Fig 3221

Mode of action of Mapleson D breathing system during controlled ventilation. See text for details. PGF, fresh gas flow.

of anaesthetic and alveolar gas from the corrugated outer tube into the patient's lungs; during expiration, the ventilator gas and some of the alveolar gas are vented through the exhaust valve of the ventilator. The degree of rebreathing is regulated by the anaesthetic gas flow rate; a flow of 70-80 ml kg-1 min-1 should result in normocapnia and a flow of 100 ml kg"1 min-l in moderate hypocapnia. A secure connection between the Bain system and the anaesthetic machine must be assured. If this connection is loose, a leak of fresh gas occurs; this causes rebreathing of ventilator gas and results in awareness, hypoxaemia and hypercapnia.

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