Veterinary Clinical Biochemistry and Laboratory Diagnosis-II

 ACIDOSIS

METABOLIC ACIDOSIS

• Metabolic Acidosis is also called as Primary HCo₃ deficit; and is the commonest acid-base disturbance observed clinically.
• This condition is produced by the addition of an acid or loss of bicarbonate, or dilution of bicarbonate.

Mechanisms

• If primary deficit of HCO3^- occurs, the ratio of [HCO3^- ] / [H2CO3] = 20/1 , is decreased. i.e. pH is decreased resulting in metabolic acidosis (primary bicarbonate deficit).

Primary Compensatory Mechanism

• The respiratory centre is stimulated by acidosis
• This increased ventilaion will result in CO2 loss and reduction in [H2CO3] (carbonic acid).
•  Causes of metabolic acidosis include
  • GI Loss of Bicarbonate: Diarrhea -Small bowel or pancreatic drainage -obstructed ileal loop -Anion exchange results.
  • Renal loss of Bicarbonate: Carbonic anhydrase inhibitors-Renal Tubular Acidosis (proximal RTA is difficult to treat the more bicarbonate is given, more it spills out; distal RTA is easier to treat)-Hyperparathyroidism-Hypoaldosteronism plays role in acid base balance.
  • Increased Acid Production: Lactic acidosis-Starvation ketoacidosis -Alcoholic ketoacidosis -Diabetic ketoacidosis.
  • Ingestion of Toxic Substances: Salicylate Overdose -Methanol Ingestion -Ethylene Glycol (antifreeze) ingestion.
  • Failure of Acid Excretion: Acute and chronic renal failure.
  • Respiratory Acidosis is a clinical disturbance that is due to alveolar hypoventilation which increases the level of pCO2.
  • Alveolar hypoventilation leads to an increased pCO2.
  • The increase in pCO2 in turn decreases the HCO3-/pCO2 ratio and decreases pH.

Respiratory Acidosis

• Respiratory Acidosis is a clinical disturbance that is due to alveolar hypoventilation which increases the level of pCO2.
• Alveolar hypoventilation leads to an increased pCO2.
• The increase in pCO2 in turn decreases the HCO3-/pCO2 ratio and decreases pH.

Causes

•  Depression of central nervous system Chronic respiratory acidosis may be secondary to many disorders, including COPD.
• Disease of lung
• Thoraxic muscle disorders.
• Neuromuscular disorders, interstitial fibrosis and thoracic deformities.
• Lung diseases that cause abnormality in alveolar gas exchange, cause stimulation of ventilation and hypocapnia secondary to hypoxia.
• Hypercapnia only occurs if severe disease or respiratory muscle fatigue occurs.

Physiological Response

• Failure of ventilation quickly increases the pCO2.
• In respiratory acidosis, compensation occurs in 2 steps.
  • The initial response is cellular buffering that occurs over minutes to hours.
  • The second step is renal compensation that occurs over 3-5 days.