Metabolic Acidosis in Anaesthesia and Critical Care

Question 1

What defines a strong acid

Answer 1

• pKa <4 meaning it will be >99% dissociated in normal plasma
  e.g. lactic acid pKa 3.8

Question 2

What defines a weak acid

Answer 2

• pKa 4-7 and only partially dissociated in normal plasma
  e.g. albumin pKa 6.75

Question 3

Homeostatic mechanisms for dealing with acidosis

Answer 3

Rapid
- Buffer: carbonic acid - bicarbonate equilibrium in the blood is shifted to the left
- Respiration: minute ventilation increases to excrete excess CO2

Slow
- Renal: excess H ions are excreted by the renal tubular system (sodium-hydrogen antiporter3) and HCO3 is produced
- Liver: metabolises anions such as lactate, acetate and citrate

Question 4

Causes of Acidosis

Answer 4

• An increase in endogenous acid due to increased production (e.g. lactic acidosis) or reduced excretion (e.g. Renal failure)
• Administration of exogenous acids (e.g. salicylates, ethylene glycol)
• A relative increase in chloride concentration compared to sodium concentration (e.g. acetazolamide, excessive 0.9% NaCl use)

Question 5

Common causes of metabolic acidosis seen in critical care

Answer 5

• Sepsis: lactic acidosis, hypoperfusion to kidneys and liver causing failure
• DKA: accumulation of acetoacetate and B-hydroxybutyrate
• Renal failure: accumulation of phosphates, sulphates, urates
• Hypovolaemia: Lactic acidosis
• Excessive 0.9% NaCl use: Chloride associated
• Methanol/ Ethylene glycol ingestion: Increase in formic acid and oxalic acid via alcohol deydrogenase
• Liver failure: impaired metabolism of anions
• SGLT2 Inhibitors: chloride associated as a result of sodium and bicarbonate loss, increase in ketones
• Ileostomy, diarrhoea, urethral diversion: loss of HCO3 rich fluid

Question 6

Mortality of moderate and severe acidosis

Answer 6

Moderate (pH <7.3): 21.5%
Severe (pH <7.2): 48.3%

Question 7

Adverse effects of acute acidosis

Answer 7

Respiratory
- Pulmonary vasoconstriction
- Right shift in oxy-haemoglobin dissociation curve

Cardiovascular
- Reduced myocardial contractility
- Reduced responsiveness to catecholamines
- Decreased SVR
- Arrhythmias

CNS
- Vasodilation and increased cerebral blood flow

Metabolic
- Hyperkalaemia- hydrogen ions are exchanged for intracellular K ions

GI
- Reduced splanchnic blood flow

Renal
- Increased filtration rate

Haematological
- Clotting dysfunction
- Impaired platelet aggregation

Question 8

Anion Gap Calculation

Answer 8

Anion Gap= [Na + K] - [Cl + HCO3]
Normal range 6-12 mEq/L

Question 9

Base deficit definition

Answer 9

The amount of base required to titrate a litre of arterial blood to a pH of 7.4 at 37C and a PaCO2 of 5.3kPa