Let’s get clinical: Can you do CPR too fast? The importance of Ventricular Fill Times.

A recent chat with some of our new grads the other day following an arrest we did posed the question: Can you do CPR too fast?

Surely if you can push faster you should right?

Well perhaps not so much.

To explain we’ll look at a bradycardic male. Let’s say he’s 60 years old, moderate size and weight, with a heart rate of 45 and a blood pressure of 160/95.

Obviously there’s lots of factors at play, but how can he have a blood pressure so high with a heart rate so low?

The answer is ventricular fill time.

Thanks to Frank-Starlings Law, the more blood that fills the ventricle, the more blood that is pushed out the other end (to a point anyway). See a long time between beats (such as in a slow heart rate), means a long time for those chambers in the heart to fill up, and stretch the ventricle walls. This means that when those full, stretched ventricles push that blood out, they can do so with a lot of force (think rubber bands), and there’s a lot of blood to be pushed out. The result is that you have a relatively high stroke volume (amount of blood pushed out at the end).

See a large slow pump pushing out lots of liquid slowly, can produce just as much output (if not more) than a faster pump pushing small amounts quickly.

Now imagine if our heart is pounding along at at 150 BPM. That’s not giving the ventricles much time to fill up is it? Sure a healthy heart with lots of response from your vascular system can manage fairly well for a while.

But what about that person in cardiac arrest? Their heart’s probably not so healthy (even before we had to start doing CPR) and I certainly doubt their body will have many helpful reflexes to assist in getting adequate blood flow to the brain and other vital organs now that they’re technically dead.

So for us, 100 BPM is a good rate for CPR. It’s our sweet spot: slow enough that the ventricles have sufficient time to fill up…. but fast enough that the pressure we’re generating in the chest and blood vessels doesn’t disappear between compressions.

But why doesn’t our heart always beat that fast?

Because it doesn’t have to: as eluded to earlier our body has other reflexes in our vascular system which keep the pressures up high enough that our heart can work a little easier.

So can you perform CPR too fast? Yes you can!




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