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1* AV block is defined as PR interval>200ms or 5 small boxes. This has no hemodynamic consequences and is not at significant risk for development of 3* AV block. This is sometimes debated as young, healthy adults may have a slightly longer PR interval than normal (up to 220ms); however, the definition of 1* AV block or simply longer PR interval due to baseline increased vagal tone in a healthy adult is semantics. ‘Marked’ 1* AV block is defined as PR interval>300ms. The PR interval can get up to 1000ms and still conduct to the ventricles. However, PR intervals >300ms can result in some dyssynchrony of atrial and ventricular contractions, leading to reduced cardiac output and symptoms (presyncope or syncope).
Generally, there is no specific treatment or concern from this typically and it is essentially a normal variant (exceptions may include AV nodal blocking drugs, Lyme disease, or hyperkalemia). There may be some increased concern in patients that had symptoms of higher degree AV block prior to arrival as some patients have transient 2* or 3* blocks with hemodynamic compromise.
Children have smaller hearts and a slightly different conduction system. Their PR intervals are typically shorter (a PR interval >180ms is considered long).
Note that the P-P, R-R and PR intervals are all consistent. One would differentiate this from 3* AV block in that there is a QRS for every P wave and the QRS is narrow unless a patient has an underlying bundle branch block (note that a junctional escape with a 3* AV block may appear somewhat narrow).
Examples:
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