![]() We wouldn’t want them to collapse during systole!īut why should the impulse slow down? Why do we have a PR interval? It’s to allow time for ventricular filling! Atrial systole is a end-diastolic event (the “atrial kick”) and without the delay we would not have the normal “lub dub” associated with normal cardiac function. Why does the heart have a fibrous skeleton? In the first place it gives the heart structure and provides form, shape, and stability to the AV valves. It’s worth taking a moment to think about why this should be so. The only legitimate electrical connection between the atria and ventricles is through the AV node. It should be noted that the fibrous skeleton of the heart is made of collagen (the most common structural animal protein in nature) and for the purposes of cardiac conduction it is electrically inert and insulates the ventricles from the atria. The impulse then passes through the AV node where it is slowed down and is conducted through the fibrous skeleton of the heart to the AV bundle (bundle of His) which corresponds to the PR interval on the surface ECG. The normal cardiac impulse (or depolarization wavefront) starts in the sinus node and is conducted over to the left atrium across preferred pathways known as the interatrial tract or “Bachmann’s bundle” which depolarizes the atria and corresponds to the P-wave on the surface ECG. Let’s start with a review of normal cardiac conduction (Example 1). On the left (Example 1) we see normal conduction and on the right (Example 2) we see the “WPW pattern” or ventricular pre-excitation of the ventricles across an accessory pathway. This “WPW pattern” or delta wave is visible on the resting ECG in the absence of a pre-excitation-dependent tachycardia. He also has his own blog over at My Variables Only Have 6 Letters. These amazing images were created by Christopher Watford ( who is a Sr. ![]() Normal conduction (Example 1) and conduction with an accessory pathway (Example 2) ![]()
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