Ventilatory responses to rebreathing and carbon dioxide inhalation during anaesthesia in children

In 12 spontaneously breathing intubated children (9.3-25 kg), ventilatory responses to rebreathing and to the inhalation of carbon dioxide (CO2) were investigated during halothane anaesthesia for minor surgical procedures. A T-piece (Mapleson F system) was used, modified by the insertion of a pneumotachograph and a paediatric airway adaptor of an in-line capnograph in the patient limb. Exhaled gas was collected for determination of expired CO2 content. Measurements were made when the fresh gas flow (FGF) was at the borderline for rebreathing (FGFr) and during 10 min with a mean FGF 44% lower, producing a maximal inspired CO2 (ICO2 max) (%) of 1.45 +/- 0.38% (mean +/- 1 SD). Measurements were repeated 5 min after returning to a flow exceeding FGFr and then during CO2 inhalation for 10 min after the addition of 1.24 +/- 0.32% CO2 (mean +/- 1 SD) to this flow. During both rebreathing and CO2 inhalation end-tidal CO2 (E'CO2) was unchanged and VE did not increase significantly (18%), but during CO2 inhalation alveolar ventilation increased (P less than 0.05), indicating an adequate and intact response to this level of CO2 inhalation. Estimations of ICO2 max could be made from the expression: ICO2 max (%) = -0.7 X FGF/VE + 2.5 and FGF to minute ventilation (VE) ratios lower than 1 were found to produce ICO2 max of 1.8% or higher. Such low FGF are likely to result in rebreathing within the alveolar ventilation and are thus of clinical importance. We believe that to increase the margin of safety in anaesthetized spontaneously breathing children, FGF of at least 1.5 to 2 times VE should be used.