In second-degree atrioventricular block type I, conduction through the atrioventricular node fatigues progressively with each beat until one impulse fails to get through entirely. The PR interval lengthens, lengthens again, and then a P wave arrives with no QRS behind it. The node recovers during that dropped beat, and the cycle begins again. This is usually a benign rhythm. It occurs in healthy people with high vagal tone, in athletes, during sleep, and commonly as a medication effect. Most clients are asymptomatic and need observation rather than intervention. The reason it earns a lesson is that it sits next to a rhythm that looks superficially similar and is genuinely dangerous. Distinguishing progressive PR lengthening from a fixed PR interval with sudden dropped beats is the discriminating skill, because the first is usually watched and the second is usually paced. Getting that comparison wrong in either direction causes harm: unnecessary alarm and intervention on one side, missed deterioration on the other. The nurse's contribution is therefore precise rhythm characterization, correct medication decisions, and knowing which changes convert a benign finding into...