Arterial blood gas analysis and interpretation in anesthetized patients
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.