By the time the child is 12 kg, this theoretically shouldn't be much of an issue.  In Glenn physiology, venous return from the head and upper extremities flows passively to the lungs.  Small children have heads that are large in proportion to their BSA, and the venous return from the head contributes significantly to their cardiac output.  A 4 month old who is immediately postoperative from a Glenn may have low cardiac output or low oxygen saturation if venous drainage from the head is impaired.  By the time the child is 12 kg, the contribution to total cardiac output from the head is less significant.  The oxygen saturation may be slightly lower in Trendelenburg during anesthesia, but if the sats are tolerable at the child's baseline, it will probably be fine.  Allowing mild hypercapnea during the case may be advantageous for oxygen saturation, since hypercapnea increases cerebral blood flow (and thus pulmonary blood flow in Glenn physiology).  Other techniques for improving oxygen saturation could include a fluid bolus or the use of inhaled nitric oxide. 

I agree with the previous speaker.

Is there an antegrade flow? Systemic ventricle is right or left or the patient's have two ventricle? I'm in part in accord with Dr Robert about the use of hypercarbia to make cerebral vasodylatation and increase venous retourn in the Glenn but in case of depressed function and antegrade flow, this situation can produce the risk of cerebral oedema?

Thanks

Dear recipients , thank you for your answers. The child has not antegrade flow and the cardiac function is quite good at "ophtalmic" evaluation;   at our dept the cavopulmonary anastomosis are routinely performed withoud additional source of pulmonary blood flow

Sincerely and Merry Christmas

I think that if the child went under Glenn procedure for a long time, and if the heart function is good, that's not a big issue. Correct volume and gas balance during the surgery should help.