General, spinal, regional, and local anaesthesia
General anaesthesia is when I use drugs to but your brain and your body to sleep.
If you are asleep; completely unaware; and you don’t move when the surgeon or proceduralist does something which would otherwise cause pain, then you have a general anaesthetic on board.
I am with you continuously during a general anaesthetic. I am continuously administering drugs (whether by vapour or intraveinously) to keep you asleep, and to vary the “depth of anaesthesia” as required. I am also continuously ready to give drugs to alter your other body systems to keep your body stable despite the stresses of anaesthesia and surgery being inflicted upon you.
For certain operations, we like to keep your brain awake for the procedure and just put the lower half of your body to sleep.
For other certain operations, post-operative pain is much better controlled with a spinal anaesthetic.
And for certain patients, they recover better if less medications have affected their brain.
Spinal anaesthesia involves placing a very thin needle into your lower spine, and injecting some local anaesthetic, usually combined with an opioid such as fentanyl or morphine, into your lower spinal cord. The local anaesthetic diffuses into the spinal cord, making the lower half of the body unable to feel, and also unable to move. It lasts 2 to 5 hours, usually 3 to 4 hours.
As crazy as this may sound, it is actually very safe. The delicate spinal cord tissue that one thinks of when they think of a spinal cord injury actually finishes well above where we insert the needle. The lower spinal cord contains nerves, but they are “peripheral nerves”, which are tough and robust – the same as the ulnar nerve also known as the “funny bone”. Most of us have given our “funny bone” a good whack at some stage, and it hurts, but it hasn’t caused us nerve damage. Major nerve damage from a spinal anaesthetic would occur, statistically, once every few years in all of Australia. Major spinal cord damage from a general anaesthetic can also occur, with similar frequency.
So why would I bother giving you a spinal anaesthetic rather than just putting you off to sleep?
Firstly, some operations, in particular prostate surgery (TURP), involve a lot of irrigation fluid which continuously flows in front of the surgeon’s camera. Otherwise the fluid would become blood-stained and the surgeon would not be able to see what they are doing. Your body absorbs some of this irrigation fluid – it simply flows into your circulation through veins which have been cut and are open. The irrigation fluid is not very toxic, but if your body absorbs a large amount of it (for example, more than a litre), it can become toxic. By far the earliest sign that a lot of irrigation fluid has been absorbed is that you become a bit confused or agitated – not quite your normal self. If you are awake and able to communicate then I can observe that this is occurring, if you are asleep then I cannot tell. Much later signs will appear on our monitors, but only after a lot more irrigation fluid has been absorbed and its toxicity has become much more significant.
Secondly, some operations are more painful than others, and having a spinal anaesthetic (whether or not you also have a general anaesthetic) significantly reduces the amount of post-operative pain that you experience. This effect continues even after the spinal anaesthetic has completely worn off a few hours later. Spinal anaesthesia is particularly effective at controlling post-operative pain after Caesarean sections, and after hip or knee replacements or similar surgeries. It may also be a useful strategy for less painful surgery if you have chronic pain and are tolerant to morphine and other opioids.
Thirdly, some people are at higher risk of delerium after their operation. Once people are aged in their 70s, and particularly when they are aged over 80, they are quite likely to experience delerium after their operation. Often they are fairly orientated for the rest of the day of their surgery, but become confused and disorientated that night. The cause of the delerium is multi-factorial – the stress of surgery; disturbed sleep; sleeping in a strange place; pain; pain-relief medication; and other medication such as antibiotics all contribute to delerium. One can still become delerious after a spinal anaesthetic, but spinal anaesthesia without sedation has been shown to be protective of delerium compared with general anaesthesia or spinal anaesthesia with sedation.
People with significant lung disease will have less disturbance to their lungs if they continue breathing for themselves rather than our ventilator breathing for them, and so spinal anaesthesia may be safer for them. And if we administer general anaesthesia to a lady about to have a Caesarean section, then we are also administering a general anaesthetic to their unborn baby. This is not a disaster, but it is something that we avoid if we have the option.
Epidural anaesthesia is similar to spinal anaesthesia, except that a larger needle is inserted into the back; a plastic tube is threaded through the needle, and then the needle is removed with the tube staying in. The tube lies within the spinal canal but next to the spinal cord, compared with the spinal needle which pierces the sac of the spinal cord. Despite going one layer less far than a spinal needle, epidural anaesthesia probably has a little bit more risk than spinal anaesthesia. This is because the needle is bigger, and the tube stays in for longer. The advantage of an epidural is that I can keep giving local anaesthetic down the tube for hours or even days. This makes epidurals very useful for pain relief in labour and childbirth. Outside of labour and childbirth I don’t often put epidural catheters into people, as people usually do quite well without them. There are some situations however in which they can be quite helpful.
Regional anaesthesia refers to injecting local anaesthetic next to a nerve, or next to a bunch of nerves, to put a body part to sleep.
Spinal anaesthesia is a type of regional anaesthesia, but it is usually classified separately (as “neuraxial” anaesthesia).
We can put an arm or a leg to sleep for certain surgeries. This is usually combined with general or spinal anaesthesia, but it does not have to be.
We can also put an eye to sleep by injecting local anaesthetic behind the eye (not through the eye!). This is how most cataract surgery is performed.
The main purpose of regional anaesthesia (apart from eyes) is for extended post-operative pain relief. For some surgeries such as knee replacements and shoulder surgeries, pain can sometimes be difficult to control even with large doses of morphine.
A femoral nerve block for a knee replacement lasts about 18 hours. It doesn’t provide complete pain relief as the knee joint is supplied by 3 nerves, but the femoral nerve is the main one and a femoral nerve block makes it much more comfortable. A sciatic and saphenous nerve block for ankle or foot surgery lasts about 24 hours. An ankle block for foot surgery lasts around 12 hours. And a “brachial plexus” block for shoulder or elbow surgery lasts 18-24 hours.
When the regional anaesthesia (nerve block) wears off, you will have pain, but the pain will be less than what it would have been straight after the surgery. By the time the nerve block wears off, the tissues have had many hours to settle and to begin to heal.
In addition, because the limb is asleep while the operation is proceeding, you generally don’t need the same quantity or variety of anaesthetic drugs to keep you asleep and to wake you up comfortably. People may wake up more quickly, better, and with less nausea or vomiting than if regional anaesthesia was not used.
Local anaesthesia may be used as the sole anaesthetic for some skin or other superficial surgeries (such as carpal tunnel or trigger finger release), or it may be used in combination with other pain relief strategies. It involves injecting local anaesthetic into the region that is to be operated on, rendering this region numb and painless.
Long-acting local anaesthetic is usually injected into the wound by the surgeon at the beginning or end of the procedure. This will last a few hours. It will not make all of the deep tissues operated on pain-free, but it will make the skin incision pain-free, and this will help.
Sometimes a local anaesthetic catheter is inserted into a layer of the wound by the surgeon toward the end of the surgery, and this infuses local anaesthetic into this layer of the wound for a few days. This can be quite effective for particular surgeries.
If a procedure is to be performed under local anaesthetic alone, then often my anaesthetic services aren’t required. Sometimes I will be required to assist by providing some light sedation, or by providing a short (2-3 minute) general anaesthetic while the local anaesthetic is being injected.