GENERAL ANAESTHETICS

GENERAL ANAESTHETICS

General anaesthetics are the agents which causes reversible loss of consciousness, so as to enable surgical operations to be carried out.

General anesthesia is essential to surgical practice, because it renders patients analgesic, amnesic, unconscious, and produce muscles relaxation and suppression of undesirable reflexes. No single drug is capable of achieving these effects both rapidly and safely. Pre-anesthetic medications serve to calm the patients, relieve pain, and protect against unwanted effects of the concurrently administered anesthetics or the surgical procedure. Skeletal muscle relaxant facilitates intubation and suppresses the muscle tone to the degree required for surgery. Potent general anesthetics are delivered via intravenous injection or inhalation.

Stages of anesthesia: Anesthesia can be divided into three stages: induction, maintenance, and recovery. Induction is defined as the period of time from the onset of administration of the anesthetic to the development of effective surgical anesthesia in the patient. Induction of anesthesia depends on how fast effective concentrations of the anesthetic drug reach the brain; recovery is the reverse of induction and depends on how fast the anesthetic drug diffuses from the brain. Maintenance provides a sustained surgical anesthesia. Recovery is the time from discontinuation of administration of the anesthesia until consciousness and protective physiologic reflexes are regained.

Depth of anesthesia: The depth of anesthesia has been divided into 4 stages. Each stage is characterized by increased central nervous system (CNS) depression, which is caused by accumulation of the anesthetic drug in the brain.

Stage I Analgesia: Loss of pain sensation results from interference with sensory transmission in the spinothalamic tract. The patient is conscious and conversational. Amnesia and a reduced awareness of pain occur as Stage II is approached.

Stage II Excitement: The patient experiences delirium and possibly combative behavior, violent. There is a rise and irregularity in blood pressure and increase in respiratory rate. To avoid this stage of anesthesia, a short-acting barbiturate, such as thiopental, is given intravenously before inhalation anesthesia is administered.

Stage III Surgical anesthesia: In this stage regular respiration and relaxation of the skeletal muscles occur. Eye reflexes decrease progressively, until the eye movements cease and the pupil is fixed. Surgery may proceed during this stage.

Stage IV Medullary paralysis: During this stage, severe depressions of the respiratory and vasomotor centers occur. Death can rapidly ensue unless measures are taken to maintain circulation and respiration.

Inhalation anesthetics:

Nitrous oxide: It is also known as laughing gas and it is a potent analgesic but a weak general anesthetic. Nitrous oxide is frequently employed at concentrations of 30% in combination with oxygen for analgesia, particularly in dental surgery. However, nitrous oxide at 80 percent cannot produce surgical anesthesia. It is poorly soluble in blood and other tissues, allowing it to move very rapidly in and out of the body. Under the usual circumstances, co-administration with other anesthetics, it also has moderate to no effect on the cardiovascular system or on increasing cerebral blood flow, and it is the least hepatotoxic of the inhalation anesthetics. It is therefore probably the safest of these anesthetics, provided that at least 20 percent oxygen is always administered simultaneously.

Halothane: halothane is a potent anesthetic; it is a relatively weak analgesic. This agent is the prototype to which newer inhalation anesthetics have been compared. When halothane was introduced, its ability to induce the anesthetic state rapidly and to allow quick recovery and the fact that it was non-explosive made it an anesthetic of choice.

Therapeutic uses: Halothane is usually co-administered with nitrous oxide, local anesthetics, or Opioids. Halothane relaxes both skeletal and uterine muscle, and it can be used in obstetrics when uterine relaxation is indicated.

Adverse effects: Cardiac arrhythmias, bradycardia, malignant hyperthermia, osteogenesis.

Isoflurane: This halogenated anesthetic is widely used in the United States. It is a very stable molecule that undergoes little metabolism; as a result, little fluoride is produced. Isoflurane produces concentration-dependent hypotension due to peripheral vasodilation. It also dilates the coronary vasculature, increasing coronary blood flow and oxygen consumption by the myocardium. This property may make it beneficial in patients with ischemic heart disease.

Enflurane: It has faster induction recovery than halothane. Enflurane is non inflammable and non irritating liquid with mild odour. It is contraindicated in patients with brain lesions and epilepsy.                                                                                                                

Desflurane: The rapidity with which desflurane causes anesthesia and emergence has made it a popular anesthetic for outpatient surgery. It has a low volatility and, thus, must b e delivered using a special vaporizer. It decreases vascular resistance and perfuses all major tissues very well. Because it is irritating to the airway and can cause excessive secretions and coughing.

Sevoflurane: It has low pungency, allowing rapid uptake without irritating the airway during induction, thus making it suitable for induction in children. It is replacing halothane for this purpose. The drug has low solubility in blood and is rapidly taken up and excreted. Recovery is faster than with other anesthetics. It is metabolized by the liver, releasing fluoride ions. Thus, like enflurane, it may prove to be nephrotoxic.

Intra venous anesthetics:

Barbiturates: Thiopental is a weak analgesic but a potent anesthetic. It is an ultra-short-acting barbiturate and has high lipid solubility. When agents such as thiopental and methohexital are administered intravenously, they quickly enter the CNS and depress function, often in less than 1 minute. The short duration of anesthetic action is due to the decrease of its concentration in the brain to a level below that necessary to produce anesthesia. These drugs may remain in the body for relatively long periods of time after their administration, because only about 15 percent of the dose of barbiturates entering the circulation is metabolized by the liver per hour. Thus, metabolism of thiopental is much slower than its tissue redistribution. It may contribute to severe hypotension in patients with hypovolemia or shock. All barbiturates can cause apnea, chest wall spasm, coughing, bronchospasm. Barbiturates are contraindicated in patients with acute intermittent or variegate porphyria.

Benzodiazepines/non barbiturates: The benzodiazepines are used in conjunction with anesthetics to sedate the patient. The most commonly employed is midazolam, which is available in many formulations, including oral. lorazepam and Diazepam are alternatives. All three facilitate amnesia while causing sedation.

Ketamine: It is a short-acting, non-barbiturate anesthetic, induces a dissociated state in which the patient is unconscious but appears to be awake and does not feel pain. This dissociative anesthesia provides sedation, amnesia, and immobility. Ketamine interacts with the N-methyl-D-aspartate receptor. It also stimulates the central sympathetic outflow, which, in turn, causes stimulation of the heart and increased blood pressure and cardiac output. This property is especially beneficial in patients with either hypovolemic or cardiogenic shock as well as in patients with asthma. The drug is lipophilic and enters the brain circulation very quickly. It is metabolized in the liver, but small amounts can be excreted unchanged.

Propofol: It is an intravenous sedative/hypnotic used in the induction or maintenance of anesthesia. Onset is smooth and occurs within about 40 seconds of administration. Supplementation with narcotics for analgesia is required. Whereas Propofol facilitates depression in the CNS, it is occasionally accompanied by excitatory phenomena, such as muscle twitching, spontaneous movement, or hiccups. Propofol decreases blood pressure without depressing the myocardium. It also reduces intracranial pressure.

Etomidate: It is a hypnotic agent but lacks analgesic activity and is used to induce anesthesia. Its water solubility is poor, so etomidate is formulated in a propylene glycol solution. Induction is rapid, and the drug is short-acting. It is only used for patients with coronary artery disease or cardiovascular dysfunction, such as shock. Etomidate is hydrolyzed in the liver. Its adverse effects include a decrease in plasma cortisol and aldosterone levels, which can persist for up to 8 hrs. Venous pain can occur.

Anesthesia, analgesic, amnesic, unconscious, induction, maintenance, and recovery.