Accuracy of prediction of fresh gas flow requirements during spontaneous breathing with the T-piece.

In 40 spontaneously breathing children (7.3-47.9 kg) anaesthetized with halothane for minor surgical procedures the fresh gas flow (FGF) at onset of rebreathing (FGFr) was determined and end-tidal CO2 concentration (ETCO2), minute ventilation (VE), tidal volume (VT) and respiratory rates (f) were registered. The accuracy of predicting the FGFr from 2 X VE, 3 X VE and from two formulae (FGF = 15 X kg X f and FGF = 3 X (1000 + 100 X kg) was evaluated. FGFr ranged from 3.5 to 10 l min-1. FGF calculated from 2 X VE was inadequate. Calculations of FGF from 3 X VE and with the two formulae gave an adequate FGF in more than 80% of the children. No serious under-estimations were found. In a few cases FGF level was overestimated by more than 150%. It is suggested that when the theoretical calculation of FGF results in flow rates well over 10 l min-1 an upper flow rate limit of 10 l min-1 may be used in children weighing less than 30 kg since no child required a FGF over this rate.