General Anaesthesia

INTRODUCTION

General anaesthesia has been practised in Western Medicine for approximately 150 years. The practice of anaesthesia is now a worldwide phenomenon and in Australia alone in excess of one million general anaesthetics are administered every year. Despite this a precise understanding of the mechanism of general anaesthesia, eludes the scientific community. Nonetheless great strides have been made in ensuring that the practice of anaesthesia is safe and effective. In this section an attempt will be made to give a brief description of the scientific theories behind anaesthesia and general description of how anaesthesia is conducted.

THEORIES OF ANAESTHESIA

The first problem in understanding the mechanisms of anaesthesia is that there is no clear understanding of what generates consciousness in the normal state. Furthermore general anaesthesia is a different state compared to normal sleep - one can be aroused from sleep but anaesthesia can only be ended by discontinuation of the anaesthetic drugs.

The drugs that can induce a state of anaesthesia are numerous and fall into 2 broad categories - those that can be inhaled and those that can be injected into the blood stream. Inhalational anaesthetic agents were the first to be used and they include drugs such as ether, nitrous oxide, halothane and sevoflurane. In modern anaesthetic practice there are only 4 agents that are available for use and they include nitrous oxide, isoflurane, sevoflurane and desflurane.

The drugs that can be injected were discovered later and they include agents such as thiopentone, ketamine and propofol. For decades thiopentone was the most commonly used drug but this has largely been replaced by propofol.

The fact that such different drugs administered by different routes (via the lungs or the blood stream) both cause anaesthesia points strongly to the fact that there is no single explanation as to how anaesthetics work. One of the earliest theories of anaesthesia related the ability of inhalational agents to dissolve in oil to their potency as an anaesthetic drug (Overton Meyer Hypothesis). The explanation for this observation was that the anaesthetic agents dissolved in the membranes of nerve cells and disrupted their function. However this theory has largely been dispelled and the research is now focussed on how the agents change proteins and enzyme function within nerve cells.

It is well described that certain agents that can be injected (such as midazolam) bind to specific receptors on cells in the nervous system. These receptors have an inhibitory function. However not all injectable agents bind to specific receptors so once again there does not appear to be a single mechanism of action for these drugs.

Despite the limitation of not understanding how the drugs work, the effect that these drugs have on the human body has been studied extensively and understanding this forms the core of practicing safe anaesthesia.

CONDUCT OF GENERAL ANAESTHESIA

Every patient presenting for an operation has an unique anaesthetic that is designed according to the operation they are having and their specific health needs. However there are key features of every anaesthetic that have to be followed and in this section, an outline of these steps will be described.

(A) Preoperative assessment

All anaesthetics begin with a preoperative assessment of the patient. This can be conducted days to weeks before an operation if there are significant health problems that need to be corrected before surgery. In most instances it is conducted the day before or on the day of surgery. In dire emergencies it is conducted on the way to or in theatre. A more complete description of this topic can be found in the section …….

(B) The Anaesthetic

This begins after the appropriate checks of patient identity, operation and consent have been completed. After being transferred to the operating table the appropriate monitors are connected and an intravenous cannula is inserted. Prior to this the anaesthetist will have prepared the drugs and other equipment that is going to be used in the anaesthetic.

In adults, anaesthesia is usually induced with the injection of the appropriate drugs in the intravenous cannula. This is usually a combination of different drugs that are chosen to meet the specific needs of the patient. The onset of anaesthesia is rapid but not usually as quick as the "count to 10" urban legend. The time taken to fall asleep is usually closer to 60 seconds.

Example of the essential drugs contained in the anaesthetic trolley

Typically after anaesthesia has been induced patients will lose control over the reflexes that protect their mouth and airway and may also stop breathing. This requires that the anaesthetist takes control of the patient's airway and follows the plan that has been developed to continue with the anaesthetic. The airway may be controlled with a facemask or with numerous other devices that have been developed for specific situations.

Following injection of the drugs that induced the anaesthesia, further drugs have to be administered to maintain the state of anaesthesia. This can be the use of anaesthetic gases that are inhaled and absorbed via the lungs or a continual infusion of a drug such as propofol into the veins. At the end of the surgical procedure, the administration of these drugs is stopped and the patient will be awake within 5 to 15 minutes.

The vaporizers containing the drugs (sevoflurane/desflurane) that are used to maintain anaesthesia. At the end of the procedure, the vaporiser is turned of and the patient will wake 10 t0 15 minutes later

Intravenous drip with the lines attached that deliver the anaesthetic drugs that keep a patient asleep

The infusion pumps that deliver the anaesthetic drugs during the procedure

An example of the monitoring screen reflecting some of the physiological parameters measured during an anaesthetic.

Anaesthetising a person for procedure has a number of key requirements and these as follows

(1) Achieving a state of reversible unconsciousness through the administration of drugs. This state ensures that the patient will have no recall of the procedure (amnesia) on waking

(2) Analgesia - this means the preventing the perception of pain

(3) Muscle relaxation - to facilitate the surgical access and to ensure there is no patient movement during delicate surgery

(4) Protecting the patient from harm as a patient has lost all their protective reflexes when asleep. This includes protecting the eyes and airway, preventing pressure on nerves and areas where the skin is delicate, keeping the patient warm and so on.

(5) Maintaining physiological normality - this includes keeping the heart, lungs, kidneys, liver and so forth functioning normally.

The drugs used to induce anaesthesia are extremely powerful and if administered incorrectly can cause serious harm or even death within minutes. The drugs have to be administered with precision and the anaesthetist has to know how to manage the side effects. The anaesthetics drugs are best used on combination with each other so as to achieve a balance between anaesthesia and analgesia with the least effect from any one drug.

Modern anaesthesia has an excellent safety record and this is due to the development of patient monitors. In the early days of anaesthesia, the anaesthetist would have to keep a finger on the pulse of the patient and watch their lips to see if they were turning blue from lack of oxygen. Although this made the anaesthetist keep a close eye on the patient it was very inadequate for monitoring - similar to driving a car at night while holding a torch out the window. Some of the monitors that have been developed include

(1) Electrocardiogram - measures the electrical activity in the heart

(2) Pulse Oximeter - measures the oxygen saturation of the arterial blood

(3) Capnograph - measures the carbon dioxide in the exhaled breath

(4) Blood pressure - this can be measures using a blood pressure cuff or by inserting a small cannula into an artery and transducing the pressure

(5) Temperature probes

(6) Gas analysers - measure the concentration of anaesthetic gases on the lungs

(7) BIS monitor - measures the electrical activity on the brain and this reflects the depth of the anaesthetic

There are numerous other monitors that can be used in specific circumstances where the surgery is more extensive or the patient has significant medical problems.

After the completion of an anaesthetic the patient is taken to either the recovery room (Post Anaesthesia Care Unit - PACU) or to the Intensive Care Unit (ICU) depending on their specific needs after surgery. In the recovery room patients are kept for a minimum of 30 minutes to ensure they have recovered fully from the anaesthetic. In this environment they are monitored and attended to by specially trained nursing staff who ensure that they are awake and their pain adequately controlled before being sent to the ward or the Day Surgery Unit (DSU).