EVALUATING ACID BASE DISORDERS

1.       Draw an ABG and a set of serum electrolytes at the same time.
2.       Determine if the patient is acidemic or alkalemic by looking at the pH.
3.       Determine whether the acid-base disorder is respiratory or metabolic in nature.

pH

  

Acidemic

  

― HCO3

Metabolic Acidosis
   

­ PaCO2

Respiratory Acidosis
       
         

Alkalemic

  

­ HCO3

Metabolic Alkalosis
   

― PaCO2

Respiratory Alkalosis

4.       If the disorder is respiratory, determine whether it is acute or chronic.

Respiratory Disorder

  

Acidosis

  

Acute

pH ― by 0.08(PaCO2 – 40)/10
   

Chronic

pH ― by 0.03(PaCO2 – 40)/10
       
         

Alkalosis

  

Acute

pH ­ by 0.08(40 – PaCO2)/10
   

Chronic

pH ­ by 0.03(40 – PaCO2)/10

5.       If the disorder is metabolic acidosis, determine whether it is an anion gap or a non-anion gap acidosis.
²         Anion gap (AG) = Na+ – (HCO3 + Cl-)
²         Causes of anion gap: MUDPILES

Methanol
Uremia (as in Acute Renal Failure)
Diabetic KetoAcidosis
Paraldehyde
Isoniazid (INH)
Lactic Acidosis
Ethylene glycol / Ethanol
Salicylates (Note: usually assoc. with respiratory alkalosis 20 to respiratory stimulation)

²         Causes of non-anion gap acidosis include renal tubular acidosis, diarrhea, drugs (acetazolamide, amiloride, triamterene, spironolactone, b-blockers), and ileostomy.

6.       If a metabolic disturbance is present, is the respiratory system compensating adequately?
²         Metabolic acidosis – use Winter’s formula:
expected compensatory PaCO2 in a patient in metabolic acidosis = [1.5 x HCO3] + 8 (±2)
If measured pCO2 from ABG < calculated PaCO2, then there is also a respiratory alkalosis- think Salicylates

²         Metabolic alkalosis – PaCO2 > 40 and < 50 if appropriate respiratory compensation is occurring.

7.       If the patient has an anion-gap acidosis, is there another metabolic disturbance present? 

²         Determine the corrected bicarbonate level = HCO3 + (anion gap – 12)
²         If lower than normal range (20-24), then the patient had a metabolic acidosis before the newer acid/base disturbances began; if higher than normal range, then the patient had a primary metabolic alkalosis before the acid/base disturbances began.