Investigating the effects of cardiovascular stress: Simulating heart failure

Now, let’s mimic what happens with heart failure. So once more, top off your measuring jug to 1,000 millilitres. This time, we’re going to hold the pump differently. Instead of using all three fingers to hold the pump, you’re only going to use the middle finger. So not the outside two. This mimics a heart that’s been damaged. So once more, we’re going to transfer 300 millilitres from the measuring jug to our container as fast as you possibly can, but this time only using one finger to control the pump. Reset the timer. And then when you’re ready, we can begin.

So to start with, you may find your about the same speed as you were with three fingers. You can pump quite quickly. But quite rapidly, you’ll find that the arm starts to tire and you slow down. In fact, you may find that you don’t even get through all 300 millilitres.

Keep an eye on the time.

Pause there. Make a note of the first time.

And then carrying on once more.

So our heart that we’re mimicking that’s got heart failure, it’s got some damage to it. It’s going to start struggling. It can’t keep up with this fast rate. So you find that the heart slows down. And that’s fine. Do try to keep going if you can, but try not to stop completely.

You may also find that you start cheating. You find that you’re either pushing down on the pump on the bottom of the floor. You may find that the other fingers are actually trying to hook on and help. Or before you know it, you may actually have swapped hands without even realising it. Try not to cheat if you can, but instead, slow down as much as you need to.

So we’re about halfway through here. So slow and steady.

Keep an eye on the time. Keep an eye on the volumes.

Let’s note down that second time.

So our third and final one then.

By now, you’re probably pumping much more slowly because the muscles in your arm are tireder. Remember, if you feel any pain to stop immediately.

You may also find that not only are you pumping more slowly as the arm tires, but perhaps other things are happening as well. Perhaps there’s a little break and then you have a go at a few fast ones. And then you take a break. And then a few fast ones. And then you go back to that slow rhythm again, still trying not to cheat with the other fingers if you can help it.

About halfway. So again, these things mimic what can happen in a heart that’s really struggling. It can start to beat more irregularly.

Eventually, if the heart’s unable to provide sufficient amounts of blood and oxygen to the body, of course, the body’s tissues are starved of oxygen and they die.

Nearly there.

There we go. Give that hand a bit of a shakeout. And then let’s make a note of the time. So that last time 3 minutes 15. So again, we need to convert the values from minutes into seconds. So we’ve got 1 minute 49.

That’s 109 seconds. 3 minutes 15 so that’s 180, 195. Now, we calculate the transfer time, the time each 100 millilitres took.

The first one as is. It’s 31 seconds. The second one, you take the elapsed time and we subtract the time before.

Again, we take the elapsed time. We subtract the one before and subtract the one above. So we can see the transfer time has increased with each progression. And that’s as we started to tire or as the heart started to tire. It got slower and slower. Again, we don’t need to calculate the stroke volume because it’s fixed in our mini heart, but we’ll calculate the cardiac output again. So if you remember, we take 100 millilitres. We divide it by the transfer time. And we multiply it by 60.

Let’s do that again for the second reading. 100 millilitres divided by the transfer time multiplied by 60.

And the final 100 millilitres divided by the transfer time times by 60.

So again, the cardiac output has decreased with each repetition as the heart, our mini heart, got more and more fatigued. Let’s convert it to a percentage once again so that we can compare to each other. We take the cardiac output. We divide it by the average baseline value. And we multiply it by 100 to convert to a percentage.

Same again for the second one. So we take our cardiac output. Divide it by the baseline reading. Multiply it by 100. And then the third time, our cardiac output divided by the average baseline times by 100. So we can see from our values here that we started off with a cardiac output of 180%. In other words, when we were pumping fast, we were supplying quite large amounts of blood and oxygen to the body’s tissues. So increasing the heart rate was fine to begin with. The problem with heart failure is we can’t sustain it. The heart starts to tire. And we can see that gradually the percent cardiac output has reduced, coming down below 100%.

If that percent cardiac output drops too low and is unable to provide sufficient amounts of blood and oxygen to the body, then, unfortunately, death occurs. Now the heart undergoes some amazing adaptations during heart failure, similar to the exercising heart. To begin with, we can increase the heart rate. By increasing the heart rate, we can boost the cardiac output. But as we’ve seen, we can’t sustain that for too long a period. With the real heart that can change its stroke volume, during heart failure some of the same adaptations occurs with exercise. The first adaptation, that increasing the strength, unfortunately, the heart usually can’t carry that out because it’s been damaged.

But the second one where it increased the stretch, how big the heart could expand before the contraction, a heart during heart failure does do. And it expands bigger and bigger. The third option, which is to reduce the resistance to the blood coming out of the heart by dilating the arteries and reducing the blood pressure is another strategy that the body takes with heart failure. We can see these amazing adaptations enable the heart to carry on its function for as long as possible. Eventually though, we see some of the other changes that we saw here, such as the erratic beating of the heart when the pumping was going sporadically fast and then slow again. Fast and then slow.

That irregular heart beating happens when the heart really, really starts to struggle. And eventually, when it just can’t keep going any longer, unfortunately, death occurs. I hope that you’ve enjoyed today’s practical and found it both interesting and also useful in deepening your understanding of the topic. Remember to clear away all of your equipment, washing them up, and clearing up any spills of water as it is a slip hazard. Then you need to complete the rest of your worksheet and you can upload some values to share them online and discuss them with others.