A patient with salicylic acid poisoning has the following arterial blood gas analysis report: pH = 7.12; PCO2 = 18 mmHg; HCO3 = 12 mmol/L. The resulting acid-base abnormality can be best labeled as:

Ans. is 'a' i.e. Metabolic acidosis with respiratory alkalosisRef. Harrison 17th/e p 288 & 16th/e p 2591, 266 (15th/e page 284, 286,187)Also see June 2000 Q. No. 94.In patients with salicylate poisoning, initially the acidic metabolites of salicylic acid accumulate and cause metabolic acidosis.This causes the following acid base reaction to shift to left (because of addition of H) CO2 + H20 = H2CO3 = H+ + HCO3-This causes decrease in extracellular HCO3* This in turn stimulates the medullary chemoreceptors to increase ventilation and return the ratio of / PaCO2 and thus pH towards normal* The formula given to predict the degree of compensation required for simple form of metabolic acidosis is PaCO2 = 1.5 x + 8 or you can say that the PaCO2 is expected to decrease 1.25 mm of Hg for each mmol/L decrease in The normal values of PCO2 and HCO3-Arterial PC02 - 40 +- 2 mm HgArterial - 24 +- mm HgThus a patient with metabolic acidosis and of 12 mmol/L would be expected to have a PaCO2 between 24 and 28 mm Hg. (to compensate for the acidosis)Values of PaCO2 below 24 or greater than 28 mm Hg will mean mixed disturbance.If PaCO2 is < 24 it means metabolic acidosis + respiratory alkalosis (hyperventilation has led to excess CO2 washout causing respiratory alkalosis)If PaCO2 > 28 it means - metabolic acidosis respiratory acidosisHere is a table giving predictions of compensatory response on simple acid base disturbance. Prediction of Compensatory Responses on Simple Acid-Base DisturbancesDisorderPrediction of CompensationMetabolic acidosisPaCO2 =(1.5xHCO3-) +8 OrPaCO2 will | 1.25mmHg per mmol/L | in OrPaCO22 = + 15Metabolic alkalosisPaCO2 will | 0.75 mmHg per mmol/L | in orPaCO2 will | 6 mmHg per 10-mmol/L | in OrPaCO2 = + 15Respiratory AlkalosisAcute will | 2 mmol/L per 10-mmHg | in PaCO2Chronic will | 4 mmol/L per 10-mmHg | in PaCO2Respiratory acidosisAcute will | 1 mmol/L per 10-mmHg T in PaCO2Chronic will | 4 mmol/L per 10-mmHg | in PaCO2