What ECG finding indicates a first-degree AV block?

The presence of a long PR interval with no dropped beats is a characteristic finding of a first-degree AV block on an electrocardiogram (ECG). In first-degree AV block, the conduction through the atrioventricular (AV) node is prolonged, which is reflected in the extended duration of the PR interval, typically greater than 200 milliseconds. Despite this longer PR interval, all atrial impulses are successfully conducted to the ventricles, resulting in no dropped beats. The increased interval indicates delayed conduction rather than a complete block.

In contrast, short PR intervals combined with dropped beats would suggest a different conduction issue, possibly indicative of a higher degree AV block. A normal PR interval with broad QRS complexes may point towards a bundle branch block or other ventricular conduction abnormalities rather than a first-degree AV block. Lastly, a variable PR interval is typically associated with conditions like atrial fibrillation or other arrhythmias that result in unpredictable conduction through the AV node, which does not fit the criteria for a first-degree block. Therefore, the distinctive finding of a long PR interval with consistent conduction is what confirms the diagnosis of first-degree AV block.