Venous blood gas in the cat: what it can and cannot assess
8-year-old cat with moderate tachypnoea. As arterial sampling is difficult, blood is taken from the jugular vein. Venous blood gas on room air:
| Parameter | Result | Reference |
|---|---|---|
| pH | 7.36 | 7.30–7.40 (venous) |
| PvO2 | 38 mmHg | 35–45 (venous) |
| PvCO2 | 36 mmHg | 33–45 (venous) |
Which interpretation is correct?
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A. A PvO2 of 38 mmHg indicates severe hypoxaemia requiring mechanical ventilation
A PvO2 of 35–45 mmHg is normal in venous blood; it reflects tissue oxygen extraction, not pulmonary gas exchange.
B. The alveolar-arterial gradient can be calculated from PvO2 and PvCO2
The A-a gradient is only valid with arterial PaO2; with venous values the result is meaningless.
C. PvO2 says nothing about lung function, but PvCO2 (slightly above PaCO2) rules out significant hypoventilation
Correct. PvCO2 usually exceeds PaCO2 by about 3–6 mmHg when perfusion is good, so a PvCO2 of 36 mmHg excludes hypercapnia. To assess oxygenation, use pulse oximetry or an arterial sample.
D. PvCO2 is useless because it bears no relation to PaCO2
In well-perfused patients PvCO2 is a reasonable approximation of PaCO2; it only becomes unreliable in shock or arrest, when the difference widens greatly.
lightbulb Key takeaway
To assess the lungs, oxygenation is measured in arterial blood (or by pulse oximetry); ventilation can be estimated from PvCO2 or EtCO2. In cats, the femoral or dorsal metatarsal artery is the most accessible for arterial sampling.
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Updated: October 31, 2026 · Educational content for veterinary professionals. It does not replace current clinical guidelines or the judgement of the attending veterinarian.