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 (FGF^r) 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 FGF^r 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 (ECO₂) was unchanged and $V˙E$ did not increase significantly (18%), but during CO₂ inhalation alveolar ventilation increased (P < 0.05), indicating an adequate and intact response to this level of CO₂ inhalation. Estimations of ICO₂ max could be made from the expression: lCO₂ max(%) = −0.7 x FGF/$V˙E$ +2.5 and FGF to minute ventilation ($V˙E$) ratios lower than 1 were found to produce lCO₂ 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 $V˙E$ should be used.