General anaesthetic facts for kids
General anaesthetics (or anesthetics) are often defined as compounds that induce a loss of consciousness in humans or loss of righting reflex in animals. Clinical definitions are also extended to include an induced coma that causes lack of awareness to painful stimuli, sufficient to facilitate surgical applications in clinical and veterinary practice. General anaesthetics do not act as analgesics and should also not be confused with sedatives. General anaesthetics are a structurally diverse group of compounds whose mechanisms encompass multiple biological targets involved in the control of neuronal pathways. The precise workings are the subject of some debate and ongoing research.
General anesthetics elicit a state of general anesthesia. It remains somewhat controversial regarding how this state should be defined. General anesthetics, however, typically elicit several key reversible effects: immobility, analgesia, amnesia, unconsciousness, and reduced autonomic responsiveness to noxious stimuli.
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Mode of administration
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General anaesthetics can be administered either as gases or vapours (inhalational anaesthetics), or as injections (intravenous or even intramuscular). All of these agents share the property of being quite hydrophobic (i.e., as liquids, they are not freely miscible—or mixable—in water, and as gases they dissolve in oils better than in water). It is possible to deliver anaesthesia solely by inhalation or injection, but most commonly the two forms are combined, with an injection given to induce anaesthesia and a gas used to maintain it.
Mechanism of action
Induction and maintenance of general anesthesia, and the control of the various physiological side effects is typically achieved through a combinatorial drug approach. Individual general anesthetics vary with respect to their specific physiological and cognitive effects. While general anesthesia induction may be facilitated by one general anesthetic, others may be used in parallel or subsequently to achieve and maintain the desired anesthetic state. The drug approach utilized is dependent upon the procedure and the needs of the healthcare providers.
It is postulated that general anaesthetics exert their action by the activation of inhibitory central nervous system (CNS) receptors, and the inactivation of CNS excitatory receptors. The relative roles of different receptors is still under debate, but evidence exists for particular targets being involved with certain anaesthetics and drug effects.
Stages of anesthesia
During administration of an anesthetic, the receiver goes through different stages of behavior ultimately leading to unconsciousness. This process is accelerated with intravenous anesthetics, so much so that it is negligible to consider during their use. The four stages of anesthesia are described using Guedel's signs, signifying the depth of anesthesia. These stages describe effects of anesthesia mainly on cognition, muscular activity, and respiration.
Stage I: Analgesia
The receiver of the anesthesia primarily feels analgesia followed by amnesia and a sense of confusion moving into the next stage.
Stage II: Excitement
Stage II is often characterized by the receiver being delirious and confused, with severe amnesia. Irregularities in the patterns of respiration are common at this stage of anesthesia. Nausea and vomiting are also indicators of Stage II anesthesia. Struggling and panic can sometimes occur as a result of delirium.
Stage III: Surgical Anesthesia
Normal breathing resumes at the beginnings of Stage III. Nearing the end of the stage, breathing ceases completely. Indicators for stage III anesthesia include loss of the eyelash reflex as well as regular breathing. Depth of stage III anesthesia can often be gauged by eye movement and pupil size.
Stage IV: Medullary Depression
No respiration occurs in stage IV. This is shortly followed by circulatory failure and depression of the vasomotor centers. Death is common at this stage of anesthesia if no breathing and circulatory support is available.
Physiological side effects
Aside from the clinically advantageous effects of general anesthetics, there are a number of other physiological consequences mediated by this class of drug. Notably, a reduction in blood pressure can be facilitated by a variety of mechanisms, including reduced cardiac contractility and dilation of the vasculature. This drop in blood pressure may activate a reflexive increase in heart rate, due to a baroreceptor-mediated feedback mechanism. Some anesthetics, however, disrupt this reflex.
Patients under general anesthesia are at greater risk of developing hypothermia, as the aforementioned vasodilation increases the heat lost via peripheral blood flow. By and large, these drugs reduce the internal body temperature threshold at which autonomic thermoregulatory mechanisms are triggered in response to cold. (On the other hand, the threshold at which thermoregulatory mechanisms are triggered in response to heat is typically increased.)
Anesthetics typically affect respiration. Inhalational anesthetics elicit bronchodilation, an increase in respiratory rate, and reduced tidal volume. The net effect is decreased respiration, which must be managed by healthcare providers, while the patient is under general anesthesia. The reflexes that function to alleviate airway obstructions are also dampened (e.g. gag and cough). Patients are especially prone to asphyxiation while under general anesthesia. Healthcare providers closely monitor individuals under general anesthesia and utilize a number of devices, such as an endotracheal tube, to ensure patient safety.
General anesthetics also affect the chemoreceptor trigger zone and brainstem vomiting center, eliciting nausea and vomiting following treatment.
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See also
- Local anaesthesia
- Mechanical ventilation
- Intraoperative awareness
- History of general anesthesia
