Flow requirements and rebreathing during mechanically controlled ventilation in a T-piece (Mapleson E) system

Abstract

The influence of fresh gas flow (FGF) setting on rebreathing was investigated in 15 infants and children (weight 3.5-21.8 kg) during balanced anaesthesia with mechanically controlled ventilation using a T-piece (Mapleson E) system and a Nuffield ventilator 200. Tidal volume (VT), minute volume (VE), maximal inspired (PICO2) and end-tidal (PE'CO2) carbon dioxide tensions and airway pressure were measured. VE, set to produce a PE'CO2 of about 4.5 kPa and measured at a high FGF (minimal rebreathing), was unchanged throughout the study and the regression equation for VE and weight was: VE (ml min-1) = 146 x kg + 482, r = 0.92. Measurements were then repeated at FGF:VE ratios reduced to 1.5 and 1.0. To achieve minimal rebreathing (PICO2 less 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 rebreathing occurred, indicated by increased inspired (P less than 0.001) and end-tidal (P less than 0.001) carbon dioxide tensions. At FGF:VE ratios equal to 1.0, alveolar rebreathing was more pronounced and hypercapnoea occurred with a PE'CO2 (mean +/- 1 SD) of 5.89 +/- 0.53 kPa. At this FGF setting, change in I:E ratio from 1:2 to 1:1 did not influence the level of alveolar rebreathing. A minimal FGF (ml min-1) setting of 1.5 x VE (that is, 1.5 (146 x kg + 482), approximated to the expression (200 x kg + 1000) is recommended for controlled ventilation to avoid hypercapnoea when using the T-piece system in children weighing less than 20 kg.