May. 21, 2022
Alterations in acid–base and respiratory function, which are common in both emergency patients as well as hospitalized, critically ill patients. Familiarity with obtaining and interpreting blood gases can be essential in the management of these patients. It is essential that blood gas samples be properly obtained and handled, particularly venous samples. Sample error can be introduced in a number of ways (see Potential Sample Errors).
Veterinary Blood Gas Analyzer
ㆍIf 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.
ㆍIf a limb’s vein is occluded for several minutes to obtain a sample, the sample may reflect lactic acidosis specific only to that limb.
ㆍIf 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.
ㆍIf 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.
1. Ideally, a venous sample should be taken from a central catheter (in the cranial or caudal vena cava) or by direct jugular venipuncture in order to obtain the best representation of the global acid–base and respiratory status.
2. Samples should be capped off to prevent exposure to air; then processed immediately. If the sample cannot be processed immediately, it should be placed on ice until evaluation can take place.
3. Samples can either be:
ㆍProcessed immediately without anticoagulant
ㆍDrawn into a syringe that has been coated in heparin until they can be processed.
4. To prepare a sample with heparin, coat a 3-mL syringe with a small amount of liquid heparin; then draw air up to the syringe’s 3-mL mark and forcibly expel the heparin several times. While most of the heparin will be removed from the syringe, enough heparin will remain to affect the measured ionized calcium, which will be unreliable.
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:
ㆍAwake dogs: A metatarsal branch of the dorsal pedal artery is preferred.
ㆍAnesthetized patients: Coccygeal, auricular, and radial arteries may also be used; sample collection from these arteries is not well tolerated in awake patients.
ㆍSmall 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.
ㆍCats: 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 under general anesthesia. The dorsal pedal, femoral, and coccygeal arteries are the most common sites used in anesthetized cats.
3. Patients in respiratory distress may not tolerate the positioning and restraint needed to obtain arterial samples. A veterinary blood gas analyzer may be preferable in these patients.