Clinical Aspects of Acid-Base Control Flashcards
What are the buffers of pH in the body?
- Proteins
- Haemoglobin
- Carbonic acid / bicarbonate
How is standard bicarbonate calculated?
From the actual bicarbonate but assuming 37C and a paCO2 of 5.3kPa
What happens to the base excess in a metabolic acidemia?
Becomes more negative
What is likely to be occuring if pCO2 and bicarbonate are moving in the same direction?
Compensation
What is the anion gap?
- Sum of routinely measured cations in venous blood minus routinely measured anions
- (Na+ + K+) - (Cl- + HCO3-)
What are the main anions?
- Chloride
- Bicarbonate
- Protein
- Organic acid
- Phosphate
- Sulphate
What are the main cations?
- K+
- Na+
- Ca2+
- Mg2+
What does an increased anion gap signal?
Metabolic acidaemia
What is the normal fife anion gap?
16
What are the different types of metabolic acidosis / what are they due to?
- Bodies own production (endogenous)
- Ingestion (exogenous source)
- Failure of excretion / regeneration bicarb by the kidneys
What causes a lactic acidosis?
Any conditions causing hypoperfusion - Of the whole body: shock - Or part of the body: femoral artery embolism ALSO - Severe acute hypoxia - Severe convulsions (resp arrest) - Strenuous exercise (dehydration)
What happens to the anion gap in lactic acidosis?
Increases
Where is lactate metabolised?
Liver
When does production of lactate increase?
When O2 delivery falls (consumption of lactate by the liver then falls)
When are lactate levels concerning?
When > 2 mmol/L
What does uncontrolled diabetes mellitus lead to?
Ketoacidosis (endogenous metabolic acidosis)
What can cause ketoacidosis?
- Uncontrolled diabetes mellitus
- Alcoholic ketoacidosis
- Starvation ketoacidosis
Where should blood be taken from in order to measure diabetic ketoacidosis?
Venous
What substances can cause an exogenous acid load?
- Methanol (industrial solvent, windscreen wash)
- Ethylene glycol (anti-freeze)
What is the effect of renal failure both acute and chronic on the anion gap?
Increased
What is the effect of renal tubular acidosis on the anion gap?
Normal (eGFR should be more or less normal as well)
What happens below the pylorus?
- Bicarbonate secreted into gut lumen
- For every bicarbonate secreted into gut lumen an H+ ion enters ECF
Why does the anion gap remain level in diarrhoea?
- Bicarb decreases but RAAS system stimulated due to dihidration which retains Cl-
What pathologies have a metabolic acidosis but not an increased anion gap?
- Laxative abuse
- Ileostomy
- Colostomy
How do the kidneys correct metabolic acidemia?
- Secrete more acid (therefore also make new bicarbonate) - plasma H+ decreases (pH rises) and plasma bicarbonate rises to normal
- But only if the metabolic acidosis is of non-renal origin and the kidneys are functioning effectively
What is the respiration pattern called that compensates for metabolic acidosis?
Kussmaul respiration - a laboured deep, rapid pattern of breathing
How long does maximal compensation take?
Up to 24 hours
What 2 processes have to happen in a metabolic alkalosis?
- An initiating process
- A maintaining process
What are the most common initiating processes of metabolic alkalosis?
Loss of H+ ions
- From the gut (pyloric stenosis)
- From the kidney (furosemide, thiazide)
What maintains an alkalosis?
HAV
- Hypokalaemia
- Aldosterone excess
- Volume and chloride depletion group
Describe the pathology of ploric stenosis?
- In health parietal cells secrete H+ into lumen of the stomach. The bicarb is secreted into the ECF
- Gastric fluid also contains Na+, H2O and K+ (5-10mmol/l)
- Initially excess bicarb is spilled in the urine but accompanied by a cation
- Worsening volume depletion results in aldosterone secretion (RAAS)
- Na+ and water retained exacerbating hypokalaemia
