Acid/Base 

Step 1: Look at the pH

• pH < 7.35 → acidemia

• pH > 7.45 → alkalemia

• Normal pH does not exclude a disorder (mixed disorders exist)

​

Step 2: Identify the Primary Process

Compare PaCO₂ and HCO₃⁻ with the pH:

• Primary metabolic: pH and HCO₃⁻ move in same direction

• Primary respiratory: pH and PaCO₂ move in opposite direction

 

Step 3: Assess Expected Compensation​ (see below)

• If compensation is appropriate → single disorder

• If compensation is inappropriate → mixed disorder

 

​Step 4: Calculate the Anion Gap (if metabolic acidosis)

Anion Gap (AG)=Na−(Cl+HCO3))

• Normal AG ≈ 8-12 (depends on your laboratory)

• Adjust for albumin if Albumin is <4g/dL as albumin is unmeasured in the anion gap, thereby falsely decreasing the anion gap if Albumin is low, masking a high anion gap acidosis:

     Corrected AG=AG+2.5×(4−Albumin)

• Phosphate is also an anion; low phos can also cause a narrow anion gap​

 

Step 5: Delta–Delta Gap Analysis (if HAGMA). Not a perfect test (assumes normal AG is 12, normal HCO3- is 24mEq/L).

Used to detect additional metabolic processes by comparing the anion gap with a bicarb gap since if it is pure HAGMA, the AG should be greater than than the difference in normal vs measured bicarb (the ΔHCO3) since there are increased unmeasured anions :

                      1.    ΔAG=AG−12.

                      2.   ΔHCO3​=24−HCO3​​

You then subtract the ΔHCO3 from the ΔAG. There are various ways to calculate this and none are perfect. The simplest way is to use +/- 6 as your ESTIMATED cutoff.

Interpretation:

• ΔAG ≈ ΔHCO₃ → pure HAG metabolic acidosis

• ΔAG > ΔHCO₃ by 6 (i.e. +6) → concurrent metabolic alkalosis (for example, DKA with significant vomiting)

• ΔAG < ΔHCO₃ by 6 (i.e. -6)→ concurrent non-gap metabolic acidosis​ (for example, resuscitation with 0.9% NS may generate a hyperchloremic acidosis. 

​

The question you are all asking, why bother? Well, if your delta/delta gap is more than +6, you may not have to give so much bicarbonate to recover. If its more than -6, then you have to look at what fluids you are giving or if there is an underlying chronic acidosis, e.g. CKD.

 

Compensation Rules 

​

       Metabolic Acidosis (Winter’s Formula)

           

         Expected PaCO2=(1.5×HCO3)+8±2

•       Higher PaCO₂ than expected → concurrent respiratory acidosis

•       Lower PaCO₂ than expected→ concurrent respiratory alkalosis​

   

       Metabolic Alkalosis

         Expected PaCO2=0.7×HCO3+20±5 (For every 1 mEq/L increase in HCO3− above 24,  the PaCO2 should                           increase by approximately 0.6-0.7mmHg)

•       Higher PaCO₂ than expected → concurrent respiratory acidosis

•       Lower PaCO₂ than expected→ concurrent respiratory alkalosis

 

Respiratory Disorders: Acute vs Chronic Compensation

 

The “1–2–4–5 Rule” helps determine whether metabolic compensation is appropriate or if a secondary acid–base disorder is present alongside the primary respiratory disorder.

​

​

​

​

​

​

​

​

​

Example: There is a patient in the ED who came with 5 hours of vomiting and respiratory distress. 

                          - pH 7.2, PaCO2 60,  HCO3- 22 . 

                    

    PaCo2 is 20 above 40, so would expect bicarb to be 26 (+2mEq/L). Measured bicarb is 22mEq/L, indicating  concurrent metabolic  acidosis.​