Blood gas: pH, pCO₂, bicarbonate and base excess in cats and dogs

What it shows

The analyzer measures two of these numbers directly and calculates the other two:

• pH — measured; how acidic the blood is. A lower pH means more acid (acidemia), a higher pH — alkalemia.
• pCO₂ — measured; the pressure of carbon dioxide, the respiratory component. The lungs remove CO₂: when breathing is too shallow, CO₂ builds up and acts as an acid; when breathing is too fast, it falls.
• Actual bicarbonate (HCO₃⁻, cHCO3) — calculated from pH and pCO₂; the main buffer of the blood and a metabolic component.
• Base excess (BE) — calculated from pH, pCO₂ and hematocrit; it also reflects the metabolic side, taking all buffers into account. A negative value (base deficit) goes with metabolic acidosis, a positive one with metabolic alkalosis.

Bicarbonate or total CO₂ on a chemistry panel is a separate measurement from a tube that has been in contact with air. It gives similar information about the metabolic side but says nothing about breathing.

Higher than reference — what it may be a sign of

• High pCO₂ — respiratory acidosis: breathing removes too little CO₂ — upper airway obstruction, severe lung disease with tired breathing muscles, anaesthesia, or drugs and diseases that depress the breathing centre in the brain.
• High bicarbonate and positive base excess — metabolic alkalosis: most often loss of stomach acid with vomiting; also diuretics, and rarely excess aldosterone or added bicarbonate.
• High pH — alkalemia from either kind of alkalosis.
• Compensation: the body shifts the other component the same way to bring pH back. Breathing adjusts quickly; the kidneys need more time, for example to retain bicarbonate in long-standing respiratory acidosis. Compensation usually does not bring pH all the way back.

Lower than reference

• Low pCO₂ — respiratory alkalosis: fast breathing from lack of oxygen, pain, anxiety or lung disease, or as compensation for a metabolic acidosis.
• Low bicarbonate and negative base excess — metabolic acidosis: acids building up (ketoacidosis in diabetes, lactic acid from poor circulation or lack of oxygen, kidney failure, toxins such as ethylene glycol) or bicarbonate lost (diarrhoea, kidney tubule disorders).
• Low pH — acidemia, respiratory or metabolic.
• Artefacts: air bubbles in the syringe falsely raise pH and lower pCO₂; in a sample left standing, lactate builds up and uses up bicarbonate.

A pH inside the range with abnormal pCO₂ and bicarbonate, or the two moving in opposite directions, hints at two disturbances at once — for example, ketoacidosis together with vomiting.

Look at together with

• pO₂ and sO₂ — printed on the same form; they describe oxygenation, and only in arterial blood.
• Anion gap — in metabolic acidosis, separates acids building up from bicarbonate loss.
• Lactate, glucose and blood ketones — common sources of extra acid.
• Chloride and potassium — change with vomiting and with acid-base shifts.

Units and conversion

pH has no unit. pCO₂ is reported in mmHg or kPa: 1 kPa ≈ 7.5 mmHg, so 40 mmHg is about 5.3 kPa. Actual bicarbonate and base excess are in mmol/L; base excess can be negative and may be written as BE or BEecf. In the app each row keeps the unit printed on the form.

Reference range

The reference range is on your lab’s form — for a blood gas, usually the analyzer’s own. It matters whether the blood was arterial or venous: venous blood has a higher pCO₂, a slightly lower pH and a bicarbonate a few mmol/L higher, so a venous result is read against venous intervals. Venous samples are fine for acid-base balance and for estimating breathing. The vet also considers whether the syringe had air bubbles, too much heparin or a delay before analysis — all of these distort the numbers.

Cats and dogs

Sources

1. eClinPath (Cornell University College of Veterinary Medicine). Laboratory detection — chemistry / acid-base.
2. eClinPath (Cornell University College of Veterinary Medicine). Types of disturbances — chemistry / acid-base.
3. Today’s Veterinary Practice. The Practitioner’s Acid–Base Primer: Obtaining & Interpreting Blood Gases.