A blood gas machine measures the partial pressures of oxygen (pO₂) and carbon dioxide (pCO₂) and the pH of the sample. The analyser will usually then calculate the concentration of bicarbonate and a number of other values.

There are currently three blood gas machines marketed to UK vets; all of these were originally intended for point-of-care testing ('bedside testing') in human medicine. All three include the facility to perform a variety of other tests, such as measuring haematocrit, creatinine, lactate, activated clotting time and electrolytes. However, care should be taken to ensure that each individual test has been validated in the species you are dealing with.

If the blood sample is venous and appropriately obtained the results can be used to interpret the acid-base balance. Acid-base disturbances occur commonly in critically ill patients and knowledge and understanding of this may allow better patient care. In particular, it will allow a more rational use of intravenous fluid therapy. Any improvement or deterioration in the acid-base status may be helpful in judging the patient's response to treatment. If the pCO₂ is elevated, it indicates hypoventilation, that is to say inadequate ventilation to remove sufficient carbon dioxide from the blood. If the pCO₂ is reduced it demonstrates hyperventilation.

Acid-base disturbances generally result from one of the following:

 Damage to an organ such as the kidneys or lungs, which are normally responsible for acid-base balance

 Diseases that cause an increased production of metabolic acids, e.g., diabetic ketoacidosis

 Medical intervention such as mechanical ventilation and some drugs, e.g., some diuretics

If the sample is arterial blood the results can be used to assess the adequacy of the oxygen content in the patient's blood and to detect the presence of any impairment to oxygen moving into the blood passing through the lungs. This has particular advantage when considering the need for (further) oxygen therapy.

The process of obtaining an arterial sample of blood for analysis is commonly considered to be problematic (the technique is described below). However, it should be remembered that arterial blood is only required if there are concerns regarding the patient's pulmonary function. Whether the blood is venous or arterial the presence of any air bubbles, or indeed any air within the sample container, will cause deterioration in quality. All samples should be entered into the analyser as quickly as possible--within at least 10 minutes of collection. If this is not possible, the sample should be kept in an airtight container and stored in ice.

Arterial Blood Sampling

This procedure is uncomfortable for patients and if multiple samples are required an arterial catheter should be placed. Any careful veterinary surgeon or nurse may perform sampling, with the help of an assistant. If the patient has a coagulopathy, the procedure is more dangerous.

Risks include:

 Haematoma formation--firm digital pressure should be applied for at least 4 minutes after sampling (or attempting to do so!). The sampling site should be checked again a few minutes later. It is recommended that a narrow needle such as a 25 SWG (typically orange coloured in the UK) should be used.

 As with any blood sampling, care needs to be taken to avoid infection.

 There is a small risk of creating ischaemia in the extremity served by the artery.

It is possible accidentally to obtain a venous sample. Genuine arterial samples will usually be a brighter red than a venous sample and the results of the blood gas analysis may suggest a venous sample was obtained.

It is only a useful technique if the necessary equipment is available to analyse the sample. Materials required:

 25 SWG needles, syringes, heparin:

 Pre-heparinised syringes are available, but this will add to the cost of the procedure.

 Some machines require the use of anticoagulated blood to prevent damage to the equipment. Other machines, based around disposable cartridges, will probably not absolutely require you to do so but it is still usually advisable as there is a risk the sample could clot prior to analysis. Always follow the manufacturer's recommendations.

 Clippers; the site should be prepared as for venipuncture.

 Local anaesthetic cream for the unanaesthetised patient:

 To be effective this cream must be applied to the skin, after clipping, about 30 minutes prior to sampling .

 The use of an occlusive dressing further improves the effect of the local anaesthetic.

 Many practitioners prefer to use self-filling (vented) syringes. Before use the plunger is withdrawn to give the desired sample size. Once the needle is placed within the artery it will fill automatically. The syringe fills under the pressure of the arterial blood; this helps to avoid accidentally obtaining venous blood. However, if the arterial blood pressure is particularly low, the syringe may fail to fill. The vent mechanism facilitates fast filling and results in fewer air bubbles contaminating the sample.

Approach

1.  Typically, the dorsal metatarsal artery is used. This is easily palpated over the craniomedial aspect of the metatarsal region, just distal to the hock joint. The assistant should restrain the animal in lateral recumbency. The sample will be taken from the lower leg. Palpate the artery and determine its course.

2.  Clip the hair from over the artery. Prepare the site as for venepuncture. Avoid excessive skin preparation as this can cause arterial vasoconstriction, making sampling more difficult.

3.  Whilst the limb is firmly held by the assistant, place the index and first finger of one hand over the artery. These fingers should be about 1 cm apart. Doing so will help you appreciate the course of the vessel and reduce its movement as the needle enters. To prevent contamination, avoid touching the skin where the needle will enter the skin. Do not press too firmly or you will occlude the artery. Ensure the assistant is not squeezing the leg, or unthinkingly 'raising the vein', as this will hinder the sampling process.

4.  Holding the syringe and needle in your dominant hand, penetrate the patient's skin between your index and first fingers, and then the vessel. Some prefer to hold the syringe as if it were a pen. As the needle pierces the vessel wall, you will perceive that it is somewhat thicker than that of a similar sized vein. Also the lumen of the vessel will be smaller, making it somewhat easier to pass the needle through the other side of the artery. Once 'flashback' is seen within the needle hub, the hand palpating the course of the artery can be used to withdraw the plunger to obtain a sufficient sample (the volume required will be determined by the blood gas analyser and what anticoagulant has been used). To avoid movement, steady the hand holding the syringe against the patient's leg.

5.  Remove the needle and syringe. Remember to apply firm digital pressure for at least 4 minutes after sampling. Whilst this is being done, ensure that the sample is gently 'rolled' within the syringe and that any air bubbles are promptly expelled.

The auricular arteries are also frequently used for sampling. The radial, median and femoral arteries may be used, but greater attention to haemostasis is required at these sites.

References

1.  Davis H. Arterial and venous blood gases. In: Wingfield, WE; Raffe, MR. eds. The veterinary ICU book. Wyoming: Teton NewMedia, 2002; 258-265.