The state of sedation, analgesia, amnesia and muscle paralysis is called general anesthesia. In other words, general anesthesia is an induced, reversible and controlled loss of consciousness. This state is necessary to perform surgical procedures that would otherwise cause excruciating pain and potentially generate a stress response from the body. This in turn could hamper attempts to surgically manage the patient, and more often than not, result in traumatic memories for the patient.
The mechanism of action of general anesthesia is not entirely clear. However, it is known that signals along the nerves responsible for passaging stimuli are interrupted and fail to be processed by the central nervous system after the administration of a general anesthetic.
Patients placed under general anesthesia are not capable of recalling what happens intraoperatively and are not aroused by painful stimuli. Put simply, patients are sent into a deep sleep with no consciousness of what is going on around them or happening to them. Furthermore, they require protection of the airways and/or mechanical ventilation, because muscle paralysis inhibits adequate spontaneous ventilation. Hemodynamic processes are another group of key clinical parameters that is changed when a patient is put under general anesthesia. This is usually a secondary effect of the agents used to induce or maintain general anesthesia, which may cause depressant or stimulatory cardiovascular consequences.
General anesthesia begins with pre-medicating the patient. This is medication administered prior to the surgery with the aim of relaxing the patient and possibly inducing amnesia so that they do not have any recall of even entering the operating theatre. Benzodiazepines are most commonly used for premedication. Other agents are used based on necessities that are unique to the patient. For example, a patient at risk for gastroesophageal reflux may receive antacids as part of pre-medication.
Following pre-medication, the patient is given induction anesthesia, which, as we may colloquially say, is responsible for putting the patient to sleep. The anesthesiologist (i.e. the physician-specialist responsible for administering anesthesia) ensures that the patient has a secured airway, all physiological monitors are properly functioning and maintenance and resuscitation agents are at hand. Intravenous (IV) agents like propofol, or anesthetic vapors, or a combination of the two, may be used to induce anesthesia. Analgesics (pain relievers) are also given in most cases in addition to the induction agent.
Once the effect of the drugs given to induce anesthesia begins to wane, maintenance drugs are necessary. This maintenance phase is a stable one during anesthesia and the drugs may be administered via the same routes as the induction drugs (i.e.intravenously or via inhalation). Towards the end of the surgery, the anesthesiologist begins to decrease the administered dose of these maintenance drugs or may stop them completely. Specific agents are used to reverse the muscle-relaxing drugs and the patient is administered analgesics to reduce post-operative pain. Artificial airway support is removed after the patient has demonstrated adequate spontaneous ventilation.
The most obvious advantage is the elimination of the sensory capacity to feel pain during the surgical procedure, which might otherwise be unbearable. This is not only beneficial to the patient, but also to the surgeon, who would otherwise have a hard time dealing with the body’s physiological response to stress. This response could cause significant morbidity and mortality if not pre-emptively dealt with by general anesthesia. Amnesia is necessary for reducing and/ or eliminating intraoperative recall, because the average person is not able to withstand the thought of being cut up or cut into, despite not feeling it or seeing it. This experience could subsequently be a source of intensely unpleasant memories.
General anesthesia is easily and rapidly administered, is reversible and can be used for surgeries that are unpredictable in extent.
On the other hand, some disadvantages of general anesthesia are that it may cause side effects, such as nausea, vomiting, headache and a delay in the return of normal memory functioning. Anesthesiologists may sometimes have to deal with severe fluctuations in physiological parameters once a patient is put under general anesthesia. One of the most serious, but thankfully rare, complications associated with general anesthesia is malignant hyperthermia (an inherited condition that causes hyperkalemia, metabolic acidosis, hypercarbia and lethal increase in body temperature in response to a reaction to the anesthetic medication).