Disorders of Acid - Base Balance

Question 1

What is the most common surgical acid - base disorder?

Answer 1

Metabolic acidosis - Reduction in plasma bicarbonate levels

Question 2

What are the two mechanisms?

Answer 2

1. Gain of strong acid (e.g. diabetic ketoacidosis)
2. Loss of base (e.g. from bowel in diarrhoea)
   - Classified according to the anion gap, this can be calculated by:
   (Na+ + K+) - (Cl- + HCO3-).

Question 3

In Metabolic acidosis what conditions retain a normal anion gap?

Answer 3

Normal anion gap ( = hyperchloraemic metabolic acidosis)

1. Gastrointestinal bicarbonate loss: diarrhoea, ureterosigmoidostomy, fistula
2. Renal tubular acidosis
   Drugs: e.g. acetazolamide
3. Ammonium chloride injection
4. Addison’s disease

Question 4

In Metabolic acidosis what conditions retain a raised anion gap?

Answer 4

1. Lactate: shock, hypoxia
2. Ketones: diabetic ketoacidosis, alcohol
3. Urate: renal failure
4. Acid poisoning: salicylates, methanol

Question 5

How is metabolic acidosis secondary to high lactate levels subdivided?

Answer 5

1. Lactic acidosis type A: (Perfusion disorders e.g.shock, hypoxia, burns)
2. Lactic acidosis type B: (Metabolic e.g. metformin toxicity)

Question 6

What is metabolic alkalosis?

Answer 6

1. Usually caused by a rise in plasma bicarbonate levels.
2. Rise of bicarbonate above 24 mmol/L will typically result in renal excretion of excess bicarbonate.
3. Caused by a loss of hydrogen ions or a gain of bicarbonate. It is due mainly to problems of the kidney or gastrointestinal tract

Question 7

What causes metabolic alkalosis?

Answer 7

1. Vomiting / aspiration (e.g. Peptic ulcer leading to pyloric stenosis, nasogastric suction)
2. Diuretics
3. Liquorice, carbenoxolone
4. Hypokalaemia
5. Primary hyperaldosteronism
6. Cushing’s syndrome
7. Bartter’s syndrome
8. Congenital adrenal hyperplasia

Question 8

What is the Mechanism of metabolic alkalosis?

Answer 8

1. Activation of renin-angiotensin II-aldosterone (RAA) system is a key factor
2. Aldosterone causes reabsorption of Na+ in exchange for H+ in the distal convoluted tubule
3. ECF depletion (vomiting, diuretics) → Na+ and Cl- loss → activation of RAA system → raised aldosterone levels
4. In hypokalaemia, K+ shift from cells → ECF, alkalosis is caused by shift of H+ into cells to maintain neutrality

Question 9

What is respiratory acidosis?

Answer 9

1. Rise in carbon dioxide levels usually as a result of alveolar hypoventilation
2. Renal compensation may occur leading to Compensated respiratory acidosis

Question 10

Causes of Respiratory acidosis?

Answer 10

1. COPD
2. Decompensation in other respiratory conditions e.g. Life-threatening asthma / pulmonary oedema
3. Sedative drugs: benzodiazepines, opiate overdose

Question 11

What is Respiratory alkalosis?

Answer 11

1. Hyperventilation resulting in excess loss of carbon dioxide
2. This will result in increasing pH

Question 12

What are the causes of Respiratory Alkalosis?

Answer 12

1. Psychogenic: anxiety leading to hyperventilation
2. Hypoxia causing a subsequent hyperventilation: pulmonary embolism, high altitude
3. Early salicylate poisoning*
   CNS stimulation: stroke, subarachnoid haemorrhage, encephalitis
4. Pregnancy

Question 13

Salicylate overdose ?

Answer 13

1. Salicylate overdose leads to a mixed respiratory alkalosis and metabolic acidosis.
2. Early stimulation of the respiratory centre leads to a respiratory alkalosis
3. Whilst later the direct acid effects of salicylates (combined with acute renal failure) may lead to an acidosis