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How much anaesthetic will knock you out?

Tuesday 18th August 2015 12:11 pm

When it comes to ‘pain’, I am a total weakling. A stoic I am not — not even one little bit. However, I am very happy that pain will warn me of potential dangers, such as a bee sting when near a hive, or a mild burn when near a roaring fire.

But I do not think that pain makes me a better person. And so, when it comes to any kind of surgery, I take all the anaesthetic options that I can get.

But how much anaesthetic gas is the right amount to give? Well, we learnt that by trial and error, and by ‘carving up’ healthy young US Marines — who unfortunately for their sake, were bound by duty.

Surprisingly, being comfortably numb is an exacting and complicated science.

But first, let’s dive into a few mysteries.

For example, even today, we don’t fully understand how anaesthetics work — in other words — their mechanism of action. However, rest assured, thanks to the long training that they receive, specialists know how to give anaesthetics safely.

Another fundamental mystery is that we still don’t know how the brain transitions between consciousness and unconsciousness. And we don’t know why going down into unconsciousness, follows a different pathway from coming back up into consciousness.

Now back to the big question: how do anaesthetists know how much anaesthetic gas to give?

Let me introduce you to the MAC (minimum alveolar concentration). A single MAC (1 MAC) is the concentration of an anaesthetic gas (inside the lungs) that will sedate 50 per cent of the ‘population’, so that they will not move or react to, a ‘standard surgical incision’.

In these experiments, the ‘population’ was healthy young US Marines, and the ‘standard surgical incision’ was a one-centimetre-long cut in their forearm. Talking ‘scientifically’, the MAC was derived from this experimental data.

It turns out that kids need proportionally greater doses of anaesthetics than adults — relative to their body weight. (This holds true for both anaesthetic gases and intravenous anaesthetics.)

In other words, weight for weight, kids have a higher MAC (or minimum alveolar concentration) of gas needed to make them unconscious.


It’s complicated (because normal human physiology is complicated). But we do know a few of the factors involved.

Compared to adults, kids have a proportionally greater cardiac output (the volume of blood the heart pumps in each minute). Their brains (where the anaesthetics work) are proportionally bigger. Their livers (where some of the anaesthetics get broken down) are also proportionally larger.

Kids also have more a more ‘robust’ cardiovascular system than adults. So they are less compromised by anaesthetics. Adults can easily become hypotensive under anaesthetic — this means that their blood pressure can drop to dangerously low levels.

A probable factor is that kids have a different central nervous system from adults. We’re not sure exactly what the differences are in the central nervous system — more synapses, different blood-brain barrier, etc. Time will tell …

To make it easier, let’s consider just one very commonly used anaesthetic gas, sevoflurane.

Newborn full-term babies need about 2.0-2.4 per cent of sevoflurane in their lungs to achieve 1 MAC. The remaining 98-or-so per cent of their lungs is filled with air – often with some oxygen added to give a safety margin.

The percentage level of sevoflurane needed to cause anaesthesia increases with age until around six months of age – when kids need a level of 2.5-3.0 per cent of anaesthetic gas.

The ‘average’ kid older than that would need about 2.4 per cent to achieve 1 MAC. The level needed continues to taper off, in a fairly straight-line fashion, until adulthood (when it reaches about 2.1 per cent). It then remains static in the 20s, 30s and 40s. Older people, and the unwell, need less anaesthetic to reach 1 MAC, or minimum alveolar concentration.

As an aside, when you’re really old enough, you won’t need any anaesthetic to knock you out – for good.

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This blog first appeared on Dr Karl's Great Moments in Science

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