Читать книгу Practical Cardiovascular Medicine - Elias B. Hanna - Страница 142

Approximately 2–8% of STEMI patients develop some form of new intraventricular block, LAFB being the most common block. Note the following arterial supply:¹⁵⁴

 The right bundle mainly has a single arterial supply from the LAD (first septal branch).

 The left anterior fascicle has a single arterial supply from the LAD (first septal branch).

 The His bundle, the main left bundle, and the posterior fascicle have a dual supply from the LAD septal branches and the AV nodal artery. Thus, the composite of the anterior and posterior fascicles, the branching left bundle, usually has a dual supply. This explains why it is difficult to infarct the left bundle and why most new LBBBs are non-ischemic in nature.

RBBB or LAFB are most commonly seen in anterior MI,¹⁵⁵ but may also be seen in inferior MI if the LAD has severe disease and is dependent on the RCA for collaterals. LBBB may result from either anterior or inferior MI, and is more likely seen when both RCA and LAD are com- promised, with one acutely occluded and the other chronically obstructed (in the GUSTO trial, LBBB was associated with an RCA culprit at least as much as an LAD culprit).^10,156

The conduction system is more resistant to ischemia than the myocardium, as the myocardial cells require much more O₂ for their continuous mechanical work than the electrical cells, which also frequently receive dual or collateral supply. This explains why conduction blocks are frequently due to edema or ischemia rather than necrosis and are frequently reversible (25-75% of the cases).^155,156 If not reversible, and if secondary to MI rather than degenerative disease, the myocardial injury is usually quite extensive (e.g., persistent RBBB or LBBB).

While a chronic bundle branch block (BBB) has a very low risk of progression to complete AV block, 20% of acute BBBs progress to complete AV block, and 25–40% of acute bifascicular blocks progress to complete AV block.

Beside the risk of progressing to complete AV block, a new BBB is independently associated with a two- to threefold increase in in-hospital mortality, HF, and VF, particularly because it correlates with a more extensive infarction (mortality 18% vs. 11% in GUSTO-I trial; 35–50% in the pre-reperfusion era).¹⁵⁶ Up to 75% of these blocks are transient, and transient blocks do not portend any increase in mortality.^155,156 Old BBBs do not portend any increase in mortality either. Both RBBB and LBBB are associated with the same increase in mortality. ^155,156

A standby temporary transcutaneous or transvenous pacemaker is indicated for a new BBB or bifascicular block occurring in anterior MI.