Summary of Cardiac conduction velocity
Transcript for Cardiac conduction velocity
Cardiac conduction velocity
Cardiac conduction velocity is the velocity at which a depolarization wave moves through the myocardium, the muscular middle layer of the heart, and it’s measured in meters per second. The depolarization wave travels through the sinoatrial node, or SA node, through both atria, down the atrioventricular or AV node, through the Bundle of His and the Purkinje fibers, and finally to all of the parts of the ventricles, all in about 220 milliseconds, which is less than a quarter of a second!
If we zoom in on the myocardium, the depolarization waves move across neighboring cells. It moves from one cell to the next when ions like calcium and sodium slip through gap junctions and trigger voltage-gated sodium channels in that cell over to open up, allowing a rush of more sodium into the cell and causing an action potential to occur. That then results in more sodium and calcium leaking through to the next cell, triggering an action potential, which goes on to the next, and so on. Ultimately these cellular processes determines how fast or slow a depolarization wave will move across different types of tissues. More sodium channels and gap junctions speed up the depolarization wave, Fewer gap junctions and fewer sodium channels slow down the depolarization wave.
Alright so let’s break down the conduction velocities in the different parts of the heart, starting at the SA node,i the depolarization wave moves through the myocytes in the atria at about 1 meter per second, then goes through the AV node really slowly, roughly between 0.01 and 0.05 meters per second. Which means that even though this is a short segment, it takes a long time, in fact, out of the 220 milliseconds for the entire journey of the depolarization wave, about 100 milliseconds, or nearly half of the time, is spent in the AV node. That dramatic slowdown allows the atria to contract and push blood into the ventricles while the ventricles are still relaxed. After that, the conduction velocity in the His and purkinje fibers of the ventricles goes back up to 2-4 meters per second, which is the fastest in the heart. That allows the ventricle to contract all at once in a really coordinated way, so that the blood gets pushed out rather than sloshing from one part of the ventricle to the other. The depolarization wave starts at roughly the same time throughout the ventricles and then goes a bit more slowly - about 1 meter per second - through the ventricular myocytes.