General anesthetics bring about a reversible loss of consciousness and analgesia in order for surgeons to operate on a patient. Their usage is commonplace, but how they produce their effect is still not fully understood.
General anesthetics have been widely used in surgery since 1842, when Crawford Long administered diethyl ether to a patient and performed the first painless operation.
In this article, we will cover a number of topics, including the potential side effects of general anesthesia, associated risks and some theories regarding their mode of action.
Contents of this article:
Here are some key points about general anesthesia. More detail and supporting information is in the main article.
- General anesthetic was first used in an operation in 1842
- An anesthesiologist normally administers the general anesthetic prior to an operation
- There are a large number of known general anesthetic compounds
- If a procedure is likely to be particularly lengthy, general anesthetic will often be used
- There are some risks associated with taking general anesthetics, but they are relatively safe when administered correctly
- Individuals with drug and alcohol addictions have elevated risks of negative health outcomes
- Very rarely, a patient may experience unintended intraoperative awareness
- Side effects of general anesthesia can include dizziness and nausea
- The mechanisms by which anesthesia works are still only partially understood.
What is general anesthesia?
General anesthesia is, essentially, a medically induced coma, not sleep. Drugs render a patient unresponsive and unconscious.
General anesthetics are normally administered intravenously (IV) or inhaled. Under general anesthesia, the patient is unable to feel pain and may also have amnesia.
The drugs will be administered by an anesthesiologist, a specially trained doctor who will also monitor a patient's vital signs and rate of breathing during the procedure.
Stages of general anesthesia
Guedel's classification, designed by Arthur Ernest Guedel in 1937, describes the four stages of anesthesia. Modern anesthetics and updated delivery methods have improved the speed of onset, general safety and recovery, but the four stages remain essentially the same:
General anesthesia is similar to a comatose state and different from sleep.
- Stage 1 (induction): this phase occurs between the administration of the drug and the loss of consciousness. The patient moves from analgesia without amnesia to analgesia with amnesia
- Stage 2 (excitement stage): the period following a loss of consciousness, characterized by excited and delirious activity. Breathing and heart rate becomes erratic, and nausea, pupil dilation and breath holding might occur. Because of irregular breathing and a risk of vomiting, there is a real danger of choking; modern, fast-acting drugs aim to limit the time spent in stage 2 of anesthesia
- Stage 3 (surgical anesthesia): muscles relax, vomiting stops and breathing is depressed. Eye movements slow and then cease. The patient is ready to be operated on
- Stage 4 (overdose): too much medication has been administered, leading to brain stem or medullary suppression; this results in respiratory and cardiovascular collapse.
The anesthetist's priority is to take the patient to stage 3 of anesthesia as quickly as possible and keep them there for the duration of the surgery.
Reasons for using general anesthetic
There are a number of reasons why general anesthesia may be chosen over local anesthesia (affecting only one part of the body and maintaining consciousness). In some instances, the patient is asked to make a choice between general and local anesthetic.
The main reasons for opting for general anesthetic are:1
- The procedure is likely to take a long time
- There is a likelihood of significant blood loss
- Breathing may be affected - in an operation on the chest, for example
- The procedure will make the patient feel uncomfortable.
The purpose of general anesthetic is to induce:
- Analgesia - remove natural response to pain
- Amnesia - memory loss
- Immobility - removal of motor reflexes
- Skeletal muscle relaxation.
Risks of general anesthesia
Overall, general anesthesia is very safe. Even particularly ill patients can be safely anesthetized; it is the surgical procedure itself which offers the most risk.
Modern general anesthesia is an incredibly safe intervention.
Some specific conditions increase the risk to the patient undergoing general anesthetic:
- Obstructive sleep apnea - a condition where individuals stop breathing while asleep
- Existing heart, kidney or lung conditions
- High blood pressure
- History of reactions to anesthesia
- Medications that can increase bleeding - aspirin, for example
- Drug allergies
- Obesity or overweight.
Death due to general anesthetic does occur, but only very rarely - roughly 1 in every 100,000-200,000.
