Friday, November 21, 2014

Q; Patient with history of hypertension and anxiety presented to the hospital with tachypnea. Patient 

sodium was 140meq/l, 
potassium was 3meq/l, 
chloride was 94meq/l and 
HCO3 was 34. 

 On arterial blood gas, 

PH was 7.67 and 
pCO2 was 30. 

 What is the acid base disturbance? 

 A) Respiratory alkalosis 
 B) Metabolic alkalosis 
 C) Respiratory and metabolic alkalosis 
 D) Hyperchloremic non-anion gap metabolic acidosis 


 Rationale: Patient HCO3 is high suggestive of metabolic alkalosis. Patient expected CO2 should be (pCO2=HCO3x0.9+9; 34x0.9+9=39.6); CO2 is 30, which is lower than expected pCO2, suggestive of mixed respiratory and metabolic alkalosis.


  1. patient expected CO2 will be (pCO2= 0.7x(34-24)=10 ) greater than normal which in this case will be 50 rather than 39.6 which is mentioned here.

    1. Thanks for your input. The formula for expected PCO2 in metabolic CO2 is PCO2=HCO3x.9+9, which makes the calculation correct. We always appreciate and value the comment and feed back as we are always willing to learn. It would be of value if you can please provide the refernce to the formula which you have used.
      We thank for your interest in our site as well as giving us an opportunity to learn more

      The formula you have used is for acute respiratory acidosis
      Rule 1 : The 1 for 10 Rule for Acute Respiratory Acidosis
      The [HCO3] will increase by 1 mmol/l for every 10 mmHg elevation in pCO2 above 40 mmHg.
      Expected [HCO3] = 24 + { (Actual pCO2 - 40) / 10 }
      Comment:The increase in CO2 shifts the equilibrium between CO2 and HCO3 to result in an acute increase in HCO3. This is a simple physicochemical event and occurs almost immediately.

      Example: A patient with an acute respiratory acidosis (pCO2 60mmHg) has an actual [HCO3] of 31mmol/l. The expected [HCO3] for this acute elevation of pCO2 is 24 + 2 = 26mmol/l. The actual measured value is higher than this indicating that a metabolic alkalosis must also be present.

  2. Metabolic Alkalosis - Respiratory Compensation
    Expected Paco2 = ( 0.7x HCO3 ) + 21 or
    40+ ( 0.6 x SBE)
    Ref: TEXTBOOK OF CRITICAL CARE BY FINK ( Elsevier publication ) Fifth Edition Page 59.