part 2.pdf

3) Conductivity All the cardiac muscle cells are conductive but at different rates. In general, conduction rate is decreased by vagal stimulation and increased by sympathetic stimulation. 1. SAN conduction: Very slow conduction (= 0.05 m/sec.). So, preventing any ectopic focus to depol...

3) Conductivity All the cardiac muscle cells are conductive but at different rates. In general, conduction rate is decreased by vagal stimulation and increased by sympathetic stimulation. 1. SAN conduction: Very slow conduction (= 0.05 m/sec.). So, preventing any ectopic focus to depolarize it. èa protective mechanism to keep or restore the normal pacemaker. 2. Atrial conduction of excitation wave (EW): EW spreads rapidly through atria converging on AVN. This is completed in 0.1 sec. A) Specialized atrial pathways: are freeways with rapid conduction rate (1 m/ sec) and are composed of: - Interatrial pathway: from RA near the SAN to LA near the AVN. - Internodal pathways: anterior, middle & posterior bundles from RA near the SAN to AVN. B) Atrial muscle conduction: cell-to-cell conduction at a slow rate (0.3 m / sec). 3. AVN Conduction: It has the slowest conduction rate in the heart (0.02 – 0.05 m / sec). It has 3 functional divisions: *A-N zone: with maximal delay because of very long path length. * N zone: which has the slowest conduction velocity. *N-H zone. Characteristic functions of AVN: 1. A-V nodal delay (100 - 150 msec) between atria and ventricles. 2. A-V Nodal block (Refractoriness): the refractory period protects ventricles against high atrial rhythm. The refractory period does not allow except 180 - 200 bpm to pass from atria to ventricles in case of high atrial rhythms. 3. Decreased chance for retrograde direction in conduction. This protects the SAN from a ventricular ectopic focus. 4. Ventricular Conduction (A-V bundle, bundle branches & Purkinje fibers): through the conducting pathway His bundle which divides into right and left bundle branches (conduction rate of main bundle & branches is 1-2 m/sec). Each bundle ends by small fibers, Purkinje fibers which have the maximum conduction velocity (4-5 m/sec) to ensure synchronous activation of both ventricles. The final ramifications of Purkinje fibers go rather perpendicular from the endocardial surface to reach to about 2/3 of the thickness of ventricular muscle. 5. Ventricular myocardial Conduction: The rest of ventricular muscle thickness is activated by the slow cell-to-cell conduction ( 0.3 m/sec). Conduction through left wall is slower than right wall due to more thickness of its wall. First part of the ventricle to be excited is the mid portion of ventricular septum on the left side. Last part is the epicardial surface at the base of left ventricle.