Practice Gadgets - what they do: the capnograph and pulseox

You may have seen that on our “practice facilities” page we boast about our anaesthetic monitoring equipment – capnograph, blood pressure monitoring, pulseox and our new multiparameter monitor. These are tools for helping us to keep your pet safe while they’re asleep under an anaesthetic, so in this blog we’re going to look at them in more detail.

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Why is monitoring so important under anaesthetic?

Any anaesthetic is potentially a risk - although we talk about the patient “being asleep”, in fact, it’s more like a medically induced coma. If we give them too much anaesthetic, they might not wake up, and if we give them too little, they might jump off the operating table! As a result, our expertly-trained qualified veterinary nurses are constantly keeping an eye on their heart rate, breathing rate, reflexes, eye position, blood pressure, pulse quality, and a range of other factors. However, there are some things that they can’t monitor with eyes and ears and hands, and so we have machines to help with that.

One of the biggest factors that determines how well an animal recovers from anaesthesia is how well their body is able to transport oxygen to the tissues, and carbon dioxide away from them. Anaesthetic drugs generally reduce the efficiency of the cardiovascular and respiratory systems, and so any anaesthetised animal is potentially at risk of becoming hypoxic (low blood oxygen, meaning that tissues become oxygen depleted and may even start to die) or hypercapnic (high blood carbon dioxide, which can cause damage to cells and change the acidity of the blood, causing a respiratory acidosis).

Anaesthetics also tend to drop the blood pressure, which puts the internal organs under additional stress (especially the kidneys); it’s one of the reasons cats with kidney problems used to be at such high risk under anaesthesia.

In the old days, vets and nurses could only feel the pulse and check their patient’s colour (how pink their gums were) - a nice pink colour was good, blue meant not enough oxygen and grey meant cardiac arrest). While we still do that, it isn’t terribly precise, so we have now invested in machines that will give us much more information.

What does a capnograph do?

A capnograph measures the amount of carbon dioxide in the air that the patient is breathing out; it then displays this as a graph (hence the name). It has a probe that fits around the patient’s breathing tube (their endotracheal tube), and works by shining an infra-red light across this tube. The infra-red light is absorbed by carbon dioxide but not be oxygen, so the relative amounts of oxygen and CO2 can be calculated by the built-in computer.

Why is that important?

The patient needs to be able to lose the carbon dioxide that’s building up in their body! If they aren’t breathing well enough (say they are breathing fast but very shallowly), then the amount of CO2 in the expired out air will start to go up (because it’s building up between breaths). This is sometimes called the End Tidal Carbon Dioxide, and if it’s rising, it’s a red-flag for the anaesthetist that their patient isn’t breathing as well as they should be.

How does the pulse-ox work?

This bit of kit is technically called the pulse oximeter, and it measures the amount of oxygen in the tissues. Again, it works by shining light through the tissues, and measuring the colour - blood containing lots of oxygen is bright red, whereas blood with less is dark red or even blue. By measuring the pulse as well, the pulse-ox can tell whether it’s measuring arterial blood (which should be high in oxygen) or venous blood (which is naturally lower). The pulse-ox probe itself is a little clip that we usually place over the tongue (it works best on pink skin!).

What does it tell you?

This gives us a measure of the oxygen saturation of the blood as it heads out into the tissues. In a healthy patient, their arterial oxygen saturation will be over 95% (usually 98-100%). If the levels of oxygen fall, then the patient’s tissues risk becoming oxygen starved, which could be fatal, so if the oxygen saturation drops below 95%, it needs looking at.

What about the blood pressure monitor?

This machine uses an inflatable cuff and an electronic oscillometric sensor that can “feel” the blood passing through the vessels. When the cuff is inflated to the same pressure as the blood pressure, blood can no longer flow under it, so the machine knows what the systolic (peak), diastolic (background) and Mean (average) blood pressures are.

Why do you want to know?

It’s absolutely vital that the patient’s Mean blood pressure stays over 60mmHg - because below this their kidneys start to shut down. By checking the pressure every few minutes, we will know as soon as it starts to drop, and take preventative action before it drops dangerously low.

Isn’t that a lot of machines?

Not any more! All three systems - plus an ECG to monitor the patient’s heart activity, and a thermometer to make sure they aren’t getting too cold - are combined into our new single machine. We plug it in next to the anaesthetic machine and it measures every parameter, completely automatically!

Isn’t relying on the machines dangerous?

No - they aren’t being used instead of a human monitoring, but as well as. Although the machines can give errors (e.g. the pulseox giving an abnormally low reading because the probe needs to be moved), our nurses are trained to recognise these. The advantage they give us is that they will pick up high carbon dioxide, low tissue oxygen, low blood pressure, or abnormal heartbeats, well before there are any visible signs - and even before they become risky for the patient, allowing us to make adjustments to the anaesthetic in plenty of time.

Ultimately, the use of these machines by a trained and experienced nurse or anaesthetist makes anaesthesia much, much safer for the patient.

Do you want to know more about how we’ll monitor your pet under anaesthesia? Ask to talk to one of our vets or nurses about it if you’re worried!

 

Posted on October 26, 2017 and filed under Blog.