British journal of anaesthesia
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Delayed onset of obstruction in the Oxford endotracheal tube during anaesthesia is described in five patients. The effects of intra-cuff voluem and pressure on the wall of the tube were investigated and discussed with special refernce to the ffects of body heat and repeated use on the consistency of the tube. It is concluded that inward collapse of the tube wall is caused by a combination of factors, namely: frequent use, softening of the tube wall by body heat, the gradual increasing of intra-cuff volume and pressure by diffusion of nitrous oxide into the cuff, replacing a damaged cuff by a new one and heat sterilization. Deflation and re-inflation of the cuff to minimal occlusive volume at hourly intervals is suggested as a precautionary measure in the prevention of inward collapse of the tube wall.
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Serial measurements of maternal blood-gases, alveolar-to-arterial oxygen tension difference (PAO2--PaO), calculated pulmonary venous admixture (physiological shunt), deadspace/tidal volume ratio (VD/VT), and respiratory minute volume have been made in a carefully selected group of normal pregnant patients at 12, 24, 32 and 38 weeks of gestation and 5 weeks after delivery. All measurements were made in the semi-recumbent position with a left lateral pelvic tilt. Mean arterial PO2 was consistently greater than 100 mm Hg throughout pregnancy, although the value decreased from 106.4 mm Hg at 12 weeks of gestation to 101.8 mm Hg at the 38th week. Despite this decrease there was no change in (PAO2--PaO2) VD/VT, or percentage shunt with increasing gestation; nor did these values differ significantly from non-pregnant values in the same patients.