To the Editor:-Cerebral air emboli can produce neuropsychologic deficits after cardiac surgery. [1,2]O'Brien and colleagues [3]demonstrated with ultrasonography that this risk is substantial and occurs frequently in children.

Carbon dioxide, which is some 300 times more soluble than nitrogen, (partition coefficient N20.0147; CO24.0 at 37 [degree sign]C) might substantially reduce the risk of damage with gas microemboli. [4,5]However, the most effective method for replacing air by carbon dioxide was not described. We found, [6]in models of a chest cavity, that 5 1/min CO2flowing into the field produced the highest carbon dioxide concentration (70 - 80%). Higher flows did not improve the elimination of air. Two jets larger than 0.5 cm in diameter were optimum; smaller jets were unsatisfactory, causing turbulence and entraining air. The jets should be directed downward into the thoracic cavity aimed at the central region. Although suction rapidly reduces the carbon dioxide concentration, after suction ceased, these returned within 1 min. The method could be confined to the period just before closure of the heart or the great vessels.

In 1968, we recommended that a controlled trial, using these conditions, should be undertaken. However, there were problems in measuring the outcome variables. Since then, methods for detecting neuropsychiatric lesions and cerebral microemboli have been developed. It should be relatively easy to test this simple preventative measure.

Michael Rosen, C.B.E., HON.LL.D, F.R.C.A., F.R.C.O.G., F.R.C.(Eng)

University of Wales College of Medicine; Cardiff, United Kingdom

(Accepted for publication March 16, 1998.)

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