Acid-base disturbances are present in the majority of critically ill veterinary patients. Metabolic acidosis is the most common primary disorder in emergency patients, with elevated lactate serving as both a diagnostic and prognostic marker. A systematic approach to blood gas interpretation prevents misdiagnosis of mixed disorders. Use the Bloodwork OCR for panel interpretation and the Fluid Therapy Calculator for correction planning.
A reliable, repeatable system prevents errors in blood gas interpretation. Follow these four steps in order for every blood gas you analyze:
Step 1: Assess the pH. Is it acidemic (<7.35), alkalemic (>7.45), or normal? A normal pH does not rule out an acid-base disorder; it may indicate full compensation or opposing mixed disorders.
Step 2: Identify the primary disorder. Does the primary change match the pH? If pH is low, look for either decreased HCO&sub3;¹¯ (metabolic acidosis) or increased PaCO&sub2; (respiratory acidosis). If pH is high, look for increased HCO&sub3;¹¯ (metabolic alkalosis) or decreased PaCO&sub2; (respiratory alkalosis).
Step 3: Assess compensation. The body compensates for the primary disorder. Metabolic disorders trigger respiratory compensation (changes in ventilation) within minutes to hours. Respiratory disorders trigger renal compensation (HCO&sub3;¹¯ retention or excretion) over 2-5 days. Apply the expected compensation formulas.
Step 4: Calculate the anion gap. Anion gap = Na¹+ - (Cl¹¯ + HCO&sub3;¹¯). Normal is 12-24 mEq/L in dogs. An elevated anion gap indicates unmeasured anions (lactate, ketoacids, uremic toxins, ethylene glycol metabolites).
| Disorder | pH | Primary Change | Compensation | Common Causes |
|---|---|---|---|---|
| Metabolic Acidosis | ↓ | ↓ HCO&sub3;¹¯ | ↓ PaCO&sub2; (hyperventilation) | DKA, lactic acidosis, renal failure, diarrhea, ethylene glycol |
| Metabolic Alkalosis | ↑ | ↑ HCO&sub3;¹¯ | ↑ PaCO&sub2; (hypoventilation) | Vomiting (pyloric obstruction), diuretic use, post-hypercapnic |
| Respiratory Acidosis | ↓ | ↑ PaCO&sub2; | ↑ HCO&sub3;¹¯ (renal retention) | Airway obstruction, pneumothorax, neuromuscular disease, anesthesia |
| Respiratory Alkalosis | ↑ | ↓ PaCO&sub2; | ↓ HCO&sub3;¹¯ (renal excretion) | Pain, anxiety, hypoxemia, heat stroke, hepatic encephalopathy |
Comparing the actual compensation to the expected compensation reveals whether the disorder is simple (with appropriate compensation) or mixed (compensation is more or less than expected).
| Primary Disorder | Expected Compensation |
|---|---|
| Metabolic acidosis | Expected PaCO&sub2; = 1.5 × [HCO&sub3;¹¯] + 8 (±2) (Winter's formula) |
| Metabolic alkalosis | Expected PaCO&sub2; = 0.7 × [HCO&sub3;¹¯] + 21 (±2) |
| Acute respiratory acidosis | ΔHCO&sub3;¹¯ = 0.15 × ΔPaCO&sub2; |
| Chronic respiratory acidosis | ΔHCO&sub3;¹¯ = 0.35 × ΔPaCO&sub2; |
| Acute respiratory alkalosis | ΔHCO&sub3;¹¯ = 0.25 × ΔPaCO&sub2; |
| Chronic respiratory alkalosis | ΔHCO&sub3;¹¯ = 0.55 × ΔPaCO&sub2; |
If the measured PaCO&sub2; is lower than expected for a metabolic acidosis, a concurrent respiratory alkalosis is present (mixed disorder). If higher, a concurrent respiratory acidosis exists. These mixed disorders are clinically important and change the treatment approach.
The anion gap (AG) differentiates between types of metabolic acidosis. A high anion gap metabolic acidosis (AG >25 mEq/L) indicates accumulation of unmeasured anions: lactic acidosis (shock, sepsis), diabetic ketoacidosis, uremic acidosis (renal failure), and ethylene glycol poisoning (oxalate accumulation). A normal anion gap (hyperchloremic) metabolic acidosis results from bicarbonate loss (diarrhea, renal tubular acidosis) or chloride gain (0.9% NaCl fluid therapy).
The delta-delta ratio (ΔAG / ΔHCO&sub3;¹¯) further refines diagnosis in high AG acidosis: a ratio >2 suggests concurrent metabolic alkalosis, while a ratio <1 suggests concurrent non-AG metabolic acidosis.
Diabetic Ketoacidosis (DKA): High AG metabolic acidosis with ketonemia/ketonuria. Treat with IV regular insulin, aggressive fluid therapy (0.9% NaCl initially), potassium supplementation, and phosphorus monitoring.
Gastric Dilatation-Volvulus (GDV): Mixed metabolic acidosis (lactic) with possible metabolic alkalosis (from sequestration of HCl in the stomach). Lactate >6.0 mmol/L at presentation is associated with gastric necrosis.
Chronic Kidney Disease: Normal to high AG metabolic acidosis due to failure to excrete hydrogen ions and regenerate bicarbonate. Oral sodium bicarbonate supplementation may be indicated when serum HCO&sub3;¹¯ is consistently <18 mEq/L.
Warning: IV sodium bicarbonate administration should be reserved for severe acidemia (pH <7.1) or HCO&sub3;¹¯ <8 mEq/L. Rapid correction can cause paradoxical CNS acidosis, hypokalemia, ionized hypocalcemia, and overshoot alkalosis. If needed, administer slowly: dose (mEq) = 0.3 × body weight (kg) × base deficit, giving 25-50% of the calculated dose over 15-30 minutes.
Arterial blood gas (ABG) is the gold standard for assessing oxygenation (PaO&sub2;) and ventilation (PaCO&sub2;). However, venous blood gas (VBG) is often more practical in clinical settings and provides reliable acid-base and electrolyte information. Venous pH is typically 0.03-0.05 units lower than arterial, and venous PCO&sub2; is 3-8 mmHg higher. For acid-base interpretation, these differences are clinically acceptable for most decisions.
- Always use the systematic 4-step approach: pH, primary disorder, compensation, anion gap.
- Metabolic acidosis is the most common acid-base disorder in emergency patients.
- Apply compensation formulas to identify mixed disorders that change treatment.
- High anion gap acidosis: think lactate (shock), ketoacids (DKA), uremia, toxins.
- Venous blood gas is acceptable for acid-base assessment; arterial required for oxygenation status.
- IV bicarbonate is reserved for severe acidemia (pH <7.1) due to risk of complications.