Unintended intraoperative awareness
Unintended intraoperative awareness refers to rare cases where patients report a state of awareness during an operation, after the point at which the anesthetic should have removed all sensation. Some patients are conscious of the procedure itself and some can even feel pain.
Unintended intraoperative awareness is incredibly rare, affecting an estimated 1 in every 19,000 patients undergoing general anesthetic.
Because of the muscle relaxants given alongside anesthesia, patients are unable to signal to their surgeon or anesthetist that they are still aware of what is happening.
Unintended intraoperative awareness is more likely during emergency surgery.
Patients that experience unintended intraoperative awareness can suffer long-term psychological problems. Most often, the awareness is short-lived and occurs prior to the procedure.2
According to a recent large-scale investigation of the phenomenon, patients experienced "tugging, stitching, pain, paralysis and choking," among other sensations.
Because unintended intraoperative awareness is so infrequent, it is not clear exactly why it occurs. The following are considered to be potential risk factors:
- Heart or lung problems
- Daily alcohol use
- Emergency surgery
- Cesarean section
- Anesthesiologist error
- Use of some additional medications
Before general anesthesia is administered, patients will have a pre-surgery assessment to determine the most appropriate drugs to use, the quantities of those drugs and in which combination.
Some of the factors to be explored in a pre-surgical evaluation include:
- Body mass index (BMI)
- Medical history
- Current medications
- Fasting time
- Alcohol or drug intake
- Pharmaceutical drug use
- Mouth, dental and airway inspection
- Observation of neck flexibility and head extension.
It is essential that these questions are answered accurately; for instance, if a history of alcohol or drug use is not mentioned, an inadequate amount of anesthesia might be given which could lead to dangerously high blood pressure or unintended intraoperative awareness.
Side effects of general anesthesia
Nausea is a common side effect of general anesthesia.
There are a number of potential side effects; some individuals may experience none, others a few. None of the side effects are particularly long-lasting and tend to occur straight after the anesthesia.34
Side effects of general anesthesia include:
- Confusion and memory loss - more common in the elderly, it is not necessarily permanent
- Difficulty passing urine
- Bruising or soreness from the IV drip
- Nausea and vomiting5
- Shivering and feeling cold
- Sore throat (due to the breathing tube).
How does general anesthetic work?
The exact mechanisms that conspire to produce the state of general anesthesia are not well known. The general theory is that their action is induced by altering the activity of membrane proteins in the neuronal membrane, possibly by making certain proteins expand.
Of all the drugs used in medicine, general anesthetics are a rather strange case; rather than a single molecule acting at a single site to produce a response, there is a huge variety of compounds, all of which generating quite similar but widespread effects: analgesia, amnesia and immobility.
General anesthetic drugs range from the simplicity of alcohol (CH3CH2OH) to the complexity of sevoflurane (1,1,1,3,3,3-hexafluoro-2-(fluoromethoxy)propane). It seems unlikely that just one specific receptor could be activated by such different molecules.
General anesthetics are known to act at a number of sites within the central nervous system (CNS). The importance of these sites on the induction of anesthesia is not fully understood but they include:
There are multiple sites that general anesthetics can work at in the brain.
- Cerebral cortex: the brain's outer layer involved in tasks relating to memory, attention, perception among other functions
- Thalamus: roles include relaying information from the senses to the cerebral cortex and regulating sleep, wakefulness and consciousness
- Reticular activating system: important in regulating sleep-wake cycles
- Spinal cord: passes information from the brain to the body and vice versa. It also houses circuitry that controls reflexes and other motor patterns.
A number of different neurotransmitters and receptors are also known to be involved in general anesthesia:
- N-Methyl-D-aspartic acid (NMDA) receptors: some general anesthetics bind to NMDA receptors, including ketamine and nitrous oxide (N2O). They are known to be important in controlling synaptic plasticity and memory functions
- 5-hydroxytryptamine (5-HT) receptors: normally activated by the neurotransmitter serotonin, they play a part in controlling the release of a number of other neurotransmitters and hormones
- Glycine receptor: glycine can act as a neurotransmitter and has a number of roles; it has been shown to improve sleep quality.6
Although general anesthetics hold many mysteries, their importance in surgery and the field of medicine at large cannot be overstated.
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