They are usually the result of the ischemic insult on the sinus node and / or the conduction system or reflexes mediated by the vague or both causes.In 55% of patients the right coronary artery supplies blood to the sinus node, whereas the circumflex branch of the left coronary vasculature supplies the remaining 45% of the patients. Spraying of the atrioventricular node is given by the distal branches of the right coronary artery in 90% of the subjects and by the distal part of the circumflex branch of the left coronary artery in 10%. The right branch receives blood from the piercing septal branches of the anterior descending artery, as well as the distal portion of the anterior fascicle of the left branch.
The common part of the left branch, instead, has a double vascularization, from the distal branches of the right coronary and from the proximal vessels of the circumflex branch, while the posterior fascicle of the left branch is sprayed by branches of the circumflex artery. In general, conduction disturbances associated with a lower-posterior infarction are attributable to vagal reflex hyperactivity, tend to be transient and responsive to atropine, and show a prognosis that is usually more favorable than those in the course of anterior infarction. On the other hand, the major conduction disorders related to anterior infarction see the involvement of deep septal necrosis and are associated with significant contractile LV reduction.
Sinus bradycardia and sinus pauses are usually benign. An atrioventricular block (BAV) of grade I is found in 4-13% of heart attacks. In such cases clinical observation and abstention from the use of drugs capable of prolonging AV conduction are recommended.
In the first 24 hours a heart attack can occur in 3-10% of subjects. In the second grade BAV of type I, the progressive extension of the PR interval is observed. It is often observed during inferior-posterior infarction and usually responds well to atropine. The QRS is narrow and there is no need for temporary stimulation, unless the frequency drops below 45 bpm or this leads to perfusion insufficiency. The II grade BAV of type II is characterized by the intermittent loss of a beat in the absence of progressive elongation of the PR interval; it is linked to an extensive ischemic suffering of the infranodal conduction system. The QRS may present frequently enlarged, indicating an associated branch block; progression to a complete block affects about one third of these patients. The majority of patients with anterior IMA and type II II BAV require the placement of a temporary stimulator for the non-predictive full-block evolution risk.
It occurs in 3-12% of patients with IMA. In general, patients with
inferior-posterior infarction reach the complete block after a period of
blockade II and can still show to some extent a responsiveness to treatment with
atropine or aminophylline. A junctional escapement rhythm is often present and
the normalization of the picture can take place in 3-7 days. The occurrence of a
complete block in a patient with a lower heart attack involves an increase in
the risk of in-hospital mortality of 1.54 times per se. Consequently, these
patients should be under constant observation, with prior electrode application
for possible transcutaneous pacing, or undergo prior implantation of a temporary
stimulator catheter. An eventual complete blockade during anterior infarction
usually indicates an extensive area of necrosis and leads to an unfavorable
prognosis. Most of these patients require temporary stimulation and some doctors
proceed directly to a definitive stimulator implant. In these patients,
long-term mortality is attributed more to the deterioration of contractile
capacity or to ventricular fibrillation than to a persistent advanced grade
block.
The appearance of a branch block not previously known in a patient during IMA should lead to the presumption of a large extension of the infarct with a consequent high risk of complications. The blockade of only one dossier, especially the left anterior emiblock, occurs in about 5% of patients and has a relatively benign prognosis. A complete right bundle branch block (BBD)) both left (BBS) occurs in 10-15% of patients; more frequently, in two thirds of cases, it is BBD.
Both BBD and BBS is associated with higher mortality both in hospital and long ternine.111 In the past, the sudden appearance of a BBD or BBS saw the indication to temporary stimulation. The definitive pacemaker implant is indicated in patients with complete or persistent advanced blockade, or with BAV of Persistent grade after infarct and in patients with newly developed branch block and a complete block transient during the acute phase of infarction. . In some cases an electrophysiological study may be necessary to determine the site of the block and verify the need for definitive stimulation. Pacemaker implantation may be rarely reported in patients with profound sinus dysfunction. This condition, however, rarely occurs with myocardial infarction.
They recommend:
Class I
I. Definitive ventricular pacing is indicated in cases of BAVdi II persistent
degree in the His-Purkinje system with bilateral branch bundle or BAV of III
degree in His-Purkinje system or downstream after infarct. (Level of Evidence:
B)
2, Definitive ventricular pacing is indicated in the case of transient AV
blockade of the I or III intranodal degree with associated bundle branch block.
If the site can not be defined with certainty, an electrophysiological study may
be necessary. (Level of Evidence: B)
3. Definitive ventricular pacing is indicated in the case of persistent and
symptomatic AV blocks of II and III. (Level of Evidence: C)
Class II b
Definitive ventricular pacing may be considered in the case of persistent II and
III AV blocks at the atrioventricular node level. (Level of Evidence: B)
Class III
Definitive ventricular pacing is not recommended in the case of transient AV
blockade in the absence of intraventricular conduction disturbances. (Level of
Evidence: B)
2. Definitive ventricular pacing is not recommended for transient AV blockade in
the presence of isolated left anterior hemiblock. (Level of Evidence: B)
3. Definitive ventricular pacing is not recommended for left anterior hemixis
acquired in the absence of AV block (Level of Evidence: B)
4. Definitive ventricular pacing is not recommended in the case of BAVi 1 degree
in the presence of known or non-datable branch block. (Level of Evidence: B)