Acid-Base Balance and Disorders

The normal pH range is 7.35 - 7.45. Acidosis is a pH < 7.35. Alkalosis is a pH > 7.45. The pH affects potassium levels:

• Acidosis comes with Hyperkalaemia

• Alkalosis comes with Hypokalaemia

Respiratory Acidosis

This occurs when there's CO2 retention, leading to a low blood pH, and is usually due to hypoventilation. Causes include:

• COPD

• Severe asthma attack

• Respiratory centre depression - GA, Sedatives, Cerebral trauma

• Pulmonary disease - Pulmonary fibrosis, Pneumonia, RDS

• Neuromuscular disease - MND, GBS

Metabolic compensation - Kidneys excrete more H+ and reabsorb more HCO3-

• Takes a few days and can never be fully compensated

Respiratory Alkalosis

This occurs when there's a low CO2, leading to a high blood pH, and is usually due to hyperventilation. Causes include:

• Kussmaul breathing

• High altitude

• Anxiety

• Hyperthyroidism

• Salicylate poisoning

Metabolic compensation - Kidneys reasborb H+ and excrete more HCO3- via

Metabolic Acidosis

Can either be with a Raised anion gap or a Normal anion gap. Causes include:

• Raised anion gap type is usually due to increased production or ingestion of acid - MUDPILES:

  • Methanol, Uraemia, DKA, Propylene, Iron/Isoniazid, Lactic acidosis, Ethylene, Salicylates

• Normal anion gap is usually due to excessive loss of HCO3-

  • Renal tubular acidosis, Diarrhoea, Adrenal insufficiency, Spironolactone, Pyloric stenosis

Respiratory compensation - Kussmaul breathing (deep, laboured) to excrete more CO2 and raise pH

N.B. Anion gap = (Na + K) - (Cl + HCO3) - NR is 16-20.

Metabolic Alkalosis

Causes include:

• Vomiting (loss of acid)

• Diuretics

• Cushing syndrome

• Conn’s syndrome

Respiratory compensation - hypoventilation but one can only hold breath up to a limit.