MEASUREMENT IN ANAESTHESIA Conway, C.M.
British journal of anaesthesia : BJA,
January 1978, 1978-01, 1978-01-00, Letnik:
50, Številka:
1
Journal Article
Recenzirano
Odprti dostop
Away with the cant of “measures not men” —the idle supposition that it is the harness and not the horses that draw the chariot along. If the comparison must be made, if the distinction must be taken, ...men are everything, measures comparatively nothing. George Canning, 1801
A black box and a geometrical approach have been used to deduce the relationships between alveolar oxygen and carbon dioxide concentrations when rebreathing occurs during the use of semi-closed ...anaesthetic rebreathing systems.
Equations have been derived to determine the alveolar gas concentrations which occur when the Magill (Mapleson A) circuit is used with a low fresh gas flow. Alveolar oxygen and carbon dioxide ...concentrations are determined by the fresh gas flow and composition, carbon dioxide output and oxygen uptake. Gas mixing within the circuit and alterations in the inspired gas concentrations do not affect the final equilibrium. If oxygen uptake and carbon dioxide output are constant, the alveolar gas concentrations are unaffected by alterations in ventilation.
Measurements of ventilation and of inspired gas composition were made while volunteers breathed a non-anaesthetic gas through a Bain anaesthetic system. It was found that rebreathing occurred when ...the fresh gas flow was between two-and-a-half and three times the minute volume. Fresh gas flows at least three times the minute volume appear to be necessary to prevent rebreathing when using this system.
An experimental lung model was used, with controlled ventilation, to determine the effect of different circle arrangements and varying ventilatory frequencies on the efficiency of carbon dioxide ...removal from a circle system without carbon dioxide absorption. Greater efficiency was found when fresh gas entered the system between the unidirectional inspiratory valve and the subject than when the fresh gas inlet was on the ventilator side of this valve. At any fresh gas flow and minute volume, efficiency was greater at low respiratory frequencies. Good correlations existed between carbon dioxide concentration in the model lung, fresh gas flow and minute ventilation when respiratory frequency was constant. Paradoxical results were obtained when minute volume was varied by changes in frequency at a constant tidal volume. The major cause of the various differences in performance has been ascribed to variations in the degree of mixing of fresh and expired gas within the system.
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