Overview

• Conditions characterised by changes in the concentration of hydrogen ions or bicarbonate, leading to changes in the arterial blood pH.
• Metabolic Acidosis
  • Normal Anion Gap (6-16 mmol/L) ⇒ diarrhoea, renal tubular acidosis, addison disease (hyperkalaemia = acidosis), acetazolamide
  • Raised Anion Gap (>16 mmol/L) ⇒ lactic acidosis (eg. sepsis, metformin), diabetic ketoacidosis, uraemia, alcohol (metabolic ketoacidosis with normal or low glucose - think alcohol)
• Metabolic Alkalosis (may be caused by loss of H+ or gain of HCO3-) ⇒ vomiting, diuretics, hyperaldosteronism + cushing’s (hypokalaemia = alkalosis)
• Respiratory Acidosis (may be due to hypoventilation) ⇒ airway obstruction, COPD, opioids/sedatives, weakening of respiratory muscles
• Respiratory Alkalosis (may be due to hyperventilation) ⇒ anxiety/panic attack, PE, hypoxia (eg. due to high altitude), aspirin OD (in early stages)
  • Anxiety/Panic Attack ⇒ hyperventilation leading to respiratory alkalosis, this causes hypocalcaemia which can then cause tingling in lips and fingers.
• Overview Table

Making Diagnosis

Investigations:

• Arterial Blood Gas

  1. Evaluate Blood pH ⇒ <7.35 (acidosis) or >7.45 (alkalosis)

  2. Evaluate pCO2 ⇒ if pH and CO2 change in opposite direction = respiratory disorder. If pH and CO2 change in same direction = metabolic disorder.

  3. Evaluate HCO3- ⇒ high = metabolic alkalosis or compensated respiratory acidosis. Low = metabolic acidosis or compensated respiratory alkalosis.

     Base Excess ⇒ >+3 indicates metabolic alkalosis. <-3 indicates metabolic acidosis.

  • Compensation

  <aside> 💡 ROME ⇒ Respiratory = Opposite (CO2). Metabolic = Equal (HCO3-)

  </aside>

• Anion Gap = ([Na+] + [K+]) - ([Cl] + [HCO3])

Management Plan

• Respiratory Acidosis & Alkalosis → treat underlying cause
• Metabolic Acidosis → IV Sodium Bicarbonate
• Metabolic Alkalosis → Acetazolamide (carbonic anhydrase inhibiting diuretic)