Precise Control of End-tidal Carbon Dioxide Levels Using Sequential Rebreathing Circuits

Abstract

Anaesthesiologists have traditionally been consulted to help design breathing circuits to attain and maintain target end-tidal carbon dioxide (P_ETCO₂). The methodology has recently been simplified by breathing circuits that sequentially deliver fresh gas (not containing carbon dioxide (CO₂)) and reserve gas (containing CO₂). Our aim was to determine the roles of fresh gas flow, reserve gas PCO₂ and minute ventilation in the determination of P_ETCO₂. 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_ETCO₂ in human volunteers who increased their minute ventilation from resting to five times resting levels. The optimal settings to maintain P_ETCO₂ independently of minute ventilation are 1) fresh gas flow equal to minute ventilation minus anatomical deadspace ventilation, and 2) reserve gas PCO₂ equal to alveolar PCO₂. We provide an equation to assist in identifying gas settings to attain a target PCO₂. The ability to precisely attain and maintain a target PCO₂ (isocapnia) using a sequential gas delivery circuit has multiple therapeutic and scientific applications.