IRISH TIMES HEALTH
Friday 21 September 2012
UNRAVELLING CARDIAC CONDUCTION ABNORMALITIES
Left, bundle branch block (LBBB) is a high-risk ECG pattern. Prof William Brady considered a case involving a pre-existing pattern – where a patient had probably experienced some form of significant cardiovascular insult of the left ventricle in the past. There is an extreme risk of a bad outcome; heart block, ventricular dysrhythmias, cardiogenic shock and death. Such patients have very large infarctions and have a lot to gain from being treated early.
An LBBB infarct diagnosis based on the ECG is difficult and the baseline ST segments and T waves tend to be shifted in a discordant direction with LBBB, which can mask or mimic acute myocardial infarction (AMI). A new LBBB is always pathological and can be a sign of myocardial infarction (MI).
Traditionally, it had been thought that as long as the left bundle block was pre-existing, little else could be concluded from the ECGT, in terms of MI diagnosis. A paper by Sgarbossa and colleagues (NEJM, 1996) challenged this dogma. In the Sgarbossa paper, the T wave was felt to be much less important in an acuate coronary syndrome (ACS) diagnosis. An ST depression with an upright T wave can be subtle and missed, unless the EDG is closely studied. Among LBBB pattern patients who are experiencing M1, 20 per cent will demonstrate abnormalities.
Acute MI
Ten per cent of patients with acute MI will present with a right bundle branch block (RBBB) pattern that is either new or pre-existing. RBBB is also a high risk pattern and patient outcomes are similar to LBBB in terms of death and acute cardiovascular issues. In the setting of a new RBBB, they fare less well than the LBBB patients. This pattern does not hinder infarction diagnosis from an electrophysiological point of view but it can present obstacles.
As with LBBB, the left ventricular hypertrophy (LVH) pattern can reduce ability to detect ACS and can also mimic ACS. Caution is advised in relation to the high-risk (in terms of outcome) LVH pattern. Brugada syndrome has a high association with sudden cardiac death and patients will present with syncope and often dysrhythmias – palpitations, etc. They will demonstrate a right bundle branch block pattern, which is either complete or incomplete, and there will be ST segment elevation in leads Vi and V2.
There are two forms of ST elevation, convex and saddle, and there is a possibility of sudden cardiac death. These patients tend to develop unpredictable episodes of polymorphic ventricular tachycardia (V-tach), which can be self-terminating, it may progress to ventricular fibrillation and full cardiac arrest. Ten percent of these patients will be in cardiac arrest or, if self-terminating, with some variant of syncope or complications. If it is asymptomatically discovered without any clinical correlation for dysrhythmia, patients should be referred promptly for cardiac evaluation.
If the patient presents with dysrhythmia – either obvious or with some symptom that could be linked to the pattern – he or she needs to be admitted to the hospital under the care of a cardiologist.
The natural history of this syndrome is malignant dysrhythmia. In the case of cardiac arrest, resuscitation in the standard fashion is indicated.
Placement in an Automatic Implantable Cardioverter-Defibrillator (AICD) may be a life-saving procedure.