Cardiovascular and respiratory effects of three rapidly acting barbiturates in dogs
Authors: Turner Donald M. , Ilkiw Jan E. 
Published: January 2024

The cardiovascular and respiratory effects of 3 rapidly acting barbiturates, thiopental sodium, thiamylal sodium, and methohexital sodium, were studied in dogs from completion of injection until 12.5 minutes after injection. The doses administered were 19.4 mg of thiopental/kg of body weight, 18.4 mg of thiamylal/kg, and 9.7 mg of methohexital/kg, which were chosen as equipotent doses necessary to inhibit the laryngoscopic reflex in 50% of the population. To determine the cardiovascular and respiratory effects for each drug, the values at each measurement time following injection were compared with baseline values (T0). At the 15- and 30-second measurement times following thiopental administration, stroke volume (sv) decreased; heart rate (hr), left atrial pressure, and mean pulmonary arterial pressure increased; and cardiac index (ci), myocardial contractility, and systemic and pulmonary vascular resistances were not different from baseline values. Mean arterial pressure (map) was not different from the baseline value at 15 seconds, but was increased from 30 seconds to 2 minutes. All values except hr had returned to baseline values by 7.5 minutes. At all measurement times, arterial oxygen tension and arterial pH were decreased, and arterial carbon dioxide tension increased from baseline values.

Although the cardiovascular and respiratory changes following administration of thiamylal and methohexital were similar to those described for thiopental, some differences were found. Following thiamylal administration, systemic vascular resistance increased at 1 minute, pulmonary vascular resistance increased at 1 and 2 minutes, and myocardial contractility increased at 1 and 2 minutes. Following methohexital administration, map decreased at 15 seconds, and sv decreased at all measurement times. Cardiac index increased at 30 seconds, 1 minute, and 2 minutes; myocardial contractility increased at 1, 2, and 2.5 minutes; and blood-gas and pH had returned to baseline by 12.5 minutes. To determine differences between drugs, the cardiovascular and respiratory values for each drug were compared at each measurement time. Changes in hr and sv induced by the 3 drugs were similar at all measurement times. Mean arterial pressure at 15 and 30 seconds was lower following methohexital administration than after thiopental or thiamylal administration. Cardiac index was higher at 1 minute following methohexital administration, compared with that after thiamylal administration, whereas systemic vascular resistance was higher at 1 minute following thiamylal administration, compared with that after methohexital administration. The increase in left atrial pressure was greater following thiamylal administration than after thiopental administration at 30 seconds to 5 minutes or after methohexital administration at 1 to 5 minutes. Mean pulmonary arterial pressure was significantly higher at 2 to 4 minutes following thiamylal administration than after methohexital administration. Pulmonary vascular resistance was higher at 1 minute following thiamylal administration, compared with that after thiopental and methohexital administration. At 1 and 2 minutes, myocardial contractility was significantly higher following methohexital administration, compared with that after thiobarbiturate administration. Arterial oxygen tension was lower at 12.5 minutes following administration of the thiobarbiturates, compared with that after methohexital administration. When compared with methohexital administration, arterial carbon dioxide tension was higher at 7.5 and 12.5 minutes following thiamylal administration. The decrease in pH following thiamylal administration was greater at all measurement times, compared with that after thiopental and methohexital administration.