Dromotropy and chronotropy

[blueboyscholar]

Is it possible to have positive dromotropy without a resultant positive chronotropy? They seem like one and the same to me...

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[mastamark]

Is it possible to have positive dromotropy without a resultant positive chronotropy? They seem like one and the same to me...

Most definitely. If you're considering myocardium, chronotrophy is the rate of the muscle contraction(HR). Dromotrophy is just the conduction/rhythm(through AV and beyond). Think of PEA as an example.

 

[mastamark]

Also, you can give someone something like atropine which is positively dromotrophic during PEA and HR will not increase, but it's possible to see increase electrical activity. Brady-->SR in PEA, with no return of mechanical activity. I have seen it.

Dromotrophic agents for the most part just increase the electrical conduction. The main controller is the vagus at the SA node. If you give an anti-cholinergic like atropine you basically tell the SA to back off and you have + dromotrophy. That doesn't mean the myocardial muscle will actually respond to the change. Usually it will, but there are those weird times. This is why they say never treat the ECG monitor but the patient.

Hope that helps.

 

[blueboyscholar]

thanks for the reply...it seems to me that if you're increasing conduction velocity, then you will automatically get a faster heart rate (chronotropy), since the conduction velocity is the main determinant of heart rate. If you slow conduction through the AV node, this will slow heart rate. If you increase conduction velocity through the AV node, then there is less time in between contractions = faster heart rate. I'm so confused😕

 

[mastamark]

thanks for the reply...it seems to me that if you're increasing conduction velocity, then you will automatically get a faster heart rate (chronotropy), since the conduction velocity is the main determinant of heart rate. If you slow conduction through the AV node, this will slow heart rate. If you increase conduction velocity through the AV node, then there is less time in between contractions = faster heart rate. I'm so confused😕

If you slow conduction through AV node, you will most definitely have a decrease in HR. Because the intrinisic rate below AV node is slower than that above it. So you are right, you will have a decrease when you block AV node.

It's a little different if you increase the dromotrophic conduction, you will usually see a response, but there are exceptions. Pulseless Electrical activity(PEA) is a prime example. In this case, there is electrical activity flowing through and it will show up on the monitor, but the muscle doesn't respond to it. I have seen a patient in PEA a while ago get atropine, the monitor showed an increase in rate, but they were still pulseless, no heart rate at all, let alone an increase.

I guess this is due to the fact that atropine's biggest affect is at the SA so the impulse was transmitted more rapidly. Unfortunately, whatever injury was done to the heart itself wouldn't respond.

I guess you could see this even when a person does have a pulse. I haven't heard of it, but I guess it's theoretically possible. The muscle conduction is merely coupled to the electrical activity. The electricity could still occur without a response from heart, but I don't think you will ever see a response from the myocardium without the electricity. You may have autoconduction by lower pacemakers(purkinje fibers et al), but its not a very effective amount, and it's rate will be very low.

Think of Dromotrophy and Chronotrophy as coupled mechanisms, with Chronotrophy being directly dependent on Dromotrophic input.

 

[blueboyscholar]

Thanks for taking the time to reply so thoroughly brother!

 

[mastamark]

Thanks for taking the time to reply so thoroughly brother!

No problem. Hope it helped a little, it's little stuff like that that gets annoying cuz it doesn't make sense intuitively.

Good luck and rock the step.