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[Type I and II heart block]
F Fillette1, P Aouate, T Papargyriou
1Laboratoire d'electrophysiologie de la Salpêtrière, service de cardiologie, Paris.
Second degree blocks are divided into type I blocks, with an increment in conduction time for two consecutive conducted impulses, and type II blocks which present with constant conduction times for consecutive conducted impulses. This distinction is not only of theoretical interest but also of practical interest, since type I blocks located in the node are benign as a rule, whereas type I blocks in other locations and type II blocks are of poor natural prognosis and require pacing. Major increments of the PR interval, occurrence at night and young age are in favour of type I blocks. 2/1 blocks without 2 consecutive impulses sometimes raise difficult problems. Carotid massage and change from supine to upright posture may display two consecutive conducted P waves. If this does not happen, Holter recordings will show, in almost every case, two consecutive waves which enable the block to be typed. Whenever a doubt persists, electrophysiological exploration makes it possible to determine the exact location of the block. Type I blocks imply the presence of a decremental conduction and therefore of varying conduction velocities which decrease with consecutive impulses. This is compatible with the normal functioning of the atrioventricular node which has calcium-dependent potentials, b ut everywhere else it implies a pathological state of tissues that have sodium-dependent potentials and give an all-or-nothing response. This explains why extranodal type I blocks, like types II blocks, make it necessary to fit the patients with a pacemaker, whereas type I blocks usually are of good prognosis.
Second degree blocks are divided into type I blocks, with an increment in conduction time for two consecutive conducted impulses, and type II blocks which present with constant conduction times for consecutive conducted impulses. This distinction is not only of theoretical interest but also of practical interest, since type I blocks located in the node are benign as a rule, whereas type I blocks in other locations and type II blocks are of poor natural prognosis and require pacing. Major increments of the PR interval, occurrence at night and young age are in favour of type I blocks. 2/1 blocks without 2 consecutive impulses sometimes raise difficult problems. Carotid massage and change from supine to upright posture may display two consecutive conducted P waves. If this does not happen, Holter recordings will show, in almost every case, two consecutive waves which enable the block to be typed. Whenever a doubt persists, electrophysiological exploration makes it possible to determine the exact location of the block. Type I blocks imply the presence of a decremental conduction and therefore of varying conduction velocities which decrease with consecutive impulses. This is compatible with the normal functioning of the atrioventricular node which has calcium-dependent potentials, b ut everywhere else it implies a pathological state of tissues that have sodium-dependent potentials and give an all-or-nothing response. This explains why extranodal type I blocks, like types II blocks, make it necessary to fit the patients with a pacemaker, whereas type I blocks usually are of good prognosis.