![]() While atropine is unlikely to cause harm, its lack of efficacy in patients with an acute bradycardia resulting from a trifascicular block means that its use should be avoided in favour of cardiac pacing . In the case of a complete trifascicular block (or an incomplete block such as a Mobitz II), the blocking lesion is distal to the AV node, and atropine in unlikely to have any effect. They reduce the vagal nerve’s inhibition of the AV node, and increase the SA nodal rate. This medication can decrease AV-node blockade (and hence increase heart rate) by decreasing vagal tone. įigure 14: Complete trifascicular block – RBBB + LASFB + A-V dissociation Treatment Considerations ChemicalĪs laid out in ACLS guidelines, many symptomatic bradycardias are treated with atropine. If the patient has experienced symptoms, such as syncope, they should be paced asymptomatic individuals need not be paced. Bifascicular blocks that coexist with a 1 st degree AV node block are complicated and should be seen by cardiology. Similarly, a fixed RBBB with separate ECGs demonstrating disease of both the LASF and LPIF is considered an alternating trifascicular block, and is also a Class I indication for permanent pacing . This (infra-His) block often presents as a bifascicular block with a history of syncope, indicating transient complete block, and is a Class I indication for permanent pacing. In the case where the sole remaining fascicle is unhealthy or partially damaged, a Mobitz II block can occur this is called an incomplete trifascicular block. These are normally well-tolerated and do not require treatment beyond avoiding AV-node-blocking medications if asymptomatic. The three possibilities are a LASFB and a RBBB, a LPIFB and a RBBB, or a left bundle branch block (as this is impairment of both the LPIF and LASF) . Isolated monofascicular blocks are benign and require no treatment .Ī bifascicular block occurs when two fascicles are impaired. ![]() This includes solitary LASFB, LPIFB, and right bundle branch block. Monofascicular blocks are the syndromes described above in isolation a single fascicle is damaged, but there is still conduction from the atria to the ventricles. Ventricular conduction blocks can be described in terms of the number of fascicles that are affected: monofascicular, bifascicular, or trifascicular. Since the lateral leads show the opposite morphology, this gives you both patterns!įor example, a left anterior superior fascicular block will produce an ECG with an initially positive ( superior) deflection in the inferior leads: an rS pattern (Figure 9).įigure 12: LPIFB results in an axis in the inferior quadrant Multifascicular Blocks This makes anatomical sense, as the anterior surface of the heart is superior to the posterior surface, but the reason for the renaming is mostly to set up a useful mnemonic: the direction of the initial deflection in the inferior ECG leads is the same as the name of the block. You may have noticed the emphasis on superior and inferior in the terms Left Anterior Superior Fascicle and Left Posterior Inferior Fascicle. LPIFB cannot be diagnosed until other causes of right-axis deviation are ruled out. There are other diseases that cause right-axis deviation, such as right ventricular hypertrophy, pulmonary embolism, COPD, and Wolff-Parkinson-White syndrome. Depolarization reaches right side of heart more slowly - Prolonged R-wave peak time in aVR.Depolarization in a ‘rightward’ direction - Right-axis deviation.Endo-to-epicardial lateral wall depolarization - qR pattern in the inferior leads, rS pattern in the lateral leads.Since, in the normal heart, there is more myocardial mass to depolarize in the left ventricle than in the right, the cardiac axis on ECG ends up pointing left, between 0 and +90 degrees. Finally, depolarization spreads back up to the walls of the ventricles (Figure 2) . The depolarization then spreads down toward the apex of the heart. ![]() In the normal heart, net depolarization begins at the septum and proceeds left-to-right across this structure. In reality, depolarization starts in many places simultaneously, and these individual depolarization waves sum together to form a net left-to-right septal vector . The term cardiac axis refers to the net cardiac vector, which is the summation of all of the tiny vectors that make up the cardiac depolarization wave. Normal Ventricular Depolarization and the Cardiac Axis
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