Blood oxygen: pO₂ and oxygen saturation (sO₂) in cats and dogs

What it shows

pO₂ (partial pressure of oxygen) is measured by an oxygen electrode in the blood gas analyzer. In an arterial sample it reflects oxygenation — how well the lungs move oxygen into the blood. It does not show ventilation, the removal of CO₂; that is the job of pCO₂. Breathing added oxygen raises pO₂: the expected arterial value is roughly five times the oxygen percentage of the inhaled gas.

sO₂ (oxygen saturation) is the percentage of hemoglobin that carries oxygen. On a blood gas form from arterial blood it may be written SaO₂; a pulse oximeter on the tongue or ear measures the same thing without a needle and calls it SpO₂. Saturation and pO₂ are linked by a curve: high up it is flat, so saturation changes little, but an arterial pO₂ of 60 mmHg corresponds to a saturation of about 90%, and below that saturation falls quickly.

Higher than reference — what it may be a sign of

A pO₂ well above the room-air level is expected when the animal breathes added oxygen — under anaesthesia or in an oxygen cage. An air bubble in the syringe lets oxygen move between the air and the sample, and the result becomes unreliable. Saturation cannot exceed 100%; a value at the top only means that hemoglobin is fully loaded, not that there is “extra” oxygen.

Lower than reference

• Low arterial pO₂ — hypoxemia. An arterial pO₂ of 60 mmHg or less, or a saturation of 90% or less, is described as hypoxemia. Causes include lung disease with poor gas exchange (ventilation–perfusion mismatch, impaired diffusion), shunts, too little oxygen in inhaled air and too shallow breathing.
• Low pO₂ in venous blood is expected: in dogs it is on average 46–55 mmHg lower than arterial, in cats about 73 mmHg lower. It reflects how much oxygen the tissues used, not how the lungs work.
• Artefacts: a sample left standing without cooling — blood cells keep consuming oxygen; air bubbles.
• Saturation that misleads. In methemoglobinemia hemoglobin carries oxygen poorly. A pulse oximeter overestimates saturation in this case; analyzers that use several wavelengths (co-oximetry) are more accurate.

Look at together with

• pCO₂ and pH — from the same syringe; pCO₂ shows ventilation, the other half of the picture.
• Hemoglobin and hematocrit — saturation is a share, not an amount. Anaemia lowers the oxygen-carrying capacity of the blood, which saturation alone does not show.
• Lactate — rises when cells lack oxygen because of poor circulation or hypoxemia.

Units and conversion

pO₂ is reported in mmHg or kPa; 1 kPa ≈ 7.5 mmHg, so 60 mmHg is about 8 kPa and 100 mmHg about 13.3 kPa. sO₂ is a percentage; the same line may appear as SaO₂ (arterial blood). SpO₂ is a pulse-oximeter reading, not a lab row. In the app each row keeps the unit printed on the form.

Reference range

The reference range is on your lab’s form, and for pO₂ and sO₂ it applies only to arterial blood taken while the animal breathes room air. With added oxygen, pO₂ is expected to be much higher. If the form does not say whether the sample was arterial or venous, pO₂ cannot be interpreted — it is a question for the vet. Pulse oximeter readings can be thrown off by movement, poor circulation, dark pigment of the skin or tongue and abnormal forms of hemoglobin.

Cats and dogs

Sources

1. eClinPath (Cornell University College of Veterinary Medicine). Laboratory detection — chemistry / acid-base (blood gas sampling; venous versus arterial).
2. Today’s Veterinary Practice. The Practitioner’s Acid–Base Primer: Obtaining & Interpreting Blood Gases.
3. Today’s Veterinary Practice. Anesthetic Monitoring: Devices to Use and What the Results Mean.
4. Merck Veterinary Manual. The Respiratory System in Animals (hypoxia and its causes).
5. Merck Veterinary Manual. Principles of Therapy of Respiratory Disease in Animals.
6. eClinPath (Cornell University College of Veterinary Medicine). 2020 Case #2 — methemoglobinemia in a cat, oximetry and co-oximetry.