Fresh gas flow in coaxial Mapleson A and D circuits during spontaneous breathing

Abstract

In a lung model simulating spontaneously breathing halothane anaesthesia, the rebreathing characteristics of the coaxial Mapleson A (Lack circuit) and D (Bain circuit) systems were tested. Using decreasing fresh gas flows (VF), the end-tidal carbon dioxide fraction (FACO2) was monitored and the point of rebreathing (R.P.) detected. The effects of changes in minute volume (VE), dead-space to tidal volume ratio (VD/VT) and carbon dioxide elimination (VCO2) were studied. The effect of increased tidal volumes (VT) on FACO2 was investigated for some different fresh gas flows (VF). The VF/VE ratio for R.P. in the Bain circuit was approximately 2 and in the Lack circuit 0.88. In both circuits an increase in VE and a decrease in the VD/VT ratio resulted in higher demands on VF if rebreathing was to be avoided. The latter effect was much more pronounced in the Lack circuit. In neither system did any changes in VCO2 affect the rebreathing characteristics. The conclusion was drawn that the Lack system is a much better choice concerning the fresh gas flows for anaesthesia with spontaneous breathing than the Bain system. It was also concluded that the fresh gas flows recommended by Humphrey for the Lack system (i.e. 51 ml X min-1 X kg b.w.-1) and by the manufacturers for the Bain system (i.e. 100 ml X min-1 X kg b.w.-1) are inadequate and should be increased if a considerable degree of rebreathing is to be avoided.

Predicted normocapnea in infants and children using the Bain circuit with controlled ventilation

“…Even if many factors affect the degree of rebreathing, such as the VD/VT ratio of the patient and the respiratory flow pattern (6,17,18), these factors will be insignificant when compared to the impact of the vFG when small O F , / vE ratios are used (6,8). was 7.38 f 0.05 with a range of 7.29-7.51.…”

“…One of the most important advantages of the Mapleson D circuit is its ability to maintain normocapnea with a large minute ventilation. Even if many factors affect the degree of rebreathing, such as the VD/VT ratio of the patient and the respiratory flow pattern (6,17,18), these factors will be insignificant when compared to the impact of the vFG when small O F , / vE ratios are used (6,8). In lung model studies, it has been shown that the fractional fresh gas utilization in a Mapleson D circuit will increase when the t r F G / v E ratio decreases (6,9).…”

Rebreathing in the Mapleson A, C and D breathing systems with sinusoidal and exponential flow waveforms

“…A weakness in this work was that the equations were solved only for linear flow waveforms, whereas the flow waveforms found in real patients (and those produced by many lung models) are nonlinear. Sinusoidal flow waveforms are commonly seen, especially when the respiratory rate is high [4,5]. They are also characteristic of lung models based on a rotating shaft linked to a piston [6].…”

Predictable Paco2, with two different flow settings using the Mapleson D system

“…Many factors may affect the fresh gas utilization in a Mapleson D system, such as the respiratory flow pattern and the dead-space to tidal volume ratio, but the VFG/VE ratio is by far the most important factor (13,14). Seeley et al calculated the relationship between different VFG/VE settings and the resulting Paco, (4).…”