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The effects of continuous pentobarbital anesthesia on certain features of the cardiovascular system were investigated in trained male dogs. Similar results were obtained in 10 animals using the pressure-pulse contour method and in 5 animals by the dye dilution technique. Cardiac output fell progressively during the first 2–3 hours, the maximum fall exceeding 44% of control value, and was statistically significant from the 1-hour observation throughout the remainder of the 7-hour experimental period. Except for the brief period during induction, mean blood pressure did not deviate markedly from that of the unanesthetized animal. The fall in hematocrit during the 1st hour was followed by a slow partial recovery. Plasma proteins decreased both in concentration and absolute amount, while plasma volume showed a slight rise. The widely used anesthetic dose of 30 mg/kg seems needlessly high, and may be expected to induce cardiovascular changes even greater than those reported herein. This study has demonstrated clearly that a severe decrease in cardiac output occurs under continuous pentobarbital anesthesia in the dog. While the mechanism responsible for this fall has not been elucidated, it appears likely that a direct action on the myocardium may be involved. From the data presented it seems clear that published works which did not include specific control experiments for the effect of the anesthetic agent used require critical re-evaluation. This precaution is especially necessary in cardiovascular experiments involving pentobarbital anesthesia, since the data here reported indicate that this anesthetic agent itself produces marked changes in the cardiovascular system.

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