In the past blood gas analysis and interpretation was performed primarily at university and large referral hospitals. The main argument against not 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 such as the i-STAT and IRMA, blood gas analysis and interpretation has become more common.

Blood gas analysis begins with collection of the sample. Arterial blood is preferred when assessing respiratory and metabolic status, but venous blood may be useful for assessment of some metabolic disturbances. The use of free-flowing lingual venous blood can sometimes be used to estimate arterial blood gas values in anesthetized animals when arterial blood is unobtainable. The sample should be collected into a heparinized syringe. Usually the syringe is filled with heparin and then emptied. This process coats the inside of the syringe barrel and the hub of the needle. Alternatively, syringes containing powdered heparin specifically designed for arterial blood collection are commercially available. Enough blood should be collected (~ 1 ml) to prevent the heparin from diluting the blood significantly. All visible air should be expelled from the syringe following sample collection. If the sample is not going to be analyzed immediately it should be capped and placed on ice until it is run.

Some common errors associated with improper sample collection and storage are:

2. If unchilled for a long period cellular metabolism will continue and PaO2 will be lower and PaCO2 increased.

3. If not anticoagulated the sample will cause an error.

Before a blood gas can be interpreted, information about the conditions the animal was exposed to need to be considered. The fraction of inspired oxygen (FIO2) and body temperature are often required by the blood gas machine for calculation of Alveolar-arterial (A-a) gradients and temperature corrected values respectively. It is also important to know this information for interpretation of the blood gas values in the clinical setting.

Analyzer Outputs

Many blood gas analyzers measure sodium, potassium, and calcium. Total plasma protein can be measure using a refractometer or other clinicopathological technique. This additional information will allow calculation of the anion gap or other ionic differences that can provide insight into the metabolic origin of some acid-base disturbances. These non-traditional approaches to acid-base balance are not routinely used to manage anesthetic cases intraop. However, these approaches will be encountered in the context of metabolic acid-base disturbances in Critical Care and Internal Medicine. More information about anion gap and strong ion difference theory can be found in the recommended reading (1-4).

PaCO2 is the partial pressure of CO2 in the arterial plasma. PaCO2 increases when alveolar minute ventilation is decreased and vice versa. When PaCO2 increases, ventilation is said to be depressed. Most anesthetic drugs are respiratory depressant therefore PaCO2 usually is increased from normal during anesthesia unless ventilation is controlled. Carbon dioxide is the main stimulus for respiration during anesthesia in normal patients.

PaO2 is the partial pressure of oxygen dissolved in the arterial plasma. Alone, this value does not tell you how much oxygen is in the blood. Hemoglobin is the major carrier of oxygen in blood, NOT dissolved oxygen in plasma; therefore hematocrit or hemoglobin concentration is also required before estimating oxygen content. The relationship between the PO2 and O2 content is estimated by the equation:

Oxygen Content (ml/dL)=(Hemoglobin conc. (g/dL) x %hemoglobin saturation x 1.3) + 0.003 x PaO2