PR = début de P - début de R

[Critères à vérifier +++]

ATT: Le QRS n'est pas tjrs fin (le blocage infra hissien donne un aspect de BB) mais évoquer alors un BAV3.

ATT: BAV2 Mobitz II s'aggrave.

Questions à se poser:

http://master.emedicine.com/email/ecg/ecg37/answer.html

Mobitz type II atrioventricular (AV) block with variable conduction delay: This patient had a profound bradycardia due to a second-degree type II (Mobitz II) AV block (see Image 2). This block can be differentiated from complete heart block because a P wave precedes every QRS complex with constant PR length, unlike complete heart block in which the P waves and QRS complexes are not related. This type of block also differs from a second-degree type I block (Mobitz I block or Wenckebach pattern) because the PR distance does not change. In a Wenckebach pattern, the PR interval progressively lengthens until a QRS complex is dropped. If a 2:1 block is present, type I and type II second-degree blocks are difficult to distinguish (as with the 12-lead ECG in this case); therefore, the rhythm strip (Image 1) in this case provides further information. It shows evidence of a transition to a 3:2 block, which helps differentiate the type I and II second-degree blocks. This distinction between a Wenckebach block and Mobitz II is important because the latter, as in this case, is considered a high-degree, unstable block with the potential to progress to complete heart block.

Mobitz II mimics must also be considered and ruled out. Two possibilities are nonconducting premature atrial contractions and a physiologic block of atrial tachycardia or flutter. In the first, multiple P waves precede a QRS complex, but the morphologies of the P waves typically vary, as might the P-P intervals. In the second, the physiologic refractoriness of the AV node prevents underlying atrial tachycardia from leading to a corresponding ventricular tachycardia. The ventricular rate is likely to be rapid in a physiologic block in contrast to a Mobitz II block, in which bradycardia is expected.

In addition to the rhythm, the presumed location of the block also has implications on the patient's care. Conduction delays arise from either the AV node itself or along the His-Purkinje system. Unlike first-degree AV blocks and Mobitz I second-degree blocks, which occur in the AV node itself, Mobitz II blocks typically occur in an infranodal location, usually in the bundle branches or Purkinje system. As a consequence, QRS widening or a bundle-branch pattern is often present. In 25% of patients, the block is in the His bundle, and the QRS complexes have normal morphology and duration.

Conduction delays can occur as a result of several medical conditions. Fibrosis, sclerodegenerative changes, and ischemic disease are the most common. Medications, including digitalis, verapamil, and beta-blockers, are also common causes. Other causes are cardiomyopathies, familial disease, increased vagal tone, hyperkalemia, and many other rare conditions. Coronary ischemia must be of highest concern because of heart block secondary to infarction is associated with a high mortality rate.

The treatment for a Mobitz II block consists of atropine, isoproterenol, and pacing. However, pharmacologic treatment with atropine or isoproterenol carries a potential harm and should be used with caution. In general, these pharmacologic agents should be considered a temporizing or bridging measure to a stabilizing temporary pacing system. Additionally, these agents are contraindicated in the setting of acute ischemic disease because they can increase myocardial oxygen demand. Unlike a Wenckebach pattern or a first-degree AV nodal block in which the direct effects of atropine at the node improve conduction, a Mobitz II block may not significantly respond to atropine because of its infranodal location. In fact, atropine may paradoxically exacerbate the block by allowing additional sinus impulses to pass the AV node, increasing the refractoriness of the infranodal tissues. If no improvement is seen after a cumulative dose of 2 mg of atropine, it should be discontinued. Isoproterenol may be used as a constant infusion starting at 0.5-2 µg/min and titrated to 10 µg/min to maintain a heart rate of 60 bpm.

Finally, transcutaneous or transvenous pacing is the definitive treatment and should be initiated in symptomatic patients, and possibly even in asymptomatic patients, in anticipation of deterioration. A permanent pacemaker is placed after the patient's condition is sufficiently stabilized. A cardiologist should be consulted on an urgent basis for all patients with a Mobitz II block.

Bloc auriculo-ventriculaire du 2ème degré (BAV 2 ou Mobitz)

http://www.estem-dbd.com/ECG/glossaire/blocAV_2.htm

C'est une interruption complète de la conduction A.V. après certaines impulsions auriculaires, mais jamais après deux ou plusieurs impulsions auriculaires consécutives. Le tracé électrique montre plus d’ondes P que de complexes QRS.

Dans le Mobitz type I (ou phénomène de Wenckebach), l’intervalle PR des impulsions auriculaires conduites s’allongent progressivement. L'incrément lors d'une période de Wenckebach peut être faible, entraînant parfois le diagnostic erroné de Mobitz II. Le phénomène de Wenckebach est généralement secondaire à une atteinte du nœud AV, mais il peut plus rarement, provenir d’une lésion haute dans le réseau de His (dans tous ces cas, les QRS restent fins). Ailleurs, le blocage est infra-hisien et les QRS prennent l’aspect d’un bloc de branche droit ou gauche. Notons qu’en faveur d’un bloc intra nodal (de meilleur pronostic), le PR se raccourcit fréquemment de plus de 100 ms après l’onde P bloquée ce qui est inhabituel en cas de bloc infra nodal.

Dans le Mobitz type II, l’intervalle PR des impulsions auriculaires conduites est constant. Ce type de bloc est généralement secondaire à une lésion organique au sein du réseau de His-Purkinje (la largeur des QRS dépend là encore de la " hauteur " du siège du bloc) et son évolution naturelle se fait vers l’aggravation.

Le degré du bloc est proportionnel à la fréquence des oreillettes (chez un même malade, on pourra par exemple observer un bloc Mobitz I pour une fréquence auriculaire de 70/min et un bloc 2/1 pour une fréquence auriculaire plus élevée).