# Acid-base disorders

## Background

- Determiners of acid-base status are:
- CO2
- Weak acids (primarily albumin)
- If albumin goes up more acidotic (since albumin is an acid)

- Strong ion difference (SID)
- Primarily Na-Cl
- Normal difference is ~38 (140-102)
- If difference shrinks (i.e. more Cl) more acidotic
- Principle of electrical neutrality requires more H+ to offset the additional Cl

- If difference increases (i.e. more Na) more alkaloatic
- Principle of electrical neutrality requires more bicarb to offset the additional Na

- If difference shrinks (i.e. more Cl) more acidotic

- Normal difference is ~38 (140-102)

- Primarily Na-Cl

- Strong ion gap (SIG)
- Equivalent to anion gap
- Strong ions include Na, Cl, lactate, ketoacid, toxic alcohols

- Base Deficit
- Gets rid of respiratory component of acidosis so only left with the metabolic component
- How much base (or acid) you would have to add to get to pH 7.4
- Base excess of -6 = base deficit of 6
- Normal = -2 to +2
- If base deficit is normal but pt is acidotic must all be from CO2
- If base deficit is abnormal must explain by SID, weak acids, or unmeasured strong ions
- If no BD is available 24.2 – serum bicarb can be used as okay substitute

## How to approach an acid/base problem

- Get labs (as coincident as possible)
- VBG/ABG
- Lactate
- Albumin
- Acetone
- Chemistry

- Look at pH
- If pH >7.45 pt's primary problem is alkalosis
- If pH <7.35 pt's primary problem is acidosis
- Rmb that the body never over-corrects any acid-base disorder!

- Look at blood gas CO2
- If >45 then respiratory acidosis
- If <35 respiratory acidosis

- Calculate the strong ion difference (SID)
- SID = Na - Cl
- Low SID if <38
- Strong ion acidosis = hyperchloremic acidosis = non-gap acidosis
- Causes include:
- Fluid administration
- Any fluid that has SID of <24 can cause acidosis (e.g. NS, 1/2NS, D5W)

- Renal Tubular Acidosis
- Calculate Urine Anion Gap: (Urine Na + K – Cl); if negative, not RTA
- Type I: Urine pH <5.55
- Type II: Urine pH >5.55
- Type IV: Hyperkalemic; from aldosterone deficiency, diabetes

- Calculate Urine Anion Gap: (Urine Na + K – Cl); if negative, not RTA
- Diarrhea

- Fluid administration

- High SID if >38
- This is metabolic alkalosis
- Causes include:
- Nasogastric suction
- Diuretics
- Hyperaldosteronism
- Volume depletion

- Low SID if <38

- SID = Na - Cl
- Look at the lactate
- If >2 then pt has hyperlactatemia
- If >4 and pt has infection start Early Goal Directed Therapy (Sepsis)
- If pt not infected consider other diagnoses: Lactic Acidosis (Lactate)

- Calculate the strong ion gap (SIG) to explain the base deficit
- SIG = (Base Deficit) + (SID – 38) + 2.5 (4.2 ‐ Albumin (g/dL)) – lactate
- If SIG >2 this is a SIG metabolic acidosis = anion gap acidosis
- Causes:
- Uremia
- DKA
- AKA
- ASA
- Ethylene Glycol, methanol, propylene glycol
- Iron Toxicity
- INH
- Paraldehyde
- Lactic Acidosis (from short gut/blind loop - will not show on lactate assay)

- Causes:
- If SIG negative (very rare):
- Hypercalcemia
- Hypermagnesemia
- Hyperkalemia
- Immunoglobulins
- Bromide
- Nitrates
- Lithium

- If SIG >2 this is a SIG metabolic acidosis = anion gap acidosis

- SIG = (Base Deficit) + (SID – 38) + 2.5 (4.2 ‐ Albumin (g/dL)) – lactate
- Think about compensations
- If primary is respiratory calculate the expected metabolic compensation
- Expected ΔBE (or expected decrease of SID) = 0.4 x (Chronic change in CO2)

- If primary is metabolic acidosis calculate the expected respiratory compensation:
- Expected ↓CO2 = Base Deficit

- If primary is metabolic alkalosis calculate the expected respiratory compensation:
- Expected ↑ CO2 = 0.6 x Base Excess

- Winter's Formula useful for figuring out PaCO2 in COPD pt:
- 0.8 decrease in pH for every 10 mmHg increase in PaCO2 acutely

- If primary is respiratory calculate the expected metabolic compensation
- Calculate the osmolar gap
- Indicated if have elevated SIG without explanation
- Osm Gap = Measured Osmal – (2 Na + Gluc/18 + BUN/2.8 + ETOH/3.7)
- Positive if osm gap >10 (if Osm gap >50 almost certainly toxic alcohol induced)
- Causes:
- Methanol
- Ethylene glycol
- Mannitol
- Isopropanol (isopropyl alcohol)
- Propylene glycol
- Lithium

- Causes:

- Positive if osm gap >10 (if Osm gap >50 almost certainly toxic alcohol induced)

- Osm Gap = Measured Osmal – (2 Na + Gluc/18 + BUN/2.8 + ETOH/3.7)

- Indicated if have elevated SIG without explanation

## IV Fluids

- Normal SIG (Na-Cl) is 38
- Fluid that has SIG of 38 would be basic b/c it would dilute out the albumin (weak acid)
- Fluid that has SIG identical to pt's serum bicarb is pH neutral
- If SID of fluid is greater than pt's bicarb level then it is alkalotic
- If SID of fluid is less than pt's bicarb level then it is acidotic

### Examples

- NS or 1/2NS
- (SID = 0) so is acidotic so causes hyperchloremic acidosis

- LR
- SID of 24-28

- D5W
- SID of 0

- NaBicarb
- SID is 892 (very alkalotic) is 8.4%

^Consider balanced solution (LR) in pts w/ low pH (e.g. DKA)

## See Also

- Electrolyte Abnormalities (Main)
- Metabolic Acidosis
- Metabolic Alkalosis
- Respiratory Acidosis
- Respiratory Alkalosis

## Source

- EMCrit Acid/Base Lecture