Disorders Of Acid Base Balance – Overview

pH of our body is maintained at a narrow range of 7.45 – 7.45
A buffering system of our body helps to maintain this range. 

This buffering system consists of acids, bases and buffers.
There are two compensatory mechanism

  • Renal compensation 
  • Respiratory compensation 


Arterial blood gas analysis helps to determine the type of acid-base disorder and its compensation.


Anion Gap

Anion gap is the difference between primary measured cations (sodium and potassium) and the primary measured anions (chloride and bicarbonate) in serum

Anion cap = ( Na⁺ + K⁺) – (Cl⁻ + HCO₃⁻)

Normal anion gap is 10 – 18 mmol/L


Normal Values Of ABG

pH 7.35 – 7.45
PaCO2 4.7 – 6.0 kPa /35 – 45 mmHg
PaO2 10 – 14 kPa / 75 – 100 mmHg
Base excess ± 2 mmol/L



Metabolic Acidosis

This is the most common acid base disorder encountered in surgery.

  • Results from a decrease of HCO₃⁻ or an increased H⁺
  • Compensated by decreasing PaCO₂ and and decreasing the renal loss of HCO₃⁻ 
  • Causes
    • High anion gap (Another soursce of acid production present)
      • Lactic acidosis
        • Lactic acidosis type A – (Perfusion disorders) – Hypoxia, shock  
        • Lactic acidosis type B – (Metabolic disorders) – Metformin toxicity 
      • Increased ketone production – Alcohol, DKA
      • Urate – Renal failure
      • Acid poisoning: salicylates, methanol
    • Normal anion gap (Hyperchloraemic metabolic acidosis) 
      • Gastrointestinal bicarbonate loss: Diarrhoea, GI Fistula, Ureterosigmoidostomy 
      • Renal tubular acidosis
      • Drugs – Acetazolamide 
      • Parenteral nutrition 
      • Ammonium chloride injection
      • Addison’s disease



Metabolic Alkalosis

  • Results from an increase of HCO₃⁻ or a decreased H⁺
  • Compensated by increasing PaCO₂ and increasing the renal loss of HCO₃⁻ (If HCO₃⁻ is more than 24 mmol/L) 
  • Causes
    • Vomiting / Nasogastric suction
    • Diuretics
    • Alkali abuse
    • Over treatment for acidosis
    • Hypokalaemia
    • Primary hyperaldosteronism
    • Cushing’s syndrome
    • Bartter’s syndrome
    • Congenital adrenal hyperplasia



Respiratory Acidosis

  • Results from an increase in PaCO₂ due to alveolar hypoventilation 
  • Compensated by decreasing the renal loss of HCO₃⁻ 
  • Causes – Type II respiratory failure
    • COPD
    • Myasthenia 
    • Ankylosing spondylitis
    • Pulmonary oedema
    • Life threatening astham
    • Sedatives – Benzodiazepines, opiate overdose



Respiratory Alkalosis

  • Results from a decrease in PaCO₂ due to alveolar hyperventilation 
  • Compensated by increasing the renal loss of HCO₃⁻ 
  • Causes
    • Psychogenic: Panic attacks, anxiety
    • High altitude
    • Pulmonary embolism 
    • Hypermetabolic state – Fever, hyperthyroidism, pregnancy
    • CNS stimulation: Stroke, subarachnoid haemorrhage, encephalitis
Salicylate poisoning is a mixture of respiratory alkalosis and metabolic acidosis.
Acid-base_nomogram
Latest posts by Dr. Pasindu Suriapperuma (see all)
0 0 vote
Article Rating
Subscribe
Notify of
guest
0 Comments
Inline Feedbacks
View all comments