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In the past, blood gas analysis and interpretation were performed primarily at university and large referral hospitals. The
main argument against using blood gas analysis to guide case management in private practice was the cost of purchasing and
maintaining a bench-top blood gas analyzer. With the availability of relatively inexpensive point-of-care units (e.g. VetScan i-STAT 1—Abaxis; Irma 2000SL—Diametrics Medical; VetStat Electrolyte and Blood Gas Analyzer—IDEXX), blood gas analysis
and interpretation have become more common. This article introduces veterinarians to the principles of clinical blood gas
analysis and interpretation and provides two case examples (see the Related Links "Case example 1: A border collie with a possible foreign body" and "Case example 2: A diabetic husky
with a fibrosarcoma" below).
SAMPLE COLLECTION
Blood gas analysis begins with the collection of the sample. Arterial blood is preferred when assessing respiratory and metabolic
status, but venous blood may be useful for the assessment of some metabolic disturbances such as those that occur during severe
diarrhea, vomiting, and some toxin exposures. Free-flowing lingual venous blood can sometimes be used to estimate arterial
blood gas values in anesthetized animals when arterial blood is unobtainable.1 Common sites for arterial blood collection in dogs and cats include the dorsal pedal artery, femoral artery, or lingual
artery. But any large superficial artery may be used.
Sample collection is usually performed with a 1- or 3-ml syringe with a 21-ga or smaller needle. The sample should be collected
into a heparinized syringe, which is usually just a syringe that has been filled with heparin and then emptied. This process
coats the inside of the syringe barrel and the needle hub. Alternatively, syringes containing powdered heparin specifically
designed for arterial blood collection are commercially available. Collect enough blood (about 1 ml) to prevent the heparin
from diluting the blood markedly. Expel all visible air from the syringe after sample collection. If the sample will not be
analyzed immediately, cap it and place it on ice until it is run.2 However, if accurate partial pressure of oxygen in arterial blood (PaO2) values (< 10 mm Hg change) are important, samples collected into plastic syringes should be run within 10 minutes, even
if placed on ice.3,4
Common errors associated with improper sample collection and storage are
1. If the sample is left uncapped for a prolonged period, the partial pressure of carbon dioxide in the arterial blood (PaCO2) and the PaO2 may decrease. The PaO2 may increase if the sample PaO2 is less than the partial pressure of oxygen in the room (e.g. venous blood).
2. If the sample is unchilled for a long period, cellular metabolism will continue, the PaO2 will decrease, and the PaCO2 will increase.
3. If the blood is not anticoagulated, the sample will clot in the system and cause an error.
Before a blood gas analysis can be interpreted, the conditions the patient was exposed to need to be considered. The blood
gas machine often requires input of the fraction of inspired oxygen (FiO2) and body temperature to calculate alveolar-arterial (A-a) gradients and temperature corrected values, respectively. It is
also important to know this information when you interpret the blood gas values clinically.
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