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.
CPPV (continuous positive pressure ventilation) is obviously superior to IPPV (intermittent positive pressure ventilation) for the treatment of patients with acute respiratory insufficiency (ARI) and results within a few minutes in a considerable increase in the oxygen transport. The principle is to add a positive end-expiratory plateau (PEEP) to IPPV, with a subsequent increase in FRC (functional residual capacity) resulting in re-opening in first and foremost the declive alveolae, which can then once again take part in the gas exchange and possibly re-commence the disrupted surfactant production. In this manner the ventilation/perfusion ratio in the diseases lungs is normalized and the intrapulmonary shunting of venous blood (Qs/Qt) will decrease. At the same time the dead space ventilation fraction (VD/VT) normalizes and the compliance of the lungs (CL) increases. The PEEP value, which results in a maximum oxygen transport, and the lowest dead space fraction, also appears to result in the greatest total static compliance (CT) and the greatest increase in mixed venous oxygen tension (PVO2); this value can be termed "optimal PEEP". The greater the FRC is, with an airway pressure = atmospheric pressure, the lower the PEEP value required in order to obtain maximum oxygen transport. If the optimal PEEP value is exceeded the oxygen transport will fall because of a falling Qt (cardiac output) due to a reduction in venous return. CT and PVO2 will fall and VD/VT will increase. Increasing hyperinflation of the alveolae will result in a rising danger of alveolar rupture. The critical use of CPPV treatment means that the lungs may be safeguarded against high oxygen percents. The mortality of newborn infants with RDS (respiratory distress syndrome) has fallen considerably after the general introduction of CPPV and CPAP (continuous positive airway pressures). The same appears to be the case with adults suffering from ARI (acute respiratory insufficiency).
SummaryA definite relationship between the use of contaminated anaesthetic equipment and subsequent pulmonary infection remains to be established. There is however indirect and circumstantial evidence suggesting that cross-infection may occur, and further an increased susceptibility of surgical patients to pulmonary infections has been demonstrated. Decontamination should be recommended bejore the equipment is re-used. Pasteurisation may protie sufficient and this can be obtained employing a specially designed dish-washing machine.
The bacterial content of oxygen and nitrous oxide immediately before and after passing through clean and used breathing systems (circuits) was measured using a specially constructed agar chamber (Bourdillon's slit sampler). The content per litre of oxygen from the outlet of the anaesthetic machine was 4.0 X 10-2, and 2.9 X 10-2 for nitrous oxide, corresponding to 3.5 X 10-2 for a 50% mixture of the gases. After passing through cleaned circuits, the bacterial pollution of the gas mixture had increased by 30%, but more than elevenfold after passing through used circuits. The content from cleaned circuits was less than that measured previously in the air of hospital wards and operating theatres, whereas gases from used circuits were polluted to approximately the same extent. It is concluded that used circuits may increase the risk of cross-infection. The cleaning method employed by us (dish-washer--hot airy drying) appeared to be acceptable.
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