12: Intravenous Anesthetics and Benzodiazepines

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CHAPTER 12 Intravenous Anesthetics and Benzodiazepines

Theresa C. Michel, MD

1 What qualities would the ideal intravenous induction agent possess?

The ideal agent would have a rapid onset, producing amnesia, analgesia, and hypnosis with hemodynamic stability. It would have few enduring side effects, few interactions with other medications, and allow for rapid recovery after surgery.

2 List the commonly used induction agents and their properties. Compare their cardiovascular effects

Sodium thiopental (STP) is a direct myocardial depressant, produces peripheral vasodilation resulting in decreased mean arterial pressure (MAP), and produces a reflex tachycardia.

Etomidate is an imidazole derivative the induction properties of which result from Gamma-amino butyric acid (GABA) receptor modulation. It is noted for its hemodynamic stability. Cardiac output (CO) and contractility are preserved, and only a mildly decreased MAP is noted. Side effects include pain on injection, nausea and vomiting, hiccups, myoclonus, seizures, thrombophlebitis, and most important, adrenal suppression.

Propofol: Myocardial depression and peripheral vasodilation are more profound than with thiopental and may result in a more significant decrease in MAP. There are minimal effects on heart rate (HR).

Ketamine: The sympathomimetic effects result in increased CO, MAP, and HR. It is a myocardial depressant, but usually its sympathomimetic properties dominate.

Midazolam is the principal benzodiazepine used perioperatively. Benzodiazepines provide anxiolysis, sedation, amnesia, and in high doses unconsciousness. Midazolam has minimal myocardial depression. There is no effect on MAP or CO; HR may increase slightly.

Opioids: High doses of most opioids have a vagolytic effect, producing bradycardia. The exception is meperidine, which has sympathomimetic effects that produce tachycardia. A decreased MAP may be noted secondary to bradycardia, vasodilatation, blockade of sympathetic response, and histamine release (especially evident with morphine and meperidine).

These effects are noted when these medications are individually administered. When given in concert (e.g., midazolam followed by an opioid followed by pentothal), the effects are synergistic, and hemodynamic instability is not guaranteed (Tables 12-1 and 12-2).

TABLE 12-1 Dosing Guidelines for Anesthetic Induction and Sedation*

TABLE 12-2 Cardiovascular Effects of Induction Agents

3 Instead of injecting pentothal intravenously, you have inadvertently administered it into the patient’s intra-arterial line. What is the impact on the patient and how should potential problems be addressed?

Intra-arterial STP is likely to cause significant discomfort, vasospasm, and possibly thrombus formation. Leave the catheter in place and inject dilute papaverine, procaine, or lidocaine through the catheter to inhibit smooth muscle vasospasm. If this does not re-establish perfusion, consider brachial plexus block or α-blockade. Also administer heparin to prevent thrombus formation.

4 What are contraindications to STP use?

Thiopental allergy: STP contains a sulfur molecule that results in an increased risk of histamine release. Patients allergic to sulfates should not receive STP.

Porphyria: Barbiturates increase porphyrin synthesis and may precipitate an acute porphyria attack with symptoms such as abdominal pain, weakness, autonomic dysfunction, and gastrointestinal and sleep disturbances.

Use with caution in patients with hypovolemia and poor systolic function because of its cardiovascular depressant effects. STP may be a poor choice in this scenario.

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