FLOW REQUIREMENTS AND REBREATHING DURING MECHANICALLY CONTROLLED VENTILATION IN A T-PIECE (MAPLESON E) SYSTEM

The influence of fresh gas flow (FGF) setting on rebreathingwas investigated in 15 infants and children (weight 3.5–21.8kg) during balanced anaesthesia with mechanically controlledventilation using a T-piece (Mapleson E) system and a Nuffieldventilator 200. Tidal volume (VT), minute volume (VE), maximalinspired (PlCO₂) and end-tidal (PE'_co2) carbon dioxide tensionsand airway pressure were measured. VE, set to produce a PE'_CO2of about 4.5 kPa and measured at a high FGF (minimal rebreathing),was unchanged throughout the study and the regression equationfor VE and weight was: VE (ml min^–1) = 146x kg ³482, r = 0.92. Measurements were then repeated at FGF: VE ratiosreduced to 1.5 and 1.0. To achieve minimal rebreathing (Pl_CO2less than 0.5 kPa), FGF: VE ratios greater than 1.8 (range 1.8–4.9)had to be used. At FGF: VE ratios of 1.5, some alveolar rebreathingoccurred, indicated by increased inspired (P < 0.001) andend-tidal (P < 0.001) carbon dioxide tensions. At FGF: VEratios equal to 1.0, alveolar rebreathing was more pronouncedand hypercapnoea occurred with a PE'_CO2 (mean ± 1 SD)of 5.89±0.53 kPa. At this FGF setting, change in I: Eratio from 1: 2 to 1: 1 did not influence the level of alveolarrebreathing. A minimal FGF (ml min^–1) setting of 1.5 xVE (that is, 1.5 (146 x kg ³ 482), approximated to theexpression (200 x kg ³ 1000) is recommended for controlledventilation to avoid hypercapnoea when using the T-piece systemin children weighing less than 20 kg.