Anesthetic Agents

Anesthetic agents depress the central nervous system causing a loss of consciousness. They are classified as general and local.

Anesthetic agents were introduced in surgery in the early 1800s in the form of nitrous oxide (laughing gas), which continues to be used today for dental procedures. Other anesthetic agents became widely used by the mid-1800s. These included ether and chloroform. Ether is a highly flammable liquid with a pungent odor that causes nausea and vomiting and is seldom used today. Chloroform is toxic to the liver and is no longer used.

General anesthetics are used for general surgery, cardiac surgery, neurosurgery, and pediatric surgery. They are administered by an anesthesiologist or a nurse anesthetist. They are inhaled through a mask or breathing tube.

A general anesthetic can consist of one medication or a combination of medications—called balanced anesthesia—depending on the patient's age, weight, medical history, general health, and allergies.

This balanced approach is used when administering general anesthetics to patients in phases to minimize cardiovascular problems, decrease the amount of general anesthetic needed, reduce possible post-anesthetic nausea and vomiting, minimize the disturbance of organ function, and increase recovery from anesthesia with fewer adverse reactions.

The night before the surgery, the patient is given a hypnotic to assist with a good night's sleep. On the day of the surgery, premedication may be given to the patient about one hour before surgery. Premedication typically consists of two medications. One is a benzodiazepine such as lorazepan (Ativan). This medication sedates and decreases anxiety. The other is an anticholinergic such as atropine to decrease secretions. A short-acting barbiturate such as thiopental sodium (Pentothal) is then administered in the operating room to induce anesthesia. The patient is then given inhaled gas and oxygen to maintain anesthesia. Sometimes the anesthetic is administered IV. Depending on the nature of the operation, the patient may also receive a muscle relaxant.

The patient experiences four stages of anesthesia, some of which are not observable because they occur rapidly. These stages are:

Stage one: analgesia

The patient experiences analgesia (a loss of pain sensation) but remains conscious and can carry on a conversation.

Stage two: excitement

The patient may experience delirium or become violent. Blood pressure rises and becomes irregular, and breathing rate increases. This stage is typically bypassed by administering a barbiturate such as sodium pentothal before the anesthesia.

Stage three: surgical anesthesia

Skeletal muscles relax. Breathing becomes regular. Eye movement slows then stops. It is at this point when surgery begins.

Stage four: medullary paralysis

Breathing and other vital functions cease to function because the respiratory center (medulla oblongata) is paralyzed. Death results if the patient is not revived quickly. Careful administration of the anesthesia prevents reaching this stage.

A list of Anesthetic Drugs is provided in the Appendix. Detailed tables show doses, recommendations, expectations, side effects, contraindications, and more; available on the book's Web site (see URL in Appendix).

Commonly administered intravenous anesthetic agents

Ketamine (Ketalar)

Affects the senses, and produces a dissociative anesthesia (catatonia, amnesia, analgesia) in which the patient may appear awake and reactive, but cannot respond to sensory stimuli. These properties make it especially useful in developing countries and during warfare medical treatment. Ketamine is frequently used in pediatric patients because anesthesia and analgesia can be achieved with an intramuscular injection. It is also used in high-risk geriatric patients and in shock cases, because it also provides cardiac stimulation.

Thiopental (Pentothal)

A barbiturate that induces a rapid hypnotic state of short duration. Because thiopental is slowly metabolized by the liver, toxic accumulation can occur; therefore, it should not be continuously infused. Side effects include nausea and vomiting upon awakening.

Opioids

Fentanyl, sufentanil, and alfentanil are frequently used prior to anesthesia and surgery as a sedative and analgesic, as well as a continuous infusion for primary anesthesia. Because opioids rarely affect the cardiovascular system, they are particularly useful for cardiac surgery and other high-risk cases. Opioids act directly on spinal cord receptors, and are frequently used in epidurals for spinal anesthesia. Side effects may include nausea and vomiting, itching, and respiratory depression.

Propofol (Diprivan)

Nonbarbiturate hypnotic agent and the most recently developed intravenous anesthetic. Its rapid induction and short duration of action are identical to thiopental, but recovery occurs more quickly and with much less nausea and vomiting. Also, propofol is rapidly metabolized in the liver and excreted in the urine, so it can be used for long durations of anesthesia, unlike thiopental. Hence, propofol is rapidly replacing thiopental as an anesthetic agent.

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