cardiovascular disorders
Atrioventricular Block
First degree AV block
 0.20 second—see figure First-degree atrioventricular block ). For first-degree block, conduction is slowed without skipped beats. All normal P waves are followed by QRS complexes, but the PR interval is longer than normal (>0.2 second). For third-degree block, there is no relationship between P waves and QRS complexes, and the P wave rate is greater than the QRS rate. First-degree AV block may be physiologic in younger patients with high vagal tone and in well-trained athletes. First-degree AV block is rarely symptomatic, and no treatment is required. Further investigation may be indicated when first-degree AV block accompanies another heart disorder or appears to be caused by medication. 