An introduction to anaesthetics
Anaesthesia is the controlled temporary loss of sensation or awareness where you can become unconscious, loss of response to painful stimuli or loss of reflex. There are two types of anaesthetics: local and general. Local anaesthetics are those that block nerve conductions locally. Whilst general anaesthetics work on the brain and cause loss of consciousness. Both types are frequently used experimentally and in medicine. There are two routes of administration, either through inhalation or intravenous routes (injection.)
Before anaesthetics, alcohol used to be used to induce anaesthesia. Therefore, a patient would have to drink a lot of alcohol and hope that they did not come around too quickly. Early anaesthetics used were nitric oxide, ether and chloroform. Over the years the favoured choice of anaesthetic has changed. Current anaesthetics used vary from country to country but mainly are barbiturates, benzodiazepines, ketamine and propofol.
General anaesthetics have a wide range of chemical properties and therefore do not have strict structure-activity relationships. The four stages of anaesthesia were proposed by Guedel in 1937. In stage 1 the patient is awake, but drowsy, and their perception is distorted. By the end of stage 1, they will reach analgesia. Stage 2 is where the patient loses consciousness and faces irregular breathing with cardiac dysrhythmias and loss of temperature control. They experience uncontrolled movements and exaggerated reflexes which can cause vomiting. This is why you should not eat or drink before an operation. It is important to quickly get out of stage 2 as it is dangerous and move onto stage 3. Stage 3 is where we want the patient to be. Breathing in stage 3 is back to normal, the pupils firstly constrict and then dilate, the cough and vomit reflex are depressed and the large skeletal muscles are relaxed. The operation can now commence. But it needs to finish before reaching stage 4. Stage 4 is where there is no ventilation, as the medulla oblongata is depressed, pushing the patients towards death.
The stages of anaesthesia caused problems due to it being difficult to measure. When ether was used, doctors would measure movements as a sign of being in a certain stage. This is now an ineffective method as anaesthesia can cause temporary paralysis. Most of the signs using Guedel’s model rely on watching movement and using multiples agents obscures the signs even more. These stages of anaesthesia are now thought of as out of date. A new solution is to use an EEG (electroencephalogram) to measure brain waves to indicate the stage of anaesthesia. An anaesthesia deepens, the amplitude of high frequency components of the EEG lower and at lower frequencies increase. But, these changes in EEG are agent dependent. Additionally, many pathophysiological occurrences can alter EEG, for example hypoxia, hypotension and hypercapnia. The patient state index is a method of comparing EEGs during induction, maintenance and emergence. Additionally the cerebral function motor and Bispectral index can be used to measure brain activity and other parameters for measuring anaesthesia.