Today's Veterinary Practice provides comprehensive information to keep every small animal practitioner up to date on companion animal medicine and surgery as well as practice building and management.
Issue link: http://todaysveterinarypractice.epubxp.com/i/133843
| The PracTiTioner's acid–Base Primer: oBTaining & inTerPreTing Blood gases POTENTIAl SAMPlE ERRORS • If a sample is obtained from a peripheral vein in patients with poor perfusion, the sample may reflect the acid–base status of that limb alone rather than that of the whole body. • If a limb's vein is occluded for several minutes to obtain a sample, the sample may reflect lactic acidosis specific only to that limb. • If the sample is not immediately evaluated or placed on ice until evaluation, ongoing cellular metabolism by red blood cells will continue to use o2, produce co2, decrease ph, and eventually, once o2 has been depleted, increase lactate concentration, which further reduces ph as well as hco3- and Be. • If the sample is exposed to air (ie, a blood tube with air in it or an uncapped syringe), the oxygen from the atmosphere will diffuse into the sample while co2 diffuses out, which directly affects Pao2, Paco2, and ph, rendering the calculated values for hco3- and Be inaccurate and making the sample worthless for acid–base interpretation. Arterial Samples For additional information, see Step by Step: Obtaining Arterial Blood Gas Samples, page 45. 1. Use of a local anesthetic will make the procedure more comfortable for awake patients. 2. Based on the patient, there are numerous sites from which an arterial sample can be taken: • Awake dogs: A metatarsal branch of the dorsal pedal artery is preferred. • Anesthetized patients: Coccygeal, auricular, and radial arteries may also be used; sample collection from these arteries is not well tolerated in awake patients.3 • Small patients: The femoral artery is typically used; however, if there is excessive bleeding after sampling, this bleeding is much harder to manage with a pressure bandage compared to other sites. Only use this site if sampling from other sites is not possible. • Cats: Arterial puncture in cats is particularly difficult due to their smaller arteries and the fact that they are hard to restrain. Therefore, cats should be sedated or dorsal pedal artery arcuate artery * dorsal metatarsal arteries Figure 3. Diagram showing the dorsal pedal artery and its branches, the dorsal metatarsal arteries. The branch between the second and third metatarsals(*) is most commonly used for arterial puncture and arterial catheter placement. under general anesthesia. The dorsal pedal, femoral, and coccygeal arteries are the most common sites used in anesthetized cats. Patients in respiratory distress may not tolerate the positioning and restraint needed to obtain arterial samples. A venous blood gas and pulse oximetry reading may be preferable in these patients. INTERPRETING BlOOD GAS RESUlTS There are 6 steps required to interpret blood gas results: 1. Determine If Sample Is Venous or Arterial Either sample type can be used to evaluate overall acid– base status, with the exception of severe shock and post arrest situations, which may result in large discrepancies between arterial and venous samples. Poor tissue perfusion can result in sizeable increases in CO2 and secondary decreases in pH on the venous side despite low to normal CO2 on the arterial side. • Although information can be gained about ventilation from a venous sample, only an arterial sample can truly assess oxygenation. • If unable to obtain an arterial sample, use: TaBle 2. exPecTed comPensaTory changes1 Disorder Changes - Compensatory Response 0.7 mm hg decrease in Paco2 for each 1 meq/l decrease in hco3 - Metabolic acidosis Metabolic alkalosis hco3 - 0.7 mm hg increase in Paco2 for each 1 meq/l increase in hco3 - Acute respiratory acidosis Paco2 1.5 meq/l increase in hco3 - for each 10 mm hg increase in Paco2 Chronic respiratory acidosis Paco2 3.5 meq/l increase in hco3 - for each 10 mm hg increase in Paco2 Acute respiratory alkalosis Paco2 2.5 meq/l decrease in hco3 - for each 10 mm hg decrease in Paco2 Chronic respiratory alkalosis 46 hco3 Paco2 5.5 meq/l decrease in hco3 - for each 10 mm hg decrease in Paco2 Today's Veterinary Practice May/June 2013