Handheld analysers are allowing blood gas analysis to become more common place in veterinary emergency and critical care. Blood gas analysis allows acid-base and respiratory function; oxygenation and respiration, to be interpreted.

The regulatory systems; chemical buffering, most importantly bicarbonate maintain homeostasis as well as active regulation from the lungs and kidneys in response to changes in the blood gases and acid-base status to alter hydrogen ion concentration. The functions of all body systems are affected by changes in the concentration of hydrogen ions and are regulated and maintained within narrow limits. pH is the measurement of the concentration of hydrogen ions. The concentration of hydrogen ions is the result of acid-base balance. Hydrogen ion concentration has an inverse relationship with pH; an increase in hydrogen ions, an acidaemia equals a lower pH (pH<7.35), and a decrease in hydrogen ions, an alkalemia equals a higher pH (pH>7.45).

The bicarbonate buffering system is expressed by the Henderson-Hasselbalch equation:

CO₂ + H₂O ↔ H₂CO₃ ↔ H⁺ + HCO₃⁻

which shows that pH (the concentration of hydrogen ions) corresponds to the amount of bicarbonate (HCO₃⁻) and to the partial pressure of carbon dioxide (PCO₂). Therefore, acidaemia (a low pH<7.35) could be due to a decrease in bicarbonate, an increase PCO₂ or both and alkalaemia (a high pH>7.45) could be due to an increase in bicarbonate, a decrease in PCO₂ or both.

The body’s buffering system acts very quickly resisting changes in hydrogen ion concentration using bicarbonate, phosphate, and plasma proteins to keep pH within normal limits, between 7.35–7.45. When required active regulation of acid-base balance will be compensated by the lungs and kidneys. For example, an increase in hydrogen ions (by ingestion or an increase in metabolic production) will create a decrease in blood pH (<7.35), an acidosis. A decrease in pH will stimulate an increased rate and depth of breathing, which will increase the pulmonary removal of carbon dioxide (CO₂); effectively ‘breathing off’ excess CO₂ (acid). Respiratory compensation is rapid. The kidneys react to compensate differently depending on if the patient has acidosis or alkalosis. During acidosis the kidneys will excrete excess hydrogen ions and produce bicarbonate (HCO₃⁻) to try to reduce the acidity. During alkalosis they will excrete bicarbonate (HCO₃⁻) to remove excess base (bicarbonate). Compensation from the lungs and kidneys will not return the pH to within normal limits but will aim to return the pH towards normal limits; important to remember when reading blood gases as over compensation does not occur.

Sampling

Arterial blood is often preferred for blood gas analysis, however, there are instances when a venous sample will also be useful. It is important to remember there will be differences in PCO₂; blood pH, between arterial and venous blood samples in the critical or emergent patient. Therefore, it is important to recognise when arterial vs. venous blood is collected. Generally arterial blood is collected from the dorsal pedal artery or the femoral artery using a commercially available blood gas syringe pre filled with anticoagulant (heparin powder), with an air dispenser and air tight bung to reduce potential sampling errors. The area is clipped and prepared prior to sampling; ensure the area is not scrubbed, instead wiped to avoid spasm of the vessel which may reduce blood flow. Blood should be collected over several respiratory cycles. Once blood is collected any visible air should be dispelled from the syringe and a cap placed immediately. The patient will require pressure to be put over the arterial sampling site and may require a firm bandage to be placed following sampling; ensure tight bandages are not left in place for longer than 5 minutes and monitor to ensure a haematoma has not formed. Samples should be run as quickly as possible but can be placed on ice if required for approximately 10 minutes. Before running the sample, the first few drops of blood should be discarded before immediately filling the blood gas analyser cartridge to the required amount.

Sampling Errors

• A clotted sample - using a syringe without coagulant.
• A sample exposed to air either left within the syringe or following sampling being left without a cap or not run promptly.
• A mixed venous sample collected mistaken for arterial blood; arterial blood tends to pulsate into the syringe when collected rather than needing to draw back on the sampling syringe.

Respiratory vs. Metabolic Acid-Base Disorders

Acid-base disorders are either respiratory or metabolic and may be one of four primary disorders: respiratory acidosis (an increase in PCO₂, a decrease in pH<7.35), respiratory alkalosis (a decrease in PCO₂, an increase in pH>7.45), metabolic acidosis (a decrease in HCO₃⁻ a decrease in pH<7.35) and metabolic alkalosis (an increase in HCO₃⁻ an increase in pH>7.45). In primary acid-base disorders there is the required amount of compensation in the opposite system; lungs vs. kidneys.

Mixed Disorders

When there is both a respiratory and a metabolic acid-base disorder a mixed disorder is present; the required amount of compensation is not happening in the patient between the lungs and the kidneys.

Reading a Blood Gas

1.  Evaluate the PaO₂ (the partial pressure of oxygen in the blood. The ideal value can be approximated by multiplying FiO₂ (fraction of inspired oxygen) by 5, e.g., if the FiO₂ = 21%, then ideally PaO₂ = 100 mm Hg) from arterial samples only - check: is the patient hypoxaemic?

2.  Evaluate the pH (concentration of hydrogen ions) - check: is the blood pH within normal limits (between 7.35–7.45), acidotic (<7.35), alkalotic (>7.45)?

3.  Evaluate the PaCO₂ (the partial pressure of carbon dioxide in the blood) - check: does it support the pH findings? Yes - primarily respiratory, No - not primarily respiratory.

4.  Evaluate the HCO₃- (bicarbonate) - check: does it support the pH findings? Yes - primarily metabolic, No - not primarily metabolic.

5.  Evaluate the parameter not responsible for the abnormal condition to assess for compensation. (Base excess (BE) gives an indication of the metabolic component of acid-base disturbances and is generally unaffected by changes in the PaCO₂ - therefore, if the base excess is not normal then it is a primary metabolic disorder).

Reference range

Examples

References

1.  DiBartola SP. Metabolic acid-base disorders. In: DiBartola SP, ed. Fluid, Electrolyte and Acid-Base Disorders in Small Animal Practice, 4th ed. St Louis, MO: Elsevier Sauders; 2012:253–286.

2.  Hooper KJ. Acid-base disorders. In: Drobatz KJ, et al. eds. Textbook of Small Animal Emergency Medicine. Hoboken, NJ: Wiley Blackwell; 2019:683–689.