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Intravenous Anesthetics: Benzodiazepines and more... (page 3)

Benzodiazepines (cont.)
The benzodiazepines have a much longer duration of action than, for example. thiopental. Therefore, they are not usually used for induction of anesthesia - although they can be used in this way for longer surgeries.

This class of drugs produces its effects by increasing, or potentiating, the effects of the neurotransmitter GABA at the GABA receptor. GABA stands for gamma amino-butyric acid and is an inhibitory neurotransmitter.

The benzodiazepines have minimal effects on the heart and blood pressure - decreases in blood pressure are usually an indication of reduced anxiety in the patient. These drugs do have some potential for respiratory depression, particularly when used in combination with narcotics.

Flumazenil reverses the effects of the benzodiazepines by competing for the GABA receptor and preventing the drug from binding.

This drug is a substituted imidazole ring in terms of chemical tructure. It is a newer agent that was introduced for induction of anesthesia with less depressant effects on the cardiovascular system. It also has less respiratory depression that the barbiturates.

In terms of onset of action, doses of 0.2-0.4 mg/kg produce unconsciousness in less than one minute. As with the barbiturates, a short duration of action is due to its rapid redistribution to other tissues from the brain.

Significant side effects prevent etomidate from being commonly used in all patients. These effects include pain on injection (due to the drug being prepared in propylene glycol), myoclonus and temporary suppression of the adrenal gland. It also seems to have a higher incidence of post-operative nausea and vomiting than other agents.

Ketamine is similar in structure to the street drug phenycyclidine (PCP) and produces what is known as a "dissociative state" - this is a state in which the patient has analgesia, amnesia and is "dissociated" from external events. Commonly the eyes will be open with a slow nystagmus - and the patient will be unresponsive to external stimuli.

Ketamine results in sympathetic stimulation - therefore heart rate, blood pressure, cardiac output, etc. all go up. Respiratory function is usually not depressed and laryngeal reflexes are maintained.

One serious drawback of ketamine is that unpleasant dreams and hallucinations may occur as the patient is waking up. Using benzodiazepines such as Versed in conjunction with ketamine can reduce or eliminate these unfortunate side effects.

Propofol is a substituted phenol which also has rapid onset and short duration of action. It can be used in a similar manner to barbiturates and etomidate as an induction agent. The usual dose for this is 1.5-2.5 mg/kg which produces unconsciousness within a minute.

Propofol is rapidly redistributed like the barbiturates but it is also rapidly metabolized by the body. The fast redistribution is reponsible for its initial short duration of action. The rapid metabolism means that, unlike the barbiturates, propofol does not build up in the tissues. As a result, propofol can be used as a continuous infusion for maintenance of unconsciousness without prolonging wake-up.

Propofol comes as a soybean emulsion. This emulsion does support bacterial growth, so care must be taken to maintain the drugs sterility and to use freshly prepared drug. This emulsion is also responsible for some pain on injection.

Propofol will decrease blood pressure by dilating blood vessels, but otherwise has few other side effects.

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