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MATER IALS AND METHODSuate hypoxia , but to correlate a biochemi cally measured value with th e outco me of hypoxia is difficult . Measure me nts of th e pu rine metabol ite hypoxanthine can be used as an ind icator of hypoxia (I). Du rin g tissue hypo xia, elevated hypoxanthine levels have been found in plasm a (2-5), CSF (6, 7), and urine (8).H owever, no clear definition of clinical hypoxia is available, and ma ny autho rs do not distinguish between the terms hypoxemia an d hypoxia, whic h adds to the confusio n. Our meanin g of hypoxia has previou sly been described ( I). In our study th e pigs were subjected to hypoxemia. We define hypoxe mia as deficient oxygenation of th e blood. Hypoxem ia may, however, not necessarily lead to tissue hypoxia.To further assess the usefuln ess of hypoxanthine as a biochemical marker of hypoxia, we repo rt how three degrees of hypoxemi a in young pigs in fluence the level of hypoxanthine, xanthine, and uric acid in the CSF and plasma , as well as th e pigs' urin ary excretion of hypoxan thine an d xanthine.Approval. The local hosp ital' s ethi cal com m ittee for an im al studies approved these experim ents.Animal model. T he anima l model was similar to th at previously described (9). T went y-two young pigs of eit her sex (the males had been castrated shortly after birth ) weighing 17-22 kg (mea n 18.1 kg) were used. Th ey were anes thetized with an intraperitoneal injection of sodium pentobar bital (30 mg/kg). An i.v. line was estab lished through an ear vein, an d a contin uo us infusion of Ringer acetate ( 10 mL/kg/h) was given during the experime nt. A further 100 mg of sodium pentobar bital was given i.v, if necessary every 30 min. A cuffed tu be was placed in the trachea via a tracheostom y. Art ificial ventilation was given by a ventilato r (Servo-ventilator 900B, Elema-Schenander, Stoc kholm, Sweden). Ventilation rate and tidal volume were adjusted unt il Paco, ranged between 4 and 5.3 kPa (30-40 mm Hg). No further adjustme nts of the ventilato r .were done du ring th e hypoxemic period.A midlin e supr apubic laparotomy was performed and a urinary catheter was placed in the bladder for measuring diuresis and urinary hypoxanthin e and xanthine.Polyeth ylene catheters were inserted into a fem oral artery and a fem oral vein. T he arterial cathe ter was used for taking blood samples and for measuring blood pressure continuously with a strai n gauge tra nsdu cer, Go uld recorde r 2600S (Gould In c. Recording System s, Cleveland, OH). Th e venous cath eter was used for giving Ringer acetate plu s additional sodium pentobarbital.Th e pigs were divided int o three hypoxemi c and one contro l group. Seven animals were made hypoxem ic by art ificial ventilation with 8% oxygen in nitrogen (FiO z = 0.08) (group 1). Seven