Why is the PR interval flat?

[tsip]

A friend asked me this question and i honestly could not answer it. I was taught that the EKG supposedly measures electric potential differences between the nodes. So, at the end of the P wave, the atria are all fully depolarized, and the ventricles are still polarized. But if you look at, for example, lead II, it becomes flat after the P wave. There is still a potential difference between the Right Arm lead, and the Left Foot lead, since the atria are depolarized and the ventricles are still polarized. So, why is it flat after the P wave?
The best answer i could come up with is that the EKG doesn't actually measure potential differences but the CURRENT through extracellular fluid that is caused by those potential differences. Since the AV node let's such little current flow through it, we just see a flat line after the P wave until we get to the QRS complex.
Anyone have a more complete explanation? Thanks

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[sylhet]

Immediately after P wave i.e. atrial depolarization starts AV nodal depolarization. Point to be noted that, ventricular depolarization doesnt start that immediately after Atrial depolarization. It must go through av nodal depolarization first. If AV node gets depolarized then why doesn't it have any positive deflection rather it has a flat line on ECG paper.

Any deflection wave on ecg paper depends on velocity of the vector, amount of current flow. We all know AV node is potentially a very very slow conductor. When current flows flows through AV node its very little and most importantly the velocity is very low and thats the reason ECG machine cant pick up that current. And so, the pointer of ecg machine doesn't go up, doesnt show any deflection wave rather just a flat line as if nothing happened in between atrial and ventricular depolarization.

 

[Militarydoc1]

A friend asked me this question and i honestly could not answer it. I was taught that the EKG supposedly measures electric potential differences between the nodes. So, at the end of the P wave, the atria are all fully depolarized, and the ventricles are still polarized. But if you look at, for example, lead II, it becomes flat after the P wave. There is still a potential difference between the Right Arm lead, and the Left Foot lead, since the atria are depolarized and the ventricles are still polarized. So, why is it flat after the P wave?
The best answer i could come up with is that the EKG doesn't actually measure potential differences but the CURRENT through extracellular fluid that is caused by those potential differences. Since the AV node let's such little current flow through it, we just see a flat line after the P wave until we get to the QRS complex.
Anyone have a more complete explanation? Thanks

Technically, the EKG does measure voltage = potential difference. The Y axis is measured in mVolts, not amp (unit of current). With that said, if you recall from physics V = IR, voltage is directly proportional to current. The flat line you are referring to is the isoelectric line and in the absence of current due to delay in AV node, voltage will be 0. Or another way to look at it: no deflection since there is no depolarization towards/away from either lead during AV conduction delay. This is how I have always understood it.

-aspiring cardiologist

 

[tsip]

That's a good point; however the V=IR thing could still work with a high V and extremely small I. You just need the Resistance to be increased, which is exactly what is happening at the AV node. The high potential difference is still there, it's just that the Resistance has increased, which is why the current flow has decreased. I am under the assumption that voltage differences cause current to flow, just like pressure differences cause fluid to flow, and both flows are modulated by the amount of resistance there is.