Acid-base disorders
(Redirected from Acid-Base)
Background
Determiners of acid-base status are:
- CO2
- Weak acids (primarily albumin)
- If albumin goes up more acidotic (since albumin is an acid)
- Strong ions
- 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 alkalotic
- 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 (BD)
- Eliminates the respiratory component of acidosis so only left with the metabolic component
- Is equivalent to the amount of 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 patient 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
Evaluation
Diagnosis is based on clinical history as well as labs:
Stuart Step Wise 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 then patient's primary problem is alkalosis
- If pH <7.35 the patient's primary problem is acidosis
- The body never over-corrects any acid-base disorder!
Evaluate blood gas
- If pCO2 >45 then respiratory acidosis
- If pCO2 <35 respiratory alkalosis
Calculate Strong Ion Difference (SID)
SID = Na - Cl
- Low SID is <38 and indicates a strong ion acidosis = hyperchloremic acidosis = non-gap acidosis and 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
- Fluid administration
- High SID is >38 and indicates a metabolic alkalosis and 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 toxicity
- 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
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
Traditional step-wise approach
Determine pH
- If pH < 7.35, then acidemia
- If pH > 7.45, then alkalemia
- If pH within normal range, then acid base disorder not likely present.
- pH may be normal in the presence of a mixed acid base disorder, particularly if other parameters of the ABG are abnormal.
Determine the Primary Diagnosis
- Acidemia
- ↓HCO3 -Metabolic acidosis
- ↑PaCO2-Respiratory acidosis
- Alkalemia
- ↑HCO3-Metabolic alkalosis
- ↓PaCO2 - Respiratory alkalosis
Calculate the Anion gap
Anion gap = [Na+]– [HCO3-] – [Cl-]
Calculate the delta gap
- ∆gap = anion gap - 12
- This is to determine a coexistent metabolic alkalosis or non-gap acidosis
Delta Ratio | Assessment Guideline |
< 0.4 | Hyperchloremic normal anion gap acidosis |
0.4 - 0.8 | Consider combined high AG & normal AG acidosis BUT note that the ratio is often <1 in acidosis associated with renal failure |
1 to 2 |
|
> 2 |
Suggests a pre-existing elevated HCO3 level so consider:
|
Calculate the starting bicarbonate
- ∆gap + (HCO3) = “starting bicarbonate”
- The purpose of this calculation is to assess the body’s ability to change HCO3 in response to a metabolic acid. In cases with a pure anion gap metabolic acidosis, the rise in anion gap from 12 should equal the fall in HCO3 from from 24
Calculate compensations
- Will allow for identification of a secondary process
Determinants of compensation
Metabolic acidosis:
- PaCO2 = 1.5 (HCO3) + 8 ± 2
- PaCO2 = last two digits of pH
- PaCO2= ↓ 1.0–1.5per ↓ 1mEq/L HCO3
Metabolic alkalosis
- PaCO2 = 0.9 (HCO3) + 9
- PaCO2= ↑ 0.5–1.0 mm per ↑ 1mEq/L HCO3
Respiratory acidosis and alkalosis (acute acid-base changes based on PCO2 and HCO3):
- ∆H+=0.8 (∆PaCO2)
- For every ↑ or ↓ of PCO2 by 1 the pH changes by 0.008
- For every ↑ or ↓ of HCO3 by 1 the pH changes by 0.015
Estimate of baseline PCO2 in patients with Acute Respiratory Acidosis:
- Estimated baseline PCO2 = 2.4 (admission measured HCO3 – 22)
Chronic respiratory acidosis[3]
- HCO3 increases by 4 for every 10 mmHg ↑ in pCO2 above 40
- ∆H+=0.4 (∆PaCO2)
- In chronic respiratory acidosis, kidneys retain HCO3, which takes a few days
Chronic respiratory alkalosis
- HCO3 decreases by 5 for every 10 mmHg decrease in pCO2 below 40
- ∆H+=0.5 (∆PaCO2)
- Takes few days also
- Maximal compensation is HCO3 ~12-15 mEq/L
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 patient's serum bicarb is pH neutral
- If SID of fluid is greater than patient's bicarb level then it is alkalotic
- If SID of fluid is less than patient'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 patients with low pH (e.g. DKA)
See Also
References
- ↑ http://emcrit.org/wp-content/uploads/acid_base_sheet_