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| The PracTiTioner's acid–Base Primer: oBTaining & inTerPreTing Blood gases concentrations of inspired oxygen to be evaluated and compared. A normal PaO2:FiO2 ratio is approximately 500 (PaO2 = 100 mm Hg, FiO2 = 0.21). If a patient is on 100% oxygen, the expected PaO2 would be 500 mm Hg. This is helpful in interpreting samples from patients under general anesthesia and those on supplemental oxygen that are too unstable to obtain samples when breathing room air. IN SUMMARY Interpretation of venous and arterial blood gases can be essential to treatment of many patients. Blood gas analyzers are becoming more common in veterinary practices and this analysis can aid in diagnosis and therapy for patients, indicating: • When fluid therapy is indicated • What fluid types are the best choices • If sodium bicarbonate should be administered • When oxygen and mechanical ventilation are needed, including when the patient can be weaned off this support. n Be = base excess/deficit; co2 = carbon dioxide; Fio2 = fractional inspired oxygen concentration; gi = gastrointestinal; hco3 - = bicarbonate; Pao2 = partial pressure of oxygen in arterial blood; Paco2 = partial pressure of carbon dioxide in arterial blood References 1. diBartola SP. introduction to acid–base disorders. in diBartola SP(ed): Fluid, Electrolyte, and Acid–Base Disorders in Small Animal Practice, 4th ed. St. Louis: elsevier Saunders, 2012, pp 231-252. 2. Middleton dJ, ilkiw Je, watson AdJ. Arterial and venous blood gas tensions in clinically healthy cats. Am J Vet Res 1981; 42:1609-1611. 3. Mazzaferro eM Hauser C. Arterial puncture and catheterization. in Burkett Creedon JM, davis H (eds): Advanced Monitoring and Procedures for Small Animal Emergency and Critical Care, Ames iowa, wiley-Blackwell, 2012, pp 69-81. 4. Mazzaferro eM. Arterial Catheterization. in Silverstein dC, Hopper K (eds): Small Animal Critical Care Medicine. St. Louis: elsevier Saunders, 2009, pp 206-208. Suggested Reading de Morias HA, Leisewitz AL. Mixed acid–base disorders. in diBartola SP (ed): Fluid, Electrolyte, and Acid–Base Disorders in Small Animal Practice, 4th ed. St. Louis: elsevier Saunders, 2012, pp 302-315. diBartola SP. Metabolic acid–base disorders. in diBartola SP (ed): Fluid, Electrolyte, and Acid–Base Disorders in Small Animal Practice, 4th ed. St. Louis: elsevier Saunders, 2012, pp 253-286. Johnson rA, de Morias HA. respiratory acid–base disorders. in diBartola SP (ed): Fluid, Electrolyte, and Acid–Base Disorders in Small Animal Practice, 4th ed. St. Louis: elsevier Saunders, 2012, pp 287-301. Lori S. Waddell, DVM, Diplomate ACVECC, is an adjunct associate professor in critical care at the University of Pennsylvania School of Veterinary Medicine, working in the Intensive Care Unit. Her areas of interest include colloid osmotic pressure, acid–base disturbances, and coagulation in critically ill patients. Dr. Waddell received her DVM from Cornell University; then completed an internship at Angell Memorial Animal Hospital in Boston, Massachusetts. After her internship, she worked as an emergency clinician in private practice until pursuing a residency in emergency medicine and critical care at University of Pennsylvania. 48 Today's Veterinary Practice May/June 2013