Heart Block or Conduction Delay (Left Anterior) in Dogs

Left anterior fascucular block (LAFB) is a conduction delay in the electrical system of the heart. The normal conduction system of the heart is comprised of sinoatrial (SA) node, atrioventricular (AV) node, the bundle of HIS, the left and right bundle branches and the purkinje fibres.

The SA is the pacemaker or impulse initiator of the heart. It starts the impulse which stimulates the atria and travels to the AV node. From there it passes slowly to the Bundle of HIS to allow atria time to contract and pump the blood into ventricles. Through bundle of HIS (a bundle of modified heart muscle that transmits the cardiac impulse from the atrioventricular node to the ventricles causing them to contract), the impulse travels to the apex of left and right ventricles through bundle branches while on the left side of the heart it travels through the Left Anterior fascicle and the left posterior fascicle. In a LAF Block the conduction through this section is delayed, and the impulse gets to the posterior fascicle first whereas normally the impulse goes to the LAF first. when this impluse is delayed there, it causes a delay in the activation of the anterior and lateral walls of the left ventricle.

This affects not only contraction of the heart muscles but also timing and frequency of the heart beat as well.  Left anterior fascicular block is mostly an incidental finding since it does not have any specific symptoms. Rather, symptoms are assoicated with underlying causes.

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