Investigating the effects of cardiovascular stress: Simulating exercise

Now, we’re going to mimic an exercising heart. Before we begin, we need to top off our jug to 1,000 millilitres again. So you can pour the water back in up to the 1,000 mL marker. When we mimic exercise, what we’re going to do is we’re going to transfer 300 millilitres from our measuring jug to our container, but this time we’re going to pump as fast as we possibly can. We’re going to use the same three-finger hold of the pump so that we can compare that to other holds later. So 300 millilitres as fast as you can noting down each time we pass 100 millilitres. Let’s reset the timer. And when you’re ready, we can go.

So we’re pumping as fast as we possibly can to mimic exercise. Be careful that you’re not getting water everywhere. It does have a tendency to spray out of the jugs.

Keep an eye on the volumes.

It can be quite hard to read the values because the water starts to move around, but when you get to that first 100 marker, pause the time and we’re going to fill this in in the second table. As soon as you can, we carry on. So we’re mimicking an exercising heart. A heart that’s working hard to deliver vital blood and oxygen to the exercising muscles. You might find that you start to tire. In which case, you can slow down. But if you can, keep going. Of course, if you feel any pain, stop immediately.

Make a note of the second time.

And then carrying on once more. So third and final time. It might be feeling a bit harder now. So remember you can take little breaks. Or try not to break, but you can slow down if you need to. And stop if there’s any pain. There we go. So the third and final one done. Let’s write down that time. Bang on 60 seconds. And then give the arm a bit of a shakeout. And we’ll take a break as we complete the second table.

Again, we need to convert the times to seconds if you went up into higher numbers. So we’ve got them all in seconds already actually so let’s calculate the transfer time by subtracting one from another. The first one was just the 21 seconds. The second one we take the elapsed time and subtract the first, which gives us, again, 21 seconds. They’re quite consistent. The third one, the elapsed time was 60 seconds minus the one before gives us 18 seconds. So our transfer time is around the 20 second mark. Now we don’t need to calculate the stroke volume this time because the stroke volume or volume pumps per contraction is fixed for this mini heart. We can’t change it.

Of course, with a real heart, the stroke volume can be changed as the heart adapts, and we’ll talk about that in a little while. What we can calculate is the cardiac output, the volume pumped per minute. Again, we know we transferred 100 millilitres each time. And we know how long it took to transfer. So if we divide 100 millilitres by the transfer time and multiply by 60 seconds, it will tell you the volume per minute. So the first one then. 100 millilitres divided by the transfer time and multiplied by 60 seconds gives us a volume of about 286 millilitres. The second one, 100 divided by the transfer time multiplied by 60. Again, the same value.

And then the third time, our transfer– well, we start with our 100 millilitres divided by the transfer time multiplied by 60 gives us 333 millilitres. So we can see our cardiac output has increased from the baseline. In order to be able to compare because our pumps will all differ slightly, we’re going to convert the cardiac output to a percentage of the baseline. To do this, we divide our cardiac output by the average baseline value and multiply it by 100. So we’ve got 286 divided by our average and then multiply by 100 gives us 265%.

Again, 286 divided by the baseline times by 100. 265%. Last one. 333 divided by the baseline multiplied by 100 gives us 308. So we can see here that when we mimic exercise, the cardiac output has increased quite a lot. We can see here it’s gone up by nearly three times the baseline or control value. Of course, an exercising heart needs to increase the volume of blood it pumps per minute to cope with the needs of the exercising muscles. Cardiac output or the volume of blood pumped per minute is calculated by combining the heart rate, the number of times the heart contracts, by the stroke volume. Our stroke volume of our mini heart was fixed. We couldn’t change it.

So what we did here was change the heart rate. We increased the speed at which we were pumping. And we can see from our results, that’s had quite a big impact. It’s increased that cardiac output a lot. With a real heart, it can also change the stroke volume through several different ways. The stroke volume can be increased by increasing the strength of the heart muscle so it’s able to contract to more strongly. The stroke volume can also be increased by increasing the size the heart stretches to. So the stretch of the heart. If it fills with more blood, it can then squeeze more blood out again.

Thirdly, the stroke volume can be increased if we reduce the resistance to the blood leaving the heart. If we reduce the resistance to the blood leaving the heart, more blood can be squeezed out. This resistance can be lowered by dilating the blood vessels or lowering the blood pressure. This is why someone who carries out regular cardiovascular exercise often has a larger, stronger heart that stretches and fills with more blood. This increases the stroke volume and enables them to have a lower resting heart rate. They also often have a lower blood pressure. Now that we’ve seen what happens with exercise, let’s see what happens when we mimic heart failure.