The P wave
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Table 8 Common types of PR interval abnormalities

Normal PR interval • Measured from the beginning of

P wave to the beginning of QRS complex. It varies from 0.12 seconds to 0.2 seconds (3-5 mm)

Prolonged PR interval • This is often referred to as first degree

heart block and the PR interval is >0.2 seconds (>5 mm)

Short PR interval • PR interval is <0.12 seconds (<3 mm).