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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