Carbon dioxide distribution in Mapleson A and D systems: an experimental study

Abstract

The distribution of CO2 in the Mapleson A and D rebreathing systems was investigated experimentally during controlled ventilation and with the expiratory valve closed during inspiration. Maximal and minimal levels of CO2-concentration obtained from capnograms along the tubing were used to construct "gas profiles". For both systems, high tidal volumes and low fresh gas flows resulted in a high degree of gas separation with a pool of alveolar gas near the expiratory valve, and longitudinal gas mixing was minimal. In this manner fresh gas loss was prevented and fresh gas utilization optimized. The end of the tubing nearest the patient was found to act as a reservoir for alveolar gas in the Mapleson A system and fresh gas in the Mapleson D system. Fresh gas utilization in the Mapleson D system was somewhat less efficient than in the Mapleson A system due to the fresh gas admixture to exhaled alveolar gas in the patient-near end of the tubing during expiration. The replacement of the usual expiratory valve of the Mapleson A system by a valve which is closed during inspiration makes the A system an alternative to the D system for controlled ventilation.

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

“…The lowest pH was found in an 11month-old boy who had both a mild respiratory acidosis (Paco,=6.0 kPa (45 mmHg)) and a metabolic acidosis with a base deficit of 5.4 mmol-1-'. Similar figures were found by Andersen et al (9). However, hypocapnea will induce a decrease in cardiac output, and an increase in both systemic vascular resistance and oxygen consumption (16).…”

“…If the v,,/v, ratio is SO.67, then f could be set to 0.72 (6,9) Table 1). If the v,,/v, ratio is SO.67, then f could be set to 0.72 (6,9) Table 1).…”

“…where f = the fresh gas utilization. If the v,,/v, ratio is SO.67, then f could be set to 0.72 (6,9) Approximate figures for carbon dioxide elimination were applied to equations (V) and (VI). The figures were based upon values found in children during anaesthesia with controlled ventilation (3, 10, 1 I ) , (Table 1).…”

“…This has been shown to be useful in maintaining normocapnea during controlled ventilation. With a \fFG/\jE ratio of 0.67 or less, this fresh gas utilization will be approximately 0.7 (6,9). From the theoretical analysis of Seeley et al ( 8 ) , Rose & Froese (6) concluded that the resulting alveolar fraction of carbon dioxide (F,co,) in a Bain system during rebreathing conditions will be equal to the formula VCo2/f.…”

“…the fresh gas utilization). With a \fFG/\jE ratio of 0.67 or less, this fresh gas utilization will be approximately 0.7 (6,9). It should thus be possible, using the rebreathing technique, to calculate what fresh gas flow should be used for a certain, expected Paco, level when only the carbon dioxide elimination of the patient is known.…”