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Second-degree atrioventricular (AV) block, Mobitz type I, also known as Wenckebach phenomenon, is characterized by a progressive prolongation of the PR interval on successive beats, culminating in a non-conducted P wave, which results in a dropped QRS complex. This pattern indicates a cyclical delay in the AV nodal conduction.
Analogy: “The communication between the upper and lower sections of the orchestra becomes increasingly delayed until a beat is missed.”
The ECG in Mobitz I second-degree AV block displays a characteristic pattern of progressive PR interval lengthening with each subsequent beat in a cycle. This lengthening continues until an atrial impulse (P wave) fails to conduct through the AV node to the ventricles, resulting in a dropped QRS complex. Following the non-conducted P wave, the PR interval of the next conducted beat is typically shorter than the PR interval immediately preceding the dropped beat, and the cycle then repeats. While the RR intervals become irregular due to the dropped beats, the P-P interval, reflecting the regular firing of the SA node, remains relatively constant. The greatest increase in the PR interval duration usually occurs between the first and second beats of the cycle. This type of block is typically due to a reversible conduction block at the level of the AV node.
Mobitz I second-degree AV block is often asymptomatic, particularly in individuals without underlying heart disease. Some individuals may experience mild symptoms such as dizziness or lightheadedness. Generally, Mobitz I is considered a benign rhythm with a low risk of progressing to third-degree heart block. It can be caused by various factors including certain medications that slow AV nodal conduction (e.g., beta-blockers, calcium channel blockers, digoxin), increased vagal tone (as seen in athletes), inferior myocardial infarction, and myocarditis.
“Like a musician occasionally missing their cue after increasing hesitation, the ventricles fail to contract after a progressively longer delay.”