Download CRRM2.15 - Correction of Acid/Base Disturbance

CRRM2.15: COMPENSATION AND CORRECTION OF ACID-BASE DISTURBANCE
10/03/08
LEARNING OUTCOMES
Define the major disorders of acid-base balance





Respiratory acidosis is caused by an elevated pCO2 (> 6.0kPa / 40mmHg)
o Results in reduced plasma pH and elevated plasma [HCO3-]
o Clinical causes include obstructive airway diseases, respiratory-repressing drugs
Respiratory alkalosis is caused by a decreased pCO2 (< 4.5kPa)
o Results in elevated plasma pH and reduced plasma [HCO 3-]
o Clinical causes include hyperventilation, anti-inflammatory drug overdose (e.g. aspirin)
Metabolic acidosis is caused by addition of non-carbonic acids or removal of alkali
o Results in reduced plasma pH and decreased plasma [HCO3-] (uncompensated buffer load)
o Clinical causes include ketoacidosis, lactic acidosis, diarrhoea, renal failure
Metabolic alkalosis is caused by addition of alkali or removal of non-carbonic acids
o Results in elevated plasma pH and elevated plasma [HCO3-] (more protons freed)
o Clinical causes include vomiting, hypokalaemia, excessive bicarbonate load
Basic differentiation between respiratory/metabolic acid-base disturbances can be made using pCO2:
o Respiratory disturbances manifest with significantly altered pCO2 values
o Metabolic disturbances essentially maintain a normal pCO2
 However, metabolic acidosis often induces respiratory alkalosis and vice-versa
Interpret plasma data for pH, plasma bicarbonate and pCO 2






In normal blood plasma: pH = 7.4; [HCO3-] = 24mM; pCO2 = 40mmHg
Non-bicarbonate buffers collectively drive the production of bicarbonate from carbon dioxide
Normal bicarbonate production rate is 25mM/pH – governed by non-bicarbonate buffering power
o Doubling pCO2 causes more CO2 to be converted to bicarbonate (and protons)
o Non-bicarbonate buffers consume nearly all the protons such that pH only drops marginally
o The net result is therefore decreased pH and significantly increased bicarbonate
Low non-bicarbonate buffering power (e.g. in anaemia) reduces bicarbonate production rate
o Doubling pCO2 causes more CO2 to be converted to bicarbonate (and protons)
o Non-bicarbonate buffers consume limited numbers of protons
o Newly produced bicarbonate buffers consume further protons to compensate
o The net result is therefore decreased pH and mildly increased bicarbonate
Addition of non-volatile acids at a fixed pCO2 forces [HCO3-] to drop – metabolic acidosis
Elevated pCO2 forces [HCO3-] to rise – respiratory acidosis
Explain the role of renal function in whole body pH in health and disease (compensation and correction)

Regulation of pCO2 is handled by the lungs which act very quickly
o Metabolic acidosis is compensated for by a respiratory alkalosis reducing pCO2
o Metabolic alkalosis conversely induces a respiratory acidosis increasing pCO2

Regulation of [HCO3-] is handled by the kidneys which act slowly (1-4 weeks)
o Respiratory acidosis is compensated for by increased proton excretion in the urine
o Respiratory alkalosis conversely results in decreased proton excretion in the urine