Anesthesia and Anesthetic Drug Actions
Aesthesia is the depression or numbing of nerve pathways in all or part of the nervous system. The effect of anesthesia is the loss of sensation, principally the loss of pain. Thus, anesthesia functions to keep a patent free from pain during surgery.
A second hallmark of an anesthetic is its reversibility. The loss of sensation induced by an anesthetic is only temporary; the times vary depending on the anesthetic.
Anesthesia has accompanied surgical procedures for hundreds of years. Ether—until relatively recently a popular anesthetic, until its flammable nature proved too dangerous--was first made in 1540. Injectable anesthetics were in use by the mid-seventeenth century, and nitrous oxide (also commonly called laughing gas) was inhaled as both an anesthetic and for social frivolity by the early years of the nineteenth century.
There are four basic categories of anesthesia; general, regional, local and sedation. General anesthesia affects the brain cells, producing unconsciousness. Regional anesthesia affects a large bundle of nerves to a certain area of the body. Sensation in that area of the body is lost, but consciousness is unaffected. Regional anesthesia is often employed in childbirth, producing a loss of sensation and pain to the lower body during labor and delivery. Local anesthesia produces a loss in sensation in a very specific area of the body. Anesthetizing the gums in dental surgery or the testicular area in a vasectomy are two examples of local anesthesia. Finally, application of a lower concentration of an anesthetic can produce sedation; a sleep-like state. Nitrous oxide, also commonly called laughing gas, is a sedative.
While under general anesthesia, a patient is unaware of his surroundings, is immobile, has no memory of the time they are anesthetized, and is pain-free. Both the brain and the spinal cord are affected. General anesthetics can be administered as an inhaled gas (popular examples are nitrous oxide, sevoflurane, desflurane, isoflurane and halothane) or as an injected liquid (barbiturates, propofol, ketamine, etomidate, narcotics, and Valium-like drugs like benzodiazepines).
Patients under general anesthesia must be carefully monitored, as the drugs pass through the brain and other organs of the body. Heart rate and rhythm, blood pressure and respiration rate are examples of monitored parameters.
Different anesthetic drugs have different actions in the body and exert their effects for different periods of time. For example, halothane can cause the heart rate to slow down and the blood pressure to decrease, while desflurane can have the opposite effects.
Anesthetics also vary with respect to their basis of action. A local anesthetic, which is typically applied by injection of the drug just underneath the designated area of skin, blocks nerve impulses by decreasing the permeability of nerve membranes to sodium ions. General anesthetics are thought to act on proteins and membrane channels that are involved in the transfer of information from one neuron to another (neurotransmitters). But, still after all these years of use, little is known of the exact molecular basis of action of general anesthetic drugs. Recent experiments on goldfish indicate that general anesthetics swell the proteins that form the membrane channels, and that this swelling is what blocks the transfer from neuron to neuron.
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