by Annette Turley, Anaesthetist

One of the major worries of most patients coming for surgery is will they be asleep during the cutting. Sensational reports of being aware or awake during surgery occur in New Zealand media about once a year.

The truth is the risk of awareness (being awake and aware of the surgery) during an operation is reported to be approximately one in 2000 anaesthetics. Some cases have more risk than others, for example, emergencies and very sick, frail patients do not tolerate a lot of drugs, therefore it is a balance for the anaesthetist to give enough without the patient's heart, lungs or brain being permanently affected.

There are many ways your anaesthetist monitors how deeply you are unconscious - your “depth” of anaesthesia. Your anaesthetist stays with you during the entire surgery monitoring your responses, checking you are not stressed and continually giving drugs to keep you asleep, stress free, relaxed and alive during your operation.  

Through out the history of anaesthesia, specialist anaesthetists have been searching for the “holy grail” - the monitor that tells them the drug-induced coma they have put the patient in is “deep” enough, that is, has the patient asleep enough, so they will not respond to the surgeon cutting them.

The ways of monitoring include simple things from the patient kicking the surgeon to rises in heart rate and blood pressure in response to painful stimulation.

Although these give a good indication in most people, there will always be patients who need more anaesthetic drugs and those that need less drugs to keep them asleep. A little like some people only need one glass of wine and others two or three before they feel the effects. Certainly if people are taking other drugs it can affect the amount of anaesthetic agents needed through the surgery.  

The best monitor would be one that tells anaesthetists what the brain is doing. When we are awake there is a lot of electrical activity within the brain as it processes information and we react to the inputs, (just like a computer when you are working on it). Even when we are asleep the brain has some electrical activity while it maintains your breathing, heart rate and while you are dreaming.

Scientists know what size and waveforms this electrical activity is and the differences between them. New monitors have been introduced into anaesthetic practice in the last few years that sum up the activity of these brain waves to give your anaesthetist a guide to how active your brain is.  The monitors use a scale of one to 100.  When you are awake and alert the number is between 95-100, when you are in a “natural sleep” the numbers are between 70-90, and when are under a general anaesthetic, the number should be between 30-60. If there is no brain activity the number is zero.

People do vary in the “number” their waveform generates and the way they respond to the drugs. Hopefully, with on-going research, this type of monitor will be developed further, and who knows, your anaesthetist maybe able to tell you what you were dreaming about in the future!