Select machine (eg, i-STAT, abbott.com; VitalPath, heska.com)
Determine if arterial or venous
Venous: Jugular sample provides the best idea of whole-body status (pulmonary artery sample is ideal but not practical). Limb samples may represent local tissue bed and not whole body.
- Venous sample (pulse oximetry can be used with venous blood gas to assess oxygenation)
- PvCO2 can suggest ventilation (about 5 mm Hg higher than PaCO2)
Arterial: Dorsal metatarsal artery, femoral artery, auricular artery, or caudal artery
- Arterial sample (to assess respiratory function)
- PaCO2 assesses ability to ventilate
- PaO2 assesses ability to oxygenate
Evaluate pH to determine if acidemia or alkalemia present
Related Article: Respiratory Support for Acute Intensive Care
Acidemia = PH <7.35
- Determine if metabolic or respiratory in origin
- Respiratory = PaCO2 >45 mm Hg
- Diagnosis: Respiratory Acidosis
- Depressed respiratory center
- Cervical spinal cord disease
- Neuromuscular disease
- Pleural space disease
- Airway obstruction
- Rarely severe pulmonary parenchymal disease
- Treatment
- Correct underlying problem
- Relieve airway obstruction/restrictive disease (pleural space)
- Intubate, begin PPV
- Diagnosis: Respiratory Acidosis
- Metabolic = BE <-4 mmol/L (or HCO3- <21 mEq/L)
- Diagnosis:Metabolic Acidosis
- Increased anion gap:
- Ketones
- Lactate
- Uremia
- Toxicity (ethylene glycol, salicylates)
- Normal anion gap:
- HCO3- loss through kidneys or intestinal tract
- Increased anion gap:
- Treatment
- Correct underlying cause
- Lacate: Improve oxygen delivery to the tissues
- Ketones: Insulin therapy
- NaHCO3 (if needed)
- HCO3− deficit = BE × body weight (kg) × 0.3
- Give 1/4–1/3 of dose and recheck blood gas
- Diagnosis:Metabolic Acidosis
- Respiratory = PaCO2 >45 mm Hg
Related Article:Hypochloremic Metabolic Alkalosis
Evaluate PCO2 and BE for masked disturbances if pH = 7.35–7.45
- Assess for compensation (see Rules of Compensation, below)
Alkalemia = pH >7.45
- Determine if metabolic or respiratory in origin
- Respiratory = PaCO2 <35 mm Hg
- Diagnosis: Respiratory Alkalosis
- Hypoxemia
- Pulmonary disease
- CNS disease (stimulating respiratory
center) - Exercise, pain, stress
- Treatment
- Correct underlying diseases
- Diagnosis: Respiratory Alkalosis
- Metabolic = BE >4 mmol/L (or HCO3- >27 mEq/L)
- Diagnosis: Metabolic Alkalosis
- GI obstruction with loss of H+, K+, and especially Cl- in vomitus
- Loop diuretics
- NaHCO3 administration
- Treatment
- Hypokalemic hypochloremic metabolic alkalosis: Volume expansion with 0.9% NaCl
- Loop diuretic or HCO3- therapy: No treatment, usually self-limiting
- Diagnosis: Metabolic Alkalosis
- Respiratory = PaCO2 <35 mm Hg
This algorithm reflects canine normals. For cats, substitute feline normals for pH, BE (or HCO3-), PCO2, and PO2 values (Table 1).
Table 1. Normal Values for Blood Gases