His bundle electrogram in P mitrale.

The His bundle electrogram was recorded in I9 cases with P mitrale simultaneously with standard lead II, and compared with 30 cases with normal P waves. Internodal conduction time (P1A) interval did not differ in P mitralefrom the controlgroup, confirming that prolonged atrial activation in P mitrale represents a specific 'left atrial block'. The importance of this observation is discussed with reference to preferential atrial conducting pathways between the sinus node and the atrioventricular node. The end-point of atrial activation is altered in P mitrale so that it approaches the H wave of the His bundle electrogram and 'overrides it'. This graphic feature of the His bundle electrogram is characteristic ofP mitrale within the range of normal atrioventricular conduction, the H wave preceding the termination of atrial activation (P2), and the beginning of the H wave (the P2H interval) having a negative value.