Investigating the effects of cardiovascular stress: Control test

If you have all of the required items and you’ve read the information on minimising risks, we can begin. We need our equipment, so I’ve got a measuring jug here with 1,000 millilitres, or one litre, of water in it. The water represents the blood in the body. I then have my mini pump, and this represents the heart. We’re going to hold the pump in a specific way, so that we can compare it to other ways of holding it later. You’re going to place the pump in the flat of your hand, in the palm of the hand, with the nozzle facing towards you. Hook onto the pump with three fingers, one on either side, and one hooked around the back.

Have a go at using the pump so you could prime it, ready to work. We want it filled with water.

Lovely. Check that your beaker is still at 1,000 millilitres, and then we can start by obtaining our baseline, or control values. These baseline values will represent a resting heart. So what we’re going to do is transfer 300 millilitres of water from our measuring jug to our beaker by depressing the pump once per second. So we’re going to squeeze the pump once a second. We’re going to time how long it takes, and note the time down that passes each of the 100 mL markers– so at 900 mL, 800 mL, and 700 mL. And we’ll write those down in table one of the worksheet. So when you’re ready to go, check your volume. Prime the pump, hooking on with three fingers.

Ready to start the timer– so once per second.

It’s a slow, regular rhythm. We’re representing our resting heart rate.

What we’re doing is we’re obtaining control, or baseline, values. Now it’s important to obtain control values, so that we have something that we can compare to later when we obtain the values where we mimic heart failure.

If at any stage you notice that your arms starts to tire, then you can stop and have a break. Just remember to stop the stop clock and restart it again. If any stage you develop pain, then stop immediately. see how we are doing. So we’re nearly there.

Once per second– and pause. Make a note of that time.

And then restart as soon as you can. You might find it easier if you have a friend to help you with the timing, but it is possible to do it on your own, as I’ve done.

So we’re mimicking a resting heart. The heart beats slow and steady, in a rhythmical way. What always amazes me is that the heart does this every minute, every hour, every day, every week, every year. It never gets to rest.

Again, if you feel tiredness, pause. Take a break. But if you feel pain, stop.

A little bit more to do– and again, note the time down.

Restarting for the third time. So this is our final 100 millilitres to transfer.

In scientific experiments we tend to repeat things three times, as this increases our confidence in the values that we obtain. In this experiment we’re transferring 300 millilitres, which is actually three lots of 100, so that when we calculate the stroke volume and cardiac output later, we’ll be more sure of the values we’ve obtained.

Just over halfway.

I’m writing down that third and final value.

Let’s complete table one now, giving the arm a chance to have a rest. We need to convert our times from minutes to seconds. So I’ve got those here– we’ve got one minute and five, which is 65 seconds. 1 minute 58, which would be 118, two minutes 49, which is 169. So we know our total elapsed times. We now need to calculate the transfer time for each 100 millilitres. I’m going to use my calculator. So we subtract one time from another. So the first one we know was 65 seconds. The second one, we, can subtract the 65 from 118– which gives us 53. The third, 169– subtract 118– gives us 51.

So that gives us our transfer times of each of the three replicates. Now we can calculate our stroke volume. The stroke volume is the volume pumped per beat of the heart, in this case the volume pumped per mini-heartbeat. We know that we pumped once per second, so if we have a transfer time of 65 seconds it means we pumped 65 times, and we know we transferred 100 millilitres. So if we divide 100 millilitres by the number of times we contracted, we’ll know the stroke volume. So we know we did 100 millilitres. Divide it by the transfer time, or number of contractions, and that gives us a volume of 1.5 millilitres. Let’s repeat that for the second replicate, then.

So we did 100 millilitres in 53 seconds, or 53 contractions. That’s 1.9 mL. And the third time– 100, divided by 51– that gives us 2.0, if we round up. So we’ve got stroke volumes of approximately 2 mL. So our mini heart is pumping 2 millilitres with each contraction. That’s a lot smaller than a human heart. Now we can calculate our cardiac output, or the volume that will be pumped in a minute. We know that we transferred 100 millilitres in 65 seconds, so if we take 100 and divide it by 65, and then multiply it by 60 seconds, or a minute, we’ll have the volume per minute.

So 100, divided by 65, multiplied by 60, gives us a cardiac output of 92 millilitres. Let’s do that for the second one. 100, divided by 53, and multiplied by 60, gives us 113 millilitres. And the third time– 100, divided by 51, times by 60 seconds, gives us 118 millilitres. So our mini heart is pumping between 92 and 118 millimetres per minute. Again, that’s a lot less than a human heart. Because we repeated this three times, we can now calculate an average. To do this, we add all three values together, and then divide by 3. So 92, plus 113, plus 118, and then divide by 3, because we had three replicates, gives us an average value of 108 millilitres.

So that gives us our baseline, or control value. This enables us to compare with each other, because your pump might be different to my pump. So we need to baseline, or control, to enable those comparisons. Now let’s see what happens when we mimic exercise.