Acid-base disorders

Background

1. Determiners of acid-base status are:
   1. CO2
   2. Weak acids (primarily albumin)
      1. If albumin goes up more acidotic (since albumin is an acid)
   3. Strong ion difference (SID)
      1. Primarily Na-Cl
         1. Normal difference is ~38 (140-102)
            1. If difference shrinks (i.e. more Cl) more acidotic
               1. Principle of electrical neutrality requires more H+ to offset the additional Cl
            2. If difference increases (i.e. more Na) more alkaloatic
               1. Principle of electrical neutrality requires more bicarb to offset the additional Na
2. Strong ion gap (SIG)
   1. Equivalent to anion gap
   2. Strong ions include Na, Cl, lactate, ketoacid, toxic alcohols
3. Base Deficit
   1. Gets rid of respiratory component of acidosis so only left with the metabolic component
   2. How much base (or acid) you would have to add to get to pH 7.4
   3. Base excess of -6 = base deficit of 6
   4. Normal = -2 to +2
   5. If base deficit is normal but pt is acidotic must all be from CO2
   6. If base deficit is abnormal must explain by SID, weak acids, or unmeasured strong ions
   7. If no BD is available 24.2 – serum bicarb can be used as okay substitute

How to approach an acid/base problem

1. Get labs (as coincident as possible)
   1. VBG/ABG
   2. Lactate
   3. Albumin
   4. Acetone
   5. Chemistry
2. Look at pH
   1. If pH >7.45 pt's primary problem is alkalosis
   2. If pH <7.35 pt's primary problem is acidosis
3. Look at blood gas CO2
   1. If >45 then respiratory acidosis
   2. If <35 respiratory acidosis
4. Calculate the strong ion difference (SID)
   1. SID = Na - Cl
      1. Low SID if <38
         1. Strong ion acidosis = hyperchloremic acidosis = non-gap acidosis
         2. Causes include:
            1. Fluid administration
               1. Any fluid that has SID of <24 can cause acidosis (e.g. NS, 1/2NS, D5W)
            2. Renal Tubular Acidosis
               1. Calculate Urine Anion Gap: (Urine Na + K – Cl); if negative, not RTA
                  1. Type I: Urine pH <5.55
                  2. Type II: Urine pH >5.55
                  3. Type IV: Hyperkalemic; from aldosterone deficiency, diabetes
            3. Diarrhea
      2. High SID if >38
         1. This is metabolic alkalosis
         2. Causes include:
            1. Nasogastric suction
            2. Diuretics
            3. Hyperaldosteronism
            4. Volume depletion
5. Look at the lactate
   1. If >2 then pt has hyperlactatemia
   2. If >4 and pt has infection start Early Goal Directed Therapy (Sepsis)
   3. If pt not infected consider other diagnoses: Lactic Acidosis (Lactate)
6. Calculate the strong ion gap (SIG) to explain the base deficit
   1. SIG = (Base Deficit) + (SID – 38) + 2.5 (4.2 ‐ Albumin (g/dL)) – lactate
      1. If SIG >2 this is a SIG metabolic acidosis = anion gap acidosis
         1. Causes:
            1. Uremia
            2. DKA
            3. AKA
            4. ASA
            5. Ethylene glycol, methanol, propylene glycol
            6. Iron
            7. INH
            8. Paraldehyde
            9. DLactic Acidosis (from short gut/blind loop - will not show on lactate assay)
      2. If SIG negative (very rare):
         1. Hypercalcemia
         2. Hypermagnesemia
         3. Hyperkalemia
         4. Immunoglobulins
         5. Bromide
         6. Nitrates
         7. Lithium
7. Think about compensations
   1. If primary is respiratory calculate the expected metabolic compensation
      1. Expected ΔBE (or expected decrease of SID) = 0.4 x (Chronic change in CO2)
   2. If primary is metabolic acidosis calculate the expected respiratory compensation:
      1. Expected ↓CO2 = Base Deficit
   3. If primary is metabolic alkalosis calculate the expected respiratory compensation:
      1. Expected ↑ CO2 = 0.6 x Base Excess
   4. Winter's Formula useful for figuring out PaCO2 in COPD pt:
      1. 0.8 decrease in pH for every 10 mmHg increase in PaCO2 acutely
8. Calculate the osmolar gap
   1. Indicated if have elevated SIG without explanation
      1. Osm Gap = Measured Osmal – (2 Na + Gluc/18 + BUN/2.8 + ETOH/3.7)
         1. Positive if osm gap >10 (if Osm gap >50 almost certainly toxic alcohol induced)
            1. Causes:
               1. Methanol
               2. Ethylene glycol
               3. Mannitol
               4. Isopropanol (isopropyl alcohol)
               5. Propylene glycol
               6. Lithium

Fluids

1. Normal SIG (Na-Cl) is 38
   1. Fluid that has SIG of 38 would be basic b/c it would dilute out the albumin (weak acid)
   2. Fluid that has SIG identical to pt's serum bicarb is pH neutral
      1. If SID of fluid is greater than pt's bicarb level then it is alkalotic
      2. If SID of fluid is less than pt's bicarb level then it is acidotic
2. Examples
   1. NS or 1/2NS
      1. (SID = 0) so is acidotic so causes hyperchloremic acidosis
   2. LR
      1. SID of 24-28
   3. D5W
      1. SID of 0
   4. NaBicarb
      1. SID is 892 (very alkalotic) is 8.4%
3. Consider balanced solution (LR) in pts w/ low pH (e.g. DKA)