Comparison of the Bain system and Uniflow universal anaesthetic breathing systems in spontaneously breathing young pigs

Abstract

ObjectiveTo compare minimum fresh gas flow (Vmin) requirements and respiratory resistance in the Uniflow and Bain anaesthetic breathing systems used in the Mapleson D mode. AnimalsSeven pigs, aged 8–12 weeks, anaesthetized for ophthalmic surgery. Materials and methodsAnaesthesia was maintained with halothane delivered in oxygen using a (Mapleson D) Bain breathing system. The Vmin that prevented re-breathing was found, and peak inspiratory (PIP) and peak expiratory (PEP) pressures measured. The fresh gas flow (Vf) was then increased to Vmin + 50%, then Vmin + 100%, and respiratory pressures re-measured. A heat and moisture exchanger (HME) was inserted at the endotracheal tube and the procedure repeated. The breathing system was then exchanged for a Uniflow and the protocol repeated. After final disconnection from the breathing system, the animals’ peak inspiratory and expiratory flows, tidal, and minute volumes (Vm) were measured over five respiratory cycles. ResultsThe Vmin (L minute−1; mL kg−1 minute−1) required to prevent rebreathing in the Uniflow system [8.1(mean) ±1.7 (SD); 332 ± 94] was significantly greater than the Bain system (6.5 ± 1.1; 256 ± 64). At Vmin, PEP with the Uniflow (3.5 ± 0.1 cmH2O) was significantly higher than the Bain system (2 ± 0.7 cmH2O), but PIP values did not differ (Uniflow −0.6 ± 2.1 cmH2O; Bain system −0.2 ± 0.6 cmH2O). With both systems, PEP increased significantly (p < 0.001) with each increase in Vf: Uniflow system 4.2 ± 0.4 (Vmin + 50%) and 5.5 ± 0.5 cmH2O (Vmin + 100%); Bain system 2.8 ± 0.7 (Vmin + 50%) and 3.5 ± 0.7 cmH2O (Vmin + 100%). Insertion of the HME did not alter pressures. The mean tidal volume was 6.4 ± 1.6 mL kg−1; mean Vm was 184.9 ± 69.8 mL kg−1 and mean respiratory rate was 28 ± 5 breaths minute−1. In one pig breathing with the Uniflow system PEP rose sharply; respiratory and heart rates increased, and ventricular dysrhythmias occurred. When the system was changed and Vf reduced, physiological variables became normal. ConclusionThe study discredited the hypothesis that the two breathing systems behave similarly. Values for Vmin and PEP were higher with the Uniflow system. Increasing Vf increased PEP with both systems. Insertion of an HME did not affect respiratory pressures. Clinical relevanceThe Uniflow used in Mapleson D mode is not suitable for anaesthesia in young spontaneously breathing pigs.