform measured from the beginning of the P wave to the beginning of the QRS complex. In a healthy heart the PR interval is 0.2 seconds in length. Normally this block is benign and requires no additional EMS investigation, unless drug overdose is suspected.
The second degree type I, also known as the Mobitz Type I or the Wenckebach, is where the impulse is delayed at the AV node longer than usual, increasing the interval until eventually the AV node fails to conduct the impulse on. The result is a “dropped” beat. This is commonly taught as the “going, going, going, gone” block. This block is the less dangerous of the two second degree blocks, but should be monitored appropriately.
The second degree type II, Also known as the Mobitz Type II or the Hay, is a disease of the distal conduction system where it simply fails to conduct a beat. There is no set pattern to how often or when it will occur. This is most dangerous of the second degree blocks because the AV node may drop a beat and progress into a complete heart block or fail to generate an escape beat, resulting in cardiac arrest.
These patients will present with syncope, light-headedness, or dizziness. These are all indications to perform a 12 lead ECG or at very least include cardiac monitoring. Treatments in the pre-hospital setting are limited however. It is widely accepted to attempt to correct bradycardia with atropine, however there is no guarantee that it will work, especially in second degree type II blocks, where the disease process is occurring distally to the AV node. Transcutaneous pacing is the preferred method to correct bradycardia and should be initiated immediately when the dysrhythmia is recognized.
The third degree block is the most dangerous of the three types. It is where the SA node and the AV node are working out of sync. The SA node is still sending an impulse, but the AV node is not picking it up and the Bundle of His is generating escape beats in order to keep the ventricles operating. This will be recognized in the ECG as two separate rhythms. The P-P interval (measurement from P wave to P wave) will show one rhythm and the R-R interval will show another.
In the pre-hospital setting, patients will present with ACS symptoms and presentation of this rhythm should concern us about AMI in either the inferior or anterior regions of the heart, and both can damage a part of the AV conduction system. A 12 lead ECG is always recommended. Treatment options include transcutaneous pacing and in cases where the patient does not respond to pacing or you are unable to establish a mechanical capture for whatever reason, dopamine may be given to increase both heart rate and systolic pressure.
So those are the AV blocks. I always welcome comments or corrections, or even suggestions for topics to cover and I will do my best. Stay tuned next week for another interesting topic.