Canadian Anaesthetists' Society journal
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To examine a possible mechanism which could cause arterial hypoxaemia following pulmonary embolism, we collapsed and did not ventilate one lung in each of eleven dogs, to produce hypoxic pulmonary vasoconstriction. In five dogs (Starch Group), PaO2 fell from 10 to 7.7 kPa (76.6 to 58.4 torr) as shunt fraction (Qs/Qt) rose from 19 to 31 per cent. Mean pulmonary artery pressure (ppa), paCO2 and VD/VT remained constant. ⋯ We conclude from these results that emboli are preferentially distributed to ventilated lung. After embolization PPA increases. At least in this pulmonary embolism model the increased PPA may overcome hypoxic pulmonary vasoconstriction, redistribute blood to non-ventilated lung and create arterial hypoxaemia.
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Comparative Study
An evaluation of gas density dependence of anaesthetic vaporizers.
Four commonly used vaporizers were studied for the effect of carrier gas density on vaporizer output. Vapour concentrations from a halothane Cyprane (Fluotec) Mark 2 increased in relation to the density of carrier gas, whereas the concentrations delivered by an enflurane Ohio vaporizer decreased. The halothane Cyprane (Fluotec) Mark 3 and enflurane Cyprane vaporizers were largely independent of density. Of clinical importance, nitrous oxide/oxygen (75/25), compared with oxygen alone, increased the vapour concentration outputs of the halothane Mark 2 up to 30% and decreased the outputs of the enflurane Ohio unit up to 20%.
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The volume of carbon dioxide rebreathed by spontaneously breathing patients under halothane anaesthesia at various fresh gas flow rates (FGF) with the Bain modification of the Mapleson "D" breathing circuit is measured. The effect of rebreathing on a heterogeneous patient population is shown to be unpredictable hypercapnia in those patients who cannot respond adequately to this carbon dioxide challenge. All adults rebreathe significant volumes of carbon dioxide at a FGF rate of 100 ml . kg-1 . min-1. ⋯ Rebreathing occurs because the inspired carbon dioxide load is unpredictable in a given patient and the patient's response is uncontrolled. Patients respond to this carbon dioxide challenge by increasing inspiratory flow rate (Vt/Ti), which results in increased rebreathing of carbon dioxide from the expiratory limb of the circuit. To prevent potentially dangerous rebreathing of carbon dioxide in all patients the fresh gas flow rate must be much higher than presently recommended.