Acid Base Balance & Arterial Blood Gases Flashcards
buffers that exist in the body
proteins
haemoglobin
Carbonic acid/bicarbonate
what are the three situations in which a acid-base disturbance would occur
- There is a problem with ventilation
- There is a problem with renal function
- Overwhelming acid or base load the body can’t handle
what is standard bicarbonate
calculated from the actual bicarbonate but assuming 370C and a paCO2 of 5.3kPa
this reflects the metabolic component of acid base balance
base excess
value calculated from blood pH & pCO2. Defined as the amount of acid required to restore a litre of blood to its normal pH at a PCO2 of 5.3. Normal range -2 - +2mmol/L. Becomes more negative in metabolic acidosis
Metabolic problems
overwhelming acid load
• Bodies own production (endogenous)
• Ingestion (exogenous source)
• Failure of excretion/ regeneration bicarb by the kidneys
condition causing hypoperfusion
– Of the whole body: shock (cardiogenic, septic, hypovolaemic, anaphylactic)
– Or part of the body: femoral artery embolism
increased anaerobic metabolism with
subsequent increased production of lactic acid Lactic acidaemia
Lactic acid
• Product of anaerobic metabolism
• In health lactate is metabolised in liver and overall there is no NET
production of acid
HOWEVER this process needs oxygen so therefore
Production increases whenO2 delivery falls AND consumption of lactate by the liver falls
Other causes of lactic acidaemia
- Severe acute hypoxia
- Severe convulsions (resp arrest)
- Strenuous exercise (dehydration
lactate normal marker
> 2mmol/L marker of concern
Situaons of ↓ insulin & ↑ glucagon
–Uncontrolled diabetes mellitus (severe, life- threatening)
– Alcoholic ketoacidosis (common clinically)
– Starvation ketoacidosis (mild)
Ketoacidosis
Monitoring response to treatment: KDA
bedside measurement of capillary blood ketones
• Venous blood should be used rather than arterial (unless respiratory problems dictate otherwise) in blood gas analysers
Exogenous acid load
Accidental / deliberate ingestion
• Methanol (industrial solvent, windscreen wash) • Ethylene glycol (anti freeze)
Renal causes of metabolic acidosis (variable anion gap)
Functions of the kidney re: bicarbonate
• Reabsorbs filtered bicarbonate
• Regenerates bicarbonate consumed by buffering
• Renal failure both acute and chronic (↑gap) • Renal tubular acidosis (normal gap)
Gastrointestinal causes of metabolic acidosis (normal anion gap)
- Much of gut below pylorus secretes bicarbonate into gut lumen
- For every bicarb ion into gut a H+ ion enters ECF
- Diarrhoea this process increases
- AND volume depletion renin/angiotensin/aldosterone axis stimulated retaining chloride
Compensation for metabolic problems
Slow metabolic (renal) compensation correction
–Secrete more acid (therefore also make NEW bicarbonate)
– plasma H+ decreases (pH rises) and plasma bicarbonate rises to normal
BUT ONLY IF :
1. The metabolic acidaemia is of non-renal origin
2. The kidneys are functioning effectively.
