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Blood Gas Analysis

Lori S. Waddell, DVM, DACVECC University of Pennsylvania

Clinical Pathology

|January 2012|Peer Reviewed

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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
    • 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
      • 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

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
    • 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

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

 ArterialVenous
Canine      
pH7.35–7.45 7.35–7.45
  PO2 (mm Hg)90–100 30–42
PCO2 (mm Hg)35–45 40–50
HCO3- (mEq/L)20–24 20–24
 BE (mmol/L)-4–+4-4–+4
Feline  
 pH7.34 ± 0.17.30 ± 0.08
PO2 (mm Hg)102.9 ± 1538.6 ± 11
PCO2 (mm Hg)33.6 ± 741.8 ± 9
HCO3- (mEq/L)17.5 ± 319.4 ± 4
  BE (mmol/L)-6.4 ± 5-5.7 ± 5

Rules of Compensation
1. Change in respiratory or metabolic component of the acid-base status will normally induce opposite, compensatory change in the other to return the pH toward normal.
2. Lungs compensate rapidly by changing minute ventilation (respiratory rate/tidal volume/both) within minutes.
3. Metabolic compensation occurs via the kidneys and is much slower, starting after a few hours and requiring 4 to 5 days for maximum compensation.
4. Absence or presence and degree of compensation for respiratory disturbance can give an idea of chronicity (Table 2).
5. Overcompensation does not occur.
6. If expected compensation is absent, a mixed disturbance is present. For example, if metabolic acidosis is not accompanied by compensatory respiratory alkalosis (the CO2 is normal or increased), a mixed disturbance is occurring with both metabolic acidosis and respiratory acidosis.

Table 2. Expected Compensatory Changes

DisorderPrimary ChangeCompensatory Response
 
Metabolic acidosis↓ HCO3-0.7 mm Hg decrease in PCO2 for each 1 mEq/L decrease in HCO3-
 
Metabolic alkalosis↑ HCO3-0.7 mm Hg increase in PCO2 for each 1 mEq/L increase in HCO3-
Acute respiratory acidosis↑ PCO21.5 mEq/L increase in HCO3- for each 10 mm Hg increase in PCO2
Chronic respiratory acidosis↑ PCO23.5 mEq/L increase in HCO3- for each 10 mm Hg increase in PCO2
Acute respiratory alkalosis↓ PCO22.5 mEq/L decrease in HCO3- for each 10 mm Hg decrease in PCO2
Chronic respiratory alkalosis↓ PCO25.5 mEq/L decrease in HCO3- for each 10 mm Hg decrease in PCO2

BLOOD GAS ANALYSIS • Lori S. Waddell

Suggested Reading

Arterial and venous blood gas tensions in clinically healthy cats. Middleton DJ, Ilkiw JE, Watson ADJ. Am J Vet Res 42:1609-1611, 1981
Introduction to acid-base disorders. DiBartola SP. In DiBartola SP (ed): Fluid Therapy in Small Animal Practice, 2nd ed—Philadelphia: WB Saunders, 2000, pp 189-210.
Metabolic acid-base disorders. DiBartola SP. In DiBartola SP (ed): Fluid Therapy in Small Animal Practice, 2nd ed—Philadelphia: WB Saunders, 2000, pp 211-240.
Respiratory acid-base disorders. De Morias HA, DiBartola SP. In DiBartola SP (ed): Fluid Therapy in Small Animal Practice, 2nd ed—Philadelphia: WB Saunders, 2000, pp 241-261. 

For global readers, a calculator to convert laboratory values, dosages, and other measurements to SI units can be found here.

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