Anaesthesiologists have traditionally been consulted to help design breathing circuits to attain and maintain target end-tidal carbon dioxide (P ET CO 2). The methodology has recently been simplified by breathing circuits that sequentially deliver fresh gas (not containing carbon dioxide (CO 2)) and reserve gas (containing CO 2). Our aim was to determine the roles of fresh gas flow, reserve gas PCO 2 and minute ventilation in the determination of P ET CO 2. We first used a computer model of a non-rebreathing sequential breathing circuit to determine these relationships. We then tested our model by monitoring P ET CO 2 in human volunteers who increased their minute ventilation from resting to five times resting levels. The optimal settings to maintain P ET CO 2 independently of minute ventilation are 1) fresh gas flow equal to minute ventilation minus anatomical deadspace ventilation, and 2) reserve gas PCO 2 equal to alveolar PCO 2. We provide an equation to assist in identifying gas settings to attain a target PCO 2. The ability to precisely attain and maintain a target PCO 2 (isocapnia) using a sequential gas delivery circuit has multiple therapeutic and scientific applications.
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