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ABNORMAL P-R INTERVAL
A prolonged P-R interval is one that is over 0.20 second in duration. It is indicative of auriculoventricular block, since the greater part of the P-R interval is consumed in the passage of the excitation wave through the A-V node and the junctional tissue (figs. 63 to 67).A shortened P-R interval may occur in A-V nodal rhythm, (figs. 100 and 101) when the P wave is inverted and falls just before, just after or with the Q-R-S wave, and also in a recently described rhythm consisting of functional bundle branch block, occurring in healthy young people prone to paroxysmal tachycardia. It may also appear to be shortened in cases of heart block, ventricular extrasystoles and reciprocal rhythm (escape), but in such cases it is better to speak of it as the interval between the P wave and the R wave rather than of the P-R interval
CARTER JB. THE FUNDAMENTALS OF ELECTROCARDIOGRAPHIC INTERPRETATION. JAMA. 1932;99(15):1255-1262. doi:10.1001/jama.1932.27410670004013