By means of a partial extracorporeal bypass, incorporating a bubble counting chamber, measured quantities of oxygen and of air microbubbles, having average diameters of 100 and 76 µ, respectively, were introduced into the carotid arteries of dogs. The desired number of bubbles was produced by varying the blood level and gas flow in the bubble oxygenator. Eighty per cent of dogs receiving oxygen in volumes up to 0.43 cc/kg (approximately 12.5 million microbubbles) survived without evidence of neurologic abnormalities. Seventy per cent of the control group, which received no microbubbles, survived. Of those perfused with air, all animals receiving up to 0.19 cc/kg (9.1 million bubbles) survived with no neurologic sequelae. Those perfused with air volumes greater than 0.19 cc/kg died within 24 hours, many exhibiting evidence of neurologic damage. The emphasis upon the introduction of oxygen microbubbles as the major fault of bubble oxygenators may be unwarranted. Air microbubbles are more deleterious than are oxygen microbubbles.

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