# Long PR-interval

Tracing

N° 10

Library

Pathology

Patient

68-year-old man, hypertensive, treated by beta-blocker, dyspnea;

Epigraph

A first-degree atrioventricular block corresponds to a simple prolongation of the PR-interval which exceeds physiological values; there are an equal number of P-waves and QRS-complexes as well as a fixed, constant PR-interval exceeding 200 ms in adults; a first-degree atrioventricular block most often reflects a conduction delay located at the atrioventricular node; the longer the PR-interval, the greater the probability of an intra-nodal slowdown.

The term first degree atrioventricular (AV) block corresponds to a prolongation of the PR-interval beyond physiological values (180 ms in children, 200 ms in adults). The term block is not exactly appropriate since, strictly speaking, it is not a block (interruption of the conduction) but rather a slowdown of conduction. Indeed, on the electrocardiogram, there are an equal number of P-waves and QRS-complexes, each P-wave being conducted after a fixed PR-interval exceeding 200 ms in adults. RR cycles are equal to the PP cycles, the ventricles are therefore at the same rate as the atria and there is no bradycardia if sinus function is normal.

A prolongation of the PR-interval can reflect a slowdown at any level of the "conduction chain" between the first activated atrial cell (beginning of the P-wave) and the first activated ventricular cell. A long PR can therefore be secondary to a slowdown of:

Endocardial electrophysiological study (invasive technique with insertion of catheters in the right heart chambers) enables to detect and record the electrical activity which reflects the transit of the excitation wave at the bundle of His (recording of H potential) and to determine the slowdown site by dividing the blocks into:

Various mechanisms can explain the slowdown of atrioventricular conduction: