Blood gas · Basic
Interpret arterial or venous blood gas in dogs and cats. Identifies the primary disturbance, checks whether observed compensation matches the species-specific rule of thumb, and computes anion gap when Na and Cl are supplied. Dog compensation formulas are not extrapolated to cats.
Dog compensation formulas are well-established. Cats are different: the feline kidney does not increase ammoniagenesis in response to metabolic acidosis the way dogs and humans do, and dog formulas should not be extrapolated to cats with metabolic acidosis. The calculator surfaces this caveat explicitly when relevant. Acute vs chronic only matters for respiratory disorders (renal compensation takes 2–5 days).
Enter pH, PCO₂, and HCO₃⁻ to interpret the blood gas.
Formula & compensation rules
Henderson-Hasselbalch
The foundation of acid-base interpretation. pH is determined by the ratio of bicarbonate to dissolved CO₂. Blood gas analyzers measure pH and PCO₂; HCO₃⁻ is calculated from those two using the rearranged Henderson-Hasselbalch equation.
The pKₐ of the bicarbonate buffer system is 6.1; the factor 0.0301 converts PCO₂ in mm Hg to dissolved CO₂ in mEq/L. Doubling HCO₃⁻ at constant PCO₂ raises pH by ~0.3; halving PCO₂ at constant HCO₃⁻ raises pH by ~0.3. The system buffers acutely via this ratio.
Anion gap
The difference between measured cations and measured anions. Useful in classifying metabolic acidosis as high-AG (organic acidosis: lactate, ketones, toxin metabolites, uremic acids) versus normal-AG / hyperchloremic (loss of HCO₃⁻ via GI or renal routes, dilutional, RTA).
Species reference ranges (approximate, from multiple studies cited in DiBartola Ch. 9):
- Dog: 13–25 mEq/L (mean ~19)
- Cat: 17–31 mEq/L (mean ~24, higher net protein charge)
The higher AG in cats reflects a higher net negative charge on feline plasma proteins; it does not indicate an underlying acid-base disorder. Hypoalbuminemia reduces the AG (each 1 g/dL drop in albumin lowers the AG by ~2 mEq/L in humans; similar magnitude reported in dogs).
Compensation rules of thumb
Each primary disorder produces an expected secondary (compensatory) change in the opposite component. Observed values within the expected range are consistent with a simple disorder; observed values outside the expected range suggest a mixed acid-base disorder. The textbook gives point estimates; this calculator uses a ±2 (mm Hg or mEq/L) tolerance band around the point estimate.
| Primary disorder | Dog | Cat |
|---|---|---|
| Metabolic acidosis | ↓ PCO₂ 0.7 mm Hg per 1 mEq/L ↓ HCO₃⁻ | Not established (see caveat below) |
| Metabolic alkalosis | ↑ PCO₂ 0.7 mm Hg per 1 mEq/L ↑ HCO₃⁻ | ↑ PCO₂ 0.7 mm Hg per 1 mEq/L ↑ HCO₃⁻ (limited data) |
| Respiratory acidosis (acute) | ↑ HCO₃⁻ 0.15 mEq/L per 1 mm Hg ↑ PCO₂ | ↑ HCO₃⁻ 0.15 mEq/L per 1 mm Hg ↑ PCO₂ (limited data) |
| Respiratory acidosis (chronic) | ↑ HCO₃⁻ 0.35 mEq/L per 1 mm Hg ↑ PCO₂ | Unknown |
| Respiratory alkalosis (acute) | ↓ HCO₃⁻ 0.25 mEq/L per 1 mm Hg ↓ PCO₂ | ↓ HCO₃⁻ 0.25 mEq/L per 1 mm Hg ↓ PCO₂ (anesthetized study) |
| Respiratory alkalosis (chronic) | ↓ HCO₃⁻ 0.55 mEq/L per 1 mm Hg ↓ PCO₂ | Similar to dogs (pH normalizes; exact coefficient unknown) |
Adapted from DiBartola Ch. 12, Table 12-2 (p. 304). Original dog data from de Morais HSA, DiBartola SP, J Vet Emerg Crit Care 1991;1:39–49.
Cat metabolic acidosis caveat
The feline kidney is reported to lack the adaptive increase in ammoniagenesis that dogs and humans show in response to metabolic acidosis. Cats fed an acid load do not respond with the expected respiratory alkalosis to the same extent (DiBartola Ch. 12, p. 304). Implications:
- A cat with metabolic acidosis and a normal PCO₂ may NOT have a mixed disorder. It may just be a cat.
- Do not apply the dog rule of thumb (0.7 mm Hg ↓ PCO₂ per 1 mEq/L ↓ HCO₃⁻) when interpreting a cat blood gas.
- Clinical interpretation requires the patient's history, physical findings, and ancillary labs in cats with metabolic acidosis.
Worked example
Enter pH, PCO₂, and HCO₃⁻ values to see a worked example.
Sources
-
DiBartola SP, ed. Fluid, Electrolyte, and Acid-Base
Disorders in Small Animal Practice. 4th ed. St. Louis,
MO: Elsevier Saunders; 2012.
- Chapter 9 (Introduction to Acid-Base Disorders), pp. 231–252.
- Chapter 10 (Metabolic Acid-Base Disorders), pp. 253–285.
- Chapter 11 (Respiratory Acid-Base Disorders), pp. 286–301.
- Chapter 12 (Mixed Acid-Base Disorders), pp. 302–315.
- de Morais HSA, DiBartola SP. Ventilatory and metabolic compensation in dogs with acid-base disturbances. J Vet Emerg Crit Care 1991;1:39–49. Primary source for the dog compensation coefficients in Table 12-2.
- Hopper K, Epstein SE. Incidence, nature, and etiology of metabolic acidosis in dogs and cats. J Vet Intern Med 2012;26:1107–1114. Epidemiology of metabolic acidosis as the most common acid-base disturbance in small animal practice.