Clinical approach to Acid-Base disorders

Step 1: Clinical suspicion
Step 2: Determine the acid base disorder from ABG
Step 3: Simple or mixed disorder
Step 4: Underlying cause for acid-base disorders
Step 5: Treatment




Step 1:

   Clinical suspicion by history and clinical examination.
   If diabetic → ? DKA
   If on K+-losing diuretic (frusemide) → ? Metabolic alkalosis.
   If on acetazolamide → ? Metabolic acidosis.


Look for respiratory disease (COPD, Bronchial asthma, Pneumonia …etc)

Look for GIT problem :
    Vomiting and NGT suction → loss of   acid → Metabolic alkalosis.
    Diarrhea → loss of HCO3- → Metabolic acidosis.

Look for renal failure or renal tubular acidosis → Metabolic acidosis

Look for signs of septicemia  → Lactic acidosis 

Look for respiratory disease (COPD, Bronchial asthma, Pneumonia …etc)

Look for GIT problem :
    Vomiting and NGT suction → loss of   acid → Metabolic alkalosis.
    Diarrhea → loss of HCO3- → Metabolic acidosis.

Look for renal failure or renal tubular acidosis → Metabolic acidosis

Look for signs of septicemia  → Lactic acidosis 


Ask carefully about drug overdose:
  
Salicylates overdose → respiratory alkalosis → Metabolic acidosis

Methanol ingestion → Metabolic acidosis



Step 2:

Determine the acid base disorder from arterial pH, pCO2, and [HCO3-].

↓[HCO3-] : Metabolic acidosis or compensated respiratory alkalosis.
↑[HCO3-] : Metabolic alkalosis or compensated respiratory acidosis.


↓ pCO2 : respiratory alkalosis or compensated metabolic acidosis.
↑ pCO2 : Respiratory acidosis or compensated metabolic alkalosis.

↑or↓ in both pCO2  and [HCO3-] : Mixed acid-base disorders.




Step 3 :
    
Determine whether the disorder is simple or mixed by applying :

- The rules of compensaion.


- The direction of change in pCO2  and [HCO3-].

- Change in anion gap in relation to change in [HCO3-].

    In simple disorder, the change in anion gap equals the change in [HCO3-], if not suspect mixed disorder.




Step 4 :
 
Determine the underlying cause for acid-base disorders by:

Analyzing the differential diagnosis.

Good clinical evaluation.

Calculating the anion gap in metabolic acidosis.

According to anion gap :

1- High anion gap metabolic acidosis: eg: DKA
2- Normal anion gap metabolic acidosis: RTA.



Step 5:

Treat the disorder by :

1- Treating the underlying cause.

2- Replace salt, water, and electrolyte deficits.

3- Directly altering concentration of HCO3-  and CO2.