When given orally in sedative or hypnotic doses, the barbiturates do not produce significant overt cardiovascular effects except for a slight decrease in blood pressure and heart rate such as occurs in normal sleep. Cardiovascular reflexes are obtunded by partial inhibition of ganglionic transmission; this is most evident in patients with congestive heart failure or hypovolemic shock, whose reflexes already are operating maximally and in whom barbiturates can cause an exaggerated fall in blood pressure. Because barbiturates also impair reflex cardiovascular adjustments to inflation of the lung, positive-pressure respiration should be used cautiously and only when necessary to maintain adequate pulmonary ventilation in patients who are anesthetized or intoxicated with a barbiturate.
Intravenous anesthesia with barbiturates can increase the incidence of ventricular arrhythmias, especially when epinephrine and halothane also are present. Direct depression of cardiac contractility occurs only when doses several times those required to cause anesthesia are administered.
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