Whew… Now that your patient is asleep, it’s time to grab a chair and settle in for the maintenance phase. This is the time to get some teaching.

Induction agents wear off quickly, so we need to provide a maintenance agent to keep the patient asleep. There are two main choices: inhaled volatile agents or a computer-controlled infusion of intravenous anaesthetic (usually propofol).

Inhaled Anaesthetics 🫁

Volatile anaesthetics (the -fluranes) are the standard maintenance agents. We start running them as soon as the airway device is secured. They offer a reliable dose-response curve and a way to monitor their concentration in the bloodstream. They are also expensive greenhouse gasses that like to catch fire.

• Sevoflurane is used for almost all cases
• Desflurane is even more expensive and worse for the environment
• Nitrous oxide is a weak anaesthetic gas that can reduce the requirement for -fluranes but makes it patients vomit and is still bad for the environment

Minimum Alveolar Concentration

The minimum alveolar concentration (MAC, pronounced “mack”) is the end-tidal concentration of anaesthetic agent (in %) that produces immobility in 50% of healthy middle-aged men (with no other sedative agents onboard) when a one centimetre incision is made in their forearm.

An agent’s MAC reflects its potency. A higher MAC indicates a less potent anaesthetic (more is needed for the same effect) and a lower MAC indicates a more potent anaesthetic.

MAC technically represents blunting of the spinal cord reflexes, not a lack of awareness. However, awareness is virtually unheard-of at end-tidal anaesthetic concentrations more than 70% of an agent’s MAC.

Confusingly, we also use the phase “MAC” when describing how many multiples of an agent’s MAC we are using. One could run a sevoflurane anaesthetic at “0.7 MAC”, meaning an end-tidal concentration of 1.26%, which is seven tenths of the “full” MAC of sevoflurane (1.8%).

MAC is additive. If you run half a MAC of nitrous oxide (52.5%) and half a MAC of sevoflurane (0.9%), you will achieve the same depth of anaesthesia as if you had used the full MAC of either agent.

TIVA 💉

Total intravenous anaesthesia (“TIVA” for short) is a computer-controlled infusion of intravenous anaesthetic used in place of inhaled agents. Virtually all of the TIVA you see will be a propofol target-controlled infusion (TCI).

• Pros: propofol has anti-emetic effects, smoother emergence, no direct greenhouse gas emissions, less occupational exposure to fluranes
• Cons: the patient wakes up (paralysed) during surgery if the cannula breaks, no way to measure blood concentration, uses lots of disposable tubing

Are They Asleep 🔮

MAC

MAC is a simple, reliable and non-invasive way to tell if a patient is asleep.

What About BIS?

BIS is intended to provide a numerical measure of anaesthetic depth derived from spectral analysis of a frontal electroencephalogram (EEG). There is a similar device on the market called Entropy.

There are growing concerns about how easily the BIS monitor is misled.

EEG

Manual interpretation of an un-processed frontal EEG is a reliable way to assess depth of anaesthesia. Check out ICE-TAP if you want to learn more.

Documentation 📝

The anaesthetic record is usually a paper chart filled in by the anaesthetist after the patient is safely asleep.

The anaesthetist will document the techniques they used and any complications that arose. Good documentation can save the patient from experiencing the same issues again in the future.