Cardiovascular depression may occur after the administration of barbiturates by IV bolus. The hemodynamic changes are transient in the healthy patient with good cardiovascular reserve, but they may be prolonged and/or not well tolerated in elderly patients or those with poorly compensated myocardial function. For example, thiopental decreases myocardial contractility and dilates capacitance vessels, thereby reducing venous return to the heart. The healthy normovolemic patient may compensate for these changes by an increase in heart rate to maintain stroke volume and blood pressure. The patient with myocardial disease or hypovolemia may not be capable of appropriate compensation. Serious ischemic impairment of the myocardium may occur in patients with coronary artery disease.
Respiratory depression also may occur after the administration of barbiturates by IV bolus. Respiration may be further compromised by barbiturate-induced laryngospasm, as it is with most anesthetics.
There is some tendency of the ultra-short-acting barbiturates to precipitate at biological pH once they are injected, especially if the injection solution is not given slowly enough to allow the drug to be diluted by the venous blood. If inadvertent intraarterial injection occurs and drug precipitates are formed, arterial thrombosis, vasospasm, local ischemia, and possibly tissue sloughing may occur. Methohexital precipitation is less common, since it is a more potent barbiturate and can be provided in a more dilute solution. Barbiturate solutions must not be coadministered with acidic solutions, such as those containing meperidine, morphine, or ephedrine.
Most of the adverse reactions associated with the use of the intravenous barbiturates are predictable and therefore can be controlled or avoided. Some reactions, such as hypersensitivity, are entirely unpredictable. Particularly patients with asthma, urticaria, or an-gioedema may acquire allergic hypersensitivity to the barbiturates. Acute intermittent porphyria is an absolute contraindication to the use of barbiturates.
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