Respiratory Acidosis with Renal Compensation

History. A 6-year-old male German shepherd is brought to you with complaints of weakness» exercise intolerance, and inappetence that have progressively worsened over the past 6 weeks.

Clinical Examination. The dog is recumbent and anxious. Respiration is labored» and the heart rate is rapid» but pulses are strong and regular. Crackles are heard over all lung fields. Thoracic radiographs reveal a diffuse, severe pulmonary inter­stitial and alveolar infiltrate with enlargement of the hilar lymph nodes. You obtain samples of blood and urine for a complete blood cell count (CBC)» serum chemistry panel» urinalysis, and arterial blood gas (ABG) measurement. The urine pH is 5.0, and the ABG results are as follows: pH, 7.37 (normal, 7.45); Po₂, 58 mm Hg (normal, 80-100 mm Hg); Pco₂, 70 mm Hg (normal, 31-35 mm Hg); and HCO₃^-, 37 mEq/L (normal, 18-24 mEq/L).

Comment. The dog has chronic respiratory acidosis caused by severe pulmonary infiltrates. The lung is unable to ventilate adequately, and the blood level of (LO₂ rises. The elevated CO₂ favors the production of carbonic acid, which releases H’ and lowers the blood pH. Although the increased CO₂ level con­tributes to an increase in blood HCO₃^- levels, the marked increase in the blood HCO₃^- levels in this case results from enhanced renal retention of HCO₃^- and secretion of H^t. Acidemia also enhances ammoniagenesis in the proximal tubule, thereby enhancing the generation of new bicarbonate and acid excretion in the form of ammonium ion. Respiratory acidosis activates the acid-secreting intercalated cells in the collecting duct, where HCO₃ is reabsorbed and H‘ is secreted. The blood HCO₃ concentration rises and helps return the blood pH toward normal. A steep H* gradient is established in the collecting ducts, and acid urine is excreted.

Treatment. Diagnose and correct the pulmonary disease, if possible. Bicarbonate therapy is not indicated, because the blood bicarbonate level is already high, and the blood pH is partially corrected. Oxygen therapy may improve the Pao₂ and help support the animal until specific treatment is instituted.