Evaluation of rebreathing in various modifications of the Mapleson-D system

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

OBJECTIVE

To compare minimum fresh gas flow (V(min)) requirements and respiratory resistance in the Uniflow and Bain anaesthetic breathing systems used in the Mapleson D mode. Animals Seven pigs, aged 8-12 weeks, anaesthetized for ophthalmic surgery.

MATERIALS AND METHODS

Anaesthesia was maintained with halothane delivered in oxygen using a (Mapleson D) Bain breathing system. The V(min) that prevented re-breathing was found, and peak inspiratory (PIP) and peak expiratory (PEP) pressures measured. The fresh gas flow (V(f)) was then increased to V(min) + 50%, then V(min) + 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.

RESULTS

The V(min) (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 V(min), PEP with the Uniflow (3.5 +/- 0.1 cm H(2)O) was significantly higher than the Bain system (2 +/- 0.7 cm H(2)O), but PIP values did not differ (Uniflow -0.6 +/- 2.1 cm H(2)O; Bain system -0.2 +/- 0.6 cm H(2)O). With both systems, PEP increased significantly (p < 0.001) with each increase in V(f): Uniflow system 4.2 +/- 0.4 (V(min) + 50%) and 5.5 +/- 0.5 cm H(2)O (V(min) + 100%); Bain system 2.8 +/- 0.7 (V(min) + 50%) and 3.5 +/- 0.7 cm H(2)O (V(min) + 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 V(f) reduced, physiological variables became normal.

CONCLUSION

The study discredited the hypothesis that the two breathing systems behave similarly. Values for V(min) and PEP were higher with the Uniflow system. Increasing V(f) increased PEP with both systems. Insertion of an HME did not affect respiratory pressures.

CLINICAL RELEVANCE

The Uniflow used in Mapleson D mode is not suitable for anaesthesia in young spontaneously breathing pigs.

Reduction of fresh gas flow requirements by a circle-modified bain breathing circuit

We modified a Bain circuit by placing the circuit into the Y piece of a standard carbon dioxide absorber circle, connecting the fresh gas hose on the anesthetic machine to the Bain's fresh gas inlet, and occluding the circle's fresh gas inlet. This circle-modified Bain breathing circuit was studied to evaluate whether it reduces fresh gas flow requirements. The Bain and modified Bain steady states were analyzed by mechanical and computer modeling. The mechanical model consisted of an artificial lung ventilated to steady state. Carbon dioxide was measured with capnography. Computer modeling was by compartmental analysis calculated with spread-sheet software. Steady-state solutions were obtained by numeric analysis. The circle-modified Bain greatly reduced retention of carbon dioxide. For example, with 1-liter tidal volumes, 10-liter minute volumes (10 breaths per minute), and a 2.1 L/min fresh gas flow, the steady-state end-tidal carbon dioxide values of the Bain and modified Bain were 9.3 and 4.6%, respectively, in the physical model (carbon dioxide inflow of 230 ml/min). Results from the mechanical model helped validate the computer model.