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Intravenous Anesthetics

(Originally posted 28 January 2001 on About Anesthesiology)

Intravenous anesthetics represent one broad class of drugs often used in anesthesia. This class of drugs is administered via the intravenous route - that is, directly into the patient's bloodstream. This allows them to reach a therapeutic level quickly and affect the brain quickly.

In most general anesthetics, it is standard practice to induce, or begin, anesthesia with an intravenous agent because it is quick, reliable and convenient.

The broad class of intravenous anesthetics can be further broken down into opioids (also known as narcotics), non-opioids and muscle relaxants.

Opioids
The opioid anesthetics are more commonly known as narcotics. They are used mainly for analgesia, i.e. pain relief. This class of drugs has been used throughout history for this purpose, and also in higher doses to decrease anxiety and cause unconsciousness.

The notable effects of this class of drugs are analgesia, sedation, and respiratory depression. These effects are dose related, which means that the more medication you give, the more effect you get. The effects of sedation and respiratory depression does limit, to some extent, the usefulness of these drugs. One cannot give an unlimited amount of pain relief - the dose is limited by the need to keep a patient awake and./or breathing.

In addition, narcotics are valuable in anesthesia because they do not directly depress cardiac function. Therefore, this class of drugs is useful for anesthesia in patients with heart disease.

Side effects of this class of medications include nausea and vomiting, chest wall rigidity, seizure activity and suppresion of gastrointestinal motility.

In patients who are hypovolemic, narcotics may unmask this hypovolemia through vasodilatory effects (this is especially true of morphine). In addition, narcotics cause bradycardia due to direct vagal stimulation. In normal patients this bradycardia does not drop blood pressure due to an increase in stroke volume.

The mechanism of action of opioids involves their interaction with opioid receptors in the brain (amygdala) and spinal cord. Different types of opioid receptors have been identified. Mu receptors are responsible for analgesia, respiratory depression, euphoria and physical dependence. Kappa receptors are responsible for analgesia at the spinal cord level, sedation and miosis. Other receptors are responsible for minor effects and some of the negative effects of opioids.

Commonly used drugs in this class include morphine, meperidine (Demerol), fentanyl, sufentanil, alfentanil and remifentanil. They differ in terms of potency, duration of action, etc. and different agents are selected on the basis of these different properties.

The effects of opioids can be reversed by the use of opioid antagonists - drugs that compete for the same receptors and block the opioids from exerting their efffects. Naloxone (Narcan) is one example of a drug that does this.

Next page ---> Non-opioid Agents Page 1, 2, 3, 4

 

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