Confused about ventricular systole

[DwayneEMTP]

From “Rapid interpretation of EKG's”, Dale Dubin, MD, 6th edition, pg. 27.

“Ventricular systole (contraction) begins with the QRS and persists until the end of the T wave. So ventricular contraction (systole) spans depolarization and repolarization of the ventricles. This is a convenient physiological marker.”

I know I'm going to look like a dork when someone explains this to me, but I just can't seem to get it right in my head.

Depolarization, in my mind would by synonymous with contraction would be synonymous with systole? Right?

So it would seem to follow that repolarization is synonymous with relaxation is synonymous with diastole? At least in general terms. Perhaps there is a small difference in the exact moment of activation with each? For example, I understand that repolarization happens on the cellular level, but maybe relaxation only happens on the muscular level. So 'relaxation' wouldn't happen until all myocytes in that muscle had depolarized? My point being that reploarization and relaxation might not be exactly synonymous, but maybe close enough for this question...Maybe not...but maybe.

Where I get confused is why ventricular systole is considered to persist from the beginning of the Q wave through the T wave.

Why doesn't it, by definition, terminate at the beginning of the ST segment? Wouldn't the end of the S wave to the end of the T wave be ventricular diastole? In fact, I'm reading here that “ST segment through T wave is repolarization.”, so how do I reconcile that with the statement above?

I thought perhaps it was a typo in the above mentioned book so I looked it up in my A&P book and ventricular systole is defined the same way.

How can systole and repolarization be used together unless your comparing opposites?

Gosh, I wish I was better at expressing some things. I'm hoping you can figure out what I mean. I'm quite a ways through this amazing book and can honestly say I understand each concept presented so far completely (as complete as I can without using it) but the above paragraph is making me crazy!!

What am I missing?

Thanks all, Have a great day!!

Dwayne

[BushyFromOz]

Disclaimer...i have not done cardiology yet

Why doesn't it, by definition, terminate at the beginning of the ST segment? Wouldn't the end of the S wave to the end of the T wave be ventricular diastole? In fact, I'm reading here that “ST segment through T wave is repolarization.”, so how do I reconcile that with the statement above?

Systole from QRS to end of the T wave has rapid intra/extra cellular changes in electrolyte balances and takes in phases 0, 1,2 and 3

Diastole is during the resting state where the electrolytes are at equilibrium (resting membrane potential) and there is complete rest i.e no mechanical, electrical or electrolyte movement and constitutes phase 4

One has electrolyte movement, the other does not...at least thats what i get out of the Dubin book :?

Its most likely wrong but i thought id have a crack anyway 8)

[Dustdevil]

Depolarization, in my mind would by synonymous with contraction would be synonymous with systole?

This appears to be the root of your confusion. The electrical phenomenon and the mechanical phenomenon are not synonymous. While there is a cause-and-effect relationship between the two, they are definitely not the same thing. The electrical stimulus is followed by the mechanical response. They are not simultaneous. And, as the entire depolarisation occurs in a progressive wave, the contraction follows slightly behind in a separate wave. Remember how he explained the stimulation of the SA node flowing through the atria like waves from a pebble dropped into a pond? The same thing is happening with ventricular depolarisation. Obviously, since there are progressive waves in the pond AFTER the stimulus, then depolarisation cannot be synonymous with contraction. Consequently, repolarisation begins taking place at some parts of the cycle before other parts have yet depolarised. Therefore, both processes will be taking place simultaneously at different parts of the heart. However, the QRS represents only what the majority of the myocardium is doing at a given time, so the few cells that are already repolarising (or still depolarising) will not have their actions represented on the EKG because they are basically drowned out by the rest of the heart.

[DISCLAIMER: I just made all that up out of my head on the fly. If it is incorrect, deal with it. :? ]

[AZCEP]

Actually, not bad Dust.

Consider for a moment the action potential that is created by the movement of the electrolytes. Since you have Dubin's book, you should be able to find a picture to follow along.

Phase 0-1: Sodium and potassium are moving quickly in/out of the cell, rapidly changing the membrane potential. At the end of phase 1, the QRS is formed showing ventricular depolarization. The electrical event ends.

Phase 2: Calcium moves into the myocardial cell facilitating contraction of the muscle. Notice that phase 2 begins at, or shortly after, the peak of the QRS complex. As Dust mentioned, the electrical stimulus moves through the myocardium to initiate the mechanical contraction. If the muscle can accept the stimuli, you get contraction, and movement of the blood. If the muscle can't contract, you get an electrical event without any blood movement--PEA.

Phases 3-4: Primarily deal with the electrolytes returning to the resting state. Sodium/potassium return to their original locations, calcium slowly exits the cell, and the sodium/potassium pump returns the the cell to it's resting membrane potential of roughly -90 mV.

[678 Responding]

Haha, you guys rock. You know, i've always had an issue with the same thing before, now its crystal clear. Thanks Dwayne for posting the question, and dust, azcep for giving such a clear concise explanation. Awesome.

[Ridryder 911]

This is why I wished they would bring back the term electrical mechanical dissociation (EMD). It makes more sense the PEA. Technically this is what is occurring albeit for split seconds.... People can understand the physiology better.. good posts guys.

R/r 911

[Quicksilver]

:shock:

I just started reading about this in our new EMT-I book and until further notice I am still trying to understand it.

[DwayneEMTP]

However, the QRS represents only what the majority of the myocardium is doing at a given time, so the few cells that are already repolarising (or still depolarising) will not have their actions represented on the EKG because they are basically drowned out by the rest of the heart.

So perhaps it's just the terminology that's giving me fits. I think I'm pretty comfortable with the physiology, though I didn't express it very well...

I'm still confused as to how repolarizaton, on the EKG, can be any part of systole?

Let's say, in gross terms, that the QRS complex is showing ventricular contraction (systole).

Then the ST segment and T wave are showing relaxation (diastole).

The statement that ventricular systole lasts from the begining of the QRS complex to the end of the T wave seems to be a contradiction in terms, doesn't it?

Shouldn't there be a different term to describe the systole/diastole combination? Doesn't systole end (in gross terms) when diastole begins (assuming as dust stated, that we are dealing with the majority of the myocardium being represented on the EKG)?

I guess I'm just not understanding from a terminology perspective, (perhaps it's still a physiology perspective, but I can't see it yet) how a systolic cycle can include any part of diastole and still be called systole. Again, I'm speaking in gross terms, I understand that the de/repolorization happens in waves, so the whole muscle is not completely in either state at any given time.

I'm sorry if I'm just being a complete bonehead...but isn't this as simple as the terminology being strange? Perhaps we call it ventricular systole from QRS to the end of the T wave because of tradition? (like EKG instead of ECG)?

I know I'm missing something obvious as I can't seem to even make the question clear and have not heard it asked from other, smarter people...I just don't know what it is....

Thanks for your help...Have a great day!

Dwayne

[ERDoc]

Dwayne, I think the problem that you are having is seperating the electrical from the mechanical. They are related, but not the same. What the EKG shows you is what is happening electrically. The QRS is the depolarization. This usually causes the heart muscle to contract. The T wave is repolarization, which does nothing to the heart mechanically. Systole is the contraction of the heart, which is usually caused by depolarization. Diastole is the relaxation of the heart muscle, not caused by any electrical phenomenon. Repolarization is just the electrolytes that were mixed up during depolarization resetting themselves. I said "usually" previously because as others have said, you can have electrical activity without muscular activity (PEA or EMD). You will see a normal looking EKG (depolarization and repolarization) but there will be no mechanical activity (no systole or diastole, aka asystole).

[DwayneEMTP]

Yeah, thanks Doc, I guess I understood the difference, but with them being so closely related (normaly) I tended to think of them together.

But I still have the issue with this sentence:

"So ventricular contraction (systole) spans depolarization and repolarization of the ventricles."

Wouldn't contraction (systole) cease to be present upon repolarization? Or maybe repolarization is normaly considered part of the systolic cycle? Though I couldn't find any reference to systole that referenced this.

Thanks for your help Doc...

Dwayne