Heart Blocks Explained: 1st, 2nd, and 3rd Degree for Monitor Techs

A practical guide to AV heart blocks for telemetry monitor techs. Learn to identify 1st, 2nd (Wenckebach and Mobitz II), and 3rd degree blocks on the monitor.

Understanding the Roadblock Every normal heartbeat follows the same route: the sinus node fires, the impulse travels through the atria (producing the P wave), pauses briefly at the AV node, then conducts through the His-Purkinje system to depolarize the ventricles (producing the QRS complex). Heart blocks are disruptions somewhere along that pathway — specifically at or below the AV node. Think of the AV node as a checkpoint between the atria and the ventricles. In a healthy heart, every impulse clears the checkpoint and gets through. In heart blocks, the checkpoint slows things down, intermittently refuses entry, or shuts down completely. The degree of the block tells you how severe the disruption is. As a monitor tech, heart blocks are rhythms you need to recognize quickly and categorize accurately, because the clinical urgency ranges from "document it and move on" to "call the nurse right now." First Degree Heart Block: The Slow Pass First degree heart block is the mildest form. Every impulse still gets through the AV node — no beats are dropped. The impulse is just delayed. The PR interval, which normally measures 0.12 to 0.20 seconds, stretches beyond 0.20 seconds (more than one large box on EKG paper). What it looks like on the monitor - Every P wave is followed by a QRS complex. No beats are missing. The rhythm is regular. - The PR interval is prolonged and constant. Every PR interval measures the same — maybe 0.24 seconds, maybe 0.32 seconds — but it does not change from beat to beat. - The rate and rhythm are otherwise normal. Heart rate is whatever the sinus node dictates. The only abnormality is the long PR interval. Clinical significance First degree heart block by itself is common and usually benign. You will see it frequently in older patients, patients on beta-blockers or calcium channel blockers, and patients with high vagal tone (particularly athletes). It does not cause symptoms and does not usually require treatment. When to escalate: If the PR interval is new and markedly prolonged (above 0.30 seconds), if it is progressively lengthening on serial strips, or if it accompanies other conduction changes (widening QRS, new bundle branch block). A first degree block that is getting worse may be heading toward a higher-degree block. The mnemonic If you need a memory anchor: "First degree — delayed but delivered." Every beat gets through. It is just late. Second Degree Heart Block Type I (Wenckebach): The Progressive Stall Second degree Type I, commonly called Wenckebach (after the physician who described the pattern), is more interesting than first degree but usually still benign. Here, the AV node progressively fatigues with each beat until it finally fails to conduct one impulse altogether. Then it resets and the pattern starts over. What it looks like on the monitor - The PR interval gets progressively longer with each consecutive beat. The first beat after the dropped beat has the shortest PR interval. The next beat has a slightly longer PR. The