February 22, 2008

Impulse Propagation In The Heart

spread of excitation of cardiac impulse from SA node;its relationship with the ECG: an animationIn the previous article, we discussed how the pacemaker of our heart, the SA node, was generating the rhythmic impulses. For the impulse to have any effect, it has to be travel to various parts of the heart, which will then contract as a result.

After its generation, the impulse goes to the AtrioVentricular Node. It supplies both the atria, while on their way to the AV Node. The sojourn of impulse, between the SA and the AV node, is via the internodal pathway.

There is a delay of about 0.09 second in the AV node itself. The anatomy and physiology of the AV node is responsible for it. It is said that the nodal cells have fewer gap junctions among themselves, making ionic flow highly resistive. Nevertheless, this delay serves a useful purpose. As seen in the animation, the atria contract when they are stimulated and fill the ventricles with blood. Were it not for the delay, the ventricles would fire near simultaneously, without their quota of blood from the atria, and thus producing hemodynamically ineffective cardiac output. It gives the ventricle vital time to fill. Since the junction between the atria and ventricles is electrically non-conducting (due to the presence of a fibrous partition between the two chambers of the heart. The conduction pathway is like electrical cabling between 2 floors of a building), the impulse must find a route to pass through. It has only one way, in normal physiology, through which it can travel: through the penetrating portion of the AV bundle.

So, from here onwards, the impulse is carried via the AV bundle, down the interventricular septum. The AV bundle then bifurcates into right and left bundle branches, which traverse along right and left ventricle respectively. From there, Purkinje fibres arise which ultimately supplies the ventricular muscles.

As the impulse travels down the septum, it reaches the apex of the heart, and supplies the ventricular muscles. Next, the impulse travels along the ventricular musculature upwards towards the atrioventricular septum, the top portion of the inter ventricular septum and the posterobasal portion of the heart. These are the areas to be depolarized last. While doing this part of the journey, the impulse also spreads from the endocardium to the epicardium; that is they travel from the inside of the heart to the outside. All these can be seen in the adjoining picture/animation. This animation also portrays how the ECG would be like, with the spread of the wavefront. The state of the heart valves can also be seen during systole and diastole, i.e. during the contraction/relaxation of the cardiac chambers.

The conduction system comprising of internodal tracts, AV bundle, His-Purkinje fibers are all specialized muscle cells, that carry electricity at high speeds. Also, all these depolarizations are followed by repolarizations, making the cells ready for the next impulse. The action potential characteristic of cardiac muscles are different from that of the pacemaker cells.

In some idiopathic degenerative diseases, the fibrous skeleton of the heart may be calcified and sclerosed, as in Lev's disease, or there may be sclerodegenerative changes within the conducting system sparing the myocardium or the fibrous skeleton, as in Lenegre's disease. Here the 'electric cable connecting the first and the second floor of the heart' (i.e. the piercing AV bundle); is naturally snapped (due to the interposition of non conductive sclerosed material), leading to AV block and bradycardia of varying degree.

PS: Click on the animation if it doesn't animate on its own.

Last modified: Mar 20, 2009
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