# Acid-base disorders

## Contents

## 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

- 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

## Differential Diagnosis

### Acid-base disorders

## Diagnosis

Diagnosis is based on clinical history as well as labs:

- VBG/ABG
- Lactate
- Albumin
- Acetone
- Chemistry
- Serum Osmolarity

## Stewart Method of Acid Base Approach

*Based on a stepwise approach taught about by Dr. Weingart based on the Stewart's Strong Ion Difference*^{[1]}^{[2]}

### Determine pH

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

### Evaluate blood gas

- If >45 then respiratory acidosis
- If <35 respiratory acidosis

**Calculate the strong ion difference (SID)**

### Calculate Strong Ion Difference (SID)

- SID = Na - Cl

**Low SID is <38 and indicates a 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

- Diarrhea

**High SID is >38 and indicates a metabolic alkalosis**

- Causes include:
- Nasogastric suction
- Diuretics
- Hyperaldosteronism
- Volume depletion

### Evaluate the Lactate

- If >2 then the patient has hyperlactatemia
- If >4 and the patient has an infection they should be considered Severe Sepsis
- Always consider the differential for a 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 and the causes include:
- 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)

- If SIG is negative (very rare) the differential includes:
- Hypercalcemia
- Hypermagnesemia
- Hyperkalemia
- Immunoglobulins
- Bromide
- Nitrates
- Lithium

### Consider 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 disease is a metabolic acidosis then calculate the expected respiratory compensation:
- Expected ↓CO2 = Base Deficit

- If primary disease is ametabolic alkalosis then calculate the expected respiratory compensation:
- Expected ↑ CO2 = 0.6 x Base Excess
- Winter's Formula useful for figuring out PaCO2 in COPD patients:
- pCO2 = 1.5 [HCO3] + 8 mmHg +/- 2
- 0.8 decrease in pH for every 10 mmHg increase in PaCO2 acutely

### 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 and differential includes:
- Toxic alcohols (if Osm gap >50)
- Methanol
- Ethylene glycol
- Mannitol
- Isopropanol (isopropyl alcohol)
- Propylene glycol
- Lithium

## Management

### IV Fluids

- Normal SID (Na-Cl) is 38
- Fluid that has SID of 38 would be basic b/c it would dilute out the albumin (weak acid)
- Fluid that has SID 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)