1- To understand how the various types of anesthesia work, it is first necessary to understand how pain is transmitted. It is a sign that there is something wrong with a part of the body. And that signal travels from the injured part to the brain traveling along nerves, which are like wires throughout the body. The nerves are formed by a sequence of special cells, the neurons.
2- The interior of the neuron is electrically negative and the environment around it is positive. When we injure a part of the body, the neuron closest to the site opens the so-called sodium channels in its membrane, which allow sodium ions to enter. As these ions are positive, the interior of the cell gradually loses its negative state.
3- To return to the original state, the neuron opens its potassium channels for the exit of potassium ions (positive) from the cell. This in-and-out sequence becomes a chain reaction, passing from one neuron to another until it reaches the brain. There, this sequence of changes in neuronal charge is “translated” as a pain signal.
LOCAL ANESTHESIA
Substances such as lidocaine chemically act on neurons near the wound
1- In the case of a simpler injury, doctors can interrupt the pain signal transmission mechanism by applying local anesthesia. If you cut your hand, for example, you receive an injection with anesthetics in the cut region.
2- Local anesthetics have substances, such as lidocaine and bupivacaine, which chemically react with the neurons in the injured region, preventing the opening of sodium channels. Thus, the pain transmission process is interrupted right away.
GENERAL ANESTHESIA
Cocktail of drugs acts on brain neurons, blocking pain from any part of the body
1- Many times the patient needs to be immobile or unconscious so as not to disturb the medical intervention. That’s where general anesthesia comes in, which is a mixture of substances with various functions. It is applied into a vein, because through the bloodstream anesthetics reach the brain faster.
2- One of the substances in the general anesthesia cocktail has the function of preventing the pain signal from being deciphered in the brain. This substance, which can be a drug like remifentanil, opens receptors on brain neurons through which chlorine ions enter, which are negative.
3- Full of chlorine ions, brain neurons are always “in the negative” – even with that reaction initiated at the injured site. As the cells maintain their balance, the pain signal is not transmitted in the region. And, as it is the brain that «translates» this signal, the person does not feel pain in any part of the body.
4- In the general anesthesia cocktail there are still substances that lead to unconsciousness and muscle relaxants that prevent the patient from moving. Each drug acts on several types of brain neuron receptors, and the anesthesiologist administers their doses throughout the surgery.
REGIONAL ANESTHESIA
Spinal and epidural blocks pain transmission when the signal reaches the spinal cord
1- A compromise between local and general anesthesia is anesthesia applied in strategic regions of the body, such as spinal and epidural. This type of procedure interrupts the transmission of the pain impulse in the spinal cord, which is where most of the body’s nerves pass. The patient is then immune to pain from the navel down.
2- Epidural anesthesia is applied outside the spinal canal, in a layer of fat around the dura mater. As it is injected in a less invasive and risky place, it is possible to place a catheter and apply more anesthetic for several hours, according to the need of the surgery. But it causes less relaxation than the spinal cord.
3- Spinal anesthesia is applied to the dura mater, the membrane that surrounds the spinal cord. The doctor needs to feel that he has reached the exact injection site by feel! As it acts in a key space in the transmission of sensations, with little anesthetic a great relaxation is already achieved, which is good in more delicate surgeries.
CONSULTANCY: IRIMAR DE PAULA POSSO AND MARIA CARMONA, ANESTHESIOLOGISTS AT HOSPITAL DAS CLINICS DE SÃO PAULO