The new ABCs of AV block. A revised classification to remove the mental block from recognizing AV block.
This revised classification system is designed to etch the indicators of a block into your memory. You only have to remember that the degree is the number of atrial beats getting through to the ventricles (all, some or none) and the type is where the block occurs (AV node or septum). Once you learn this system, simply practice interpreting block rhythms (see "AV Block Case Studies," p. 33). This revised block classification brings bundle branch blocks into the fold, so to speak, allowing you to focus on the block's anatomical location (see Figure 14, left). It also reinforces the fact that whenever we use the word block, we're talking about AV block. So how do you put it all together to use this new classification system? Use your usual rhythm strip analysis of rate, rhythm, P, PR, QRS, ST, T, U and summary to do the following: 1. Note any PR or QRS abnormalities. 2. Count the number of atrial and ventricular beats in a six-second ECG strip. If the ratio is 1-to-1 and the PR and QRS are normal, then there's no block present. If the ratio is 1-to-1, the PR is prolonged and the QRS is narrow, it's a first-degree, type 1 block. If the ratio is 1-to-1, the PR is normal and the QRS is wide, it's a first-degree, type 2 block. If the ratio is greater than 1-to-1 (such as 2-to-1, 3-to-1, etc.), but some of the beats are conducted, it's a some (second-degree) block. If the PR prolongs and the QRS is narrow on the conducted beats, it's considered a second-degree, type 1 block (Mobitz 1 or Wenckebach). If the PR is normal and the QRS is wide on the conducted beats, it's a second-degree, type 2 block (Mobitz 2). If none of the beats appear conducted (the PR interval is completely variable), it's a none (third-degree) block. If the QRS complex is at a normal width, then it's probably a third-degree, type 1 block. And if the QRS complex is wide, it's a third-degree, type 2 block. As with anything in medicine, no system is perfect. However, through using this revised classification of AV block--based on the heart's anatomy--you can permanently retain the features of the various AV blocks in your memory. Now that you're feeling more comfortable with block rhythms, examine Figure 15. This is a patient in sinus tachycardia who has received adenosine. Notice how the patient goes from sinus tachycardia to first-degree, type 1 to second-degree, type 1 to second-degree, type 2 and back again. It's a very rare strip that you can use to test your ability--and the ability of your crews--to interpret block rhythms.
Fowler
Raymond L
RL
University of Texas Southwestern, Dallas, USA. Drray@doctorfowler.com
D000328:Adult; D000368:Aged; D003937:Diagnosis, Differential; D004562:Electrocardiography / Q000379:methods; D004634:Emergency Medical Technicians / Q000193:education*; D006327:Heart Block / Q000145:classification / Q000175:diagnosis* / Q000503:physiopathology / Q000628:therapy; D006329:Heart Conduction System / Q000503:physiopathology; D006801:Humans; D008297:Male; D008875:Middle Aged; D010808:Physical Examination; D012720:Severity of Illness Index