Acid-Base Disorders - Anion gap, ABG, Metabolic acidosis/alkalosis, Respiratory acidosis/alkalosis Flashcards
How is metabolic acidosis classified?
By anion gap:
Normal anion gap (hyperchloraemic)
Raised anion gap
What are the causes of normal anion gap metabolic acidosis?
Gastrointestinal bicarbonate loss: Diarrhoea, ureterosigmoidostomy, fistula
Renal tubular acidosis
Drugs: e.g., acetazolamide
Ammonium chloride injection
Addison’s disease
What are common causes of respiratory alkalosis?
Anxiety leading to hyperventilation
Pulmonary embolism
Salicylate poisoning
CNS disorders: Stroke, subarachnoid haemorrhage, encephalitis
Altitude
Pregnancy
What causes metabolic alkalosis?
Loss of hydrogen ions or gain of bicarbonate
Vomiting/aspiration
Diuretics
Liquorice, carbenoxolone
Hypokalaemia
Primary hyperaldosteronism
Cushing’s syndrome
Bartter’s syndrome
Congenital adrenal hyperplasia
What are the causes of raised anion gap metabolic acidosis?
Lactate: Shock, hypoxia
Ketones: Diabetic ketoacidosis, alcohol
Urate: Renal failure
Acid poisoning: Salicylates, methanol
What are common causes of respiratory acidosis?
COPD
Decompensation in respiratory conditions (e.g., severe asthma, pulmonary oedema)
Sedative drugs (e.g., benzodiazepines, opiate overdose)
What are common causes of respiratory alkalosis?
Anxiety leading to hyperventilation
Pulmonary embolism
Salicylate poisoning
CNS disorders: Stroke, subarachnoid haemorrhage, encephalitis
Altitude
Pregnancy
How is the anion gap calculated?
Anion gap = (Sodium + Potassium) - (Bicarbonate + Chloride)
What is the normal range for the anion gap?
8-14 mmol/L
What is a potential cause of a raised anion gap metabolic acidosis related to chronic paracetamol use?
5-oxoproline
What is the first step in the Resuscitation Council’s 5-step approach to arterial blood gas interpretation?
How is the patient?
How can you assess if a patient is hypoxaemic?
Check if PaO2 on air is >10 kPa
What does a PaCO2 > 6.0 kPa indicate?
Respiratory acidosis
Or respiratory compensation for metabolic alkalosis
What does a PaCO2 < 4.7 kPa indicate?
Respiratory alkalosis
Or respiratory compensation for metabolic acidosis
What bicarbonate level suggests metabolic acidosis?
Bicarbonate < 22 mmol/L
Or base excess < -2 mmol/L
What bicarbonate level suggests metabolic alkalosis?
Bicarbonate > 26 mmol/L
Or base excess > +2 mmol/L
What does ROME stand for in arterial blood gas interpretation?
Respiratory = Opposite
Low pH + High PaCO2 = Acidosis
High pH + Low PaCO2 = Alkalosis
Metabolic = Equal
Low pH + Low bicarbonate = Acidosis
High pH + High bicarbonate = Alkalosis
What clues might indicate the need to calculate the anion gap in metabolic acidosis?
Supplied chloride level
What are the subtypes of lactic acidosis and their causes?
Type A: Sepsis, shock, hypoxia, burns
Type B: Metformin
What is a common cause of normal anion gap metabolic acidosis that can also cause hypokalaemia?
Prolonged diarrhoea
What condition is associated with a normal anion gap and is often treated with ammonium chloride?
Addison’s disease
What type of metabolic acidosis is linked to high lactate levels and associated with sepsis and shock?
Lactic acidosis Type A
What is a key drug-related cause of raised anion gap metabolic acidosis?
Metformin (Type B lactic acidosis)
Which syndrome is associated with metabolic alkalosis and involves a loss of potassium?
Primary hyperaldosteronism
How does activation of the renin-angiotensin II-aldosterone (RAA) system contribute to metabolic alkalosis?
Aldosterone causes Na+ reabsorption in exchange for H+, leading to alkalosis when Na+ and Cl- are lost (e.g., due to vomiting or diuretics)
What role does hypokalaemia play in metabolic alkalosis?
In hypokalaemia, K+ shifts from cells to ECF, causing H+ to move into cells to maintain neutrality, leading to alkalosis.
Which type of neuromuscular disease is associated with respiratory acidosis?
Neuromuscular diseases (e.g., amyotrophic lateral sclerosis)
What syndrome related to obesity can contribute to respiratory acidosis?
Obesity hypoventilation syndrome
Which sedative drugs can cause respiratory acidosis?
Benzodiazepines and opiate overdose
What other respiratory conditions might lead to respiratory acidosis if they decompensate?
Life-threatening asthma or pulmonary oedema
What is a common CNS disorder that can lead to respiratory alkalosis?
Stroke, subarachnoid haemorrhage, or encephalitis
How can pregnancy contribute to respiratory alkalosis?
Pregnancy can cause respiratory alkalosis due to increased respiratory rate and decreased CO2 levels.
How does salicylate poisoning affect blood gas levels?
Salicylate poisoning causes mixed respiratory alkalosis and metabolic acidosis; early stimulation of the respiratory centre leads to respiratory alkalosis, while later effects and acute renal failure lead to acidosis.
