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

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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 (PlCO2) 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× kg ³ 482, r = 0.92. Measurements were then repeated at FGF: VE ratios reduced to 1.5 and 1.0. To achieve minimal rebreathing (PlCO2 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 < 0.001) and end-tidal (P < 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 × VE (that is, 1.5 (146 × kg ³ 482), approximated to the expression (200 × kg ³ 1000) is recommended for controlled ventilation to avoid hypercapnoea when using the T-piece system in children weighing less than 20 kg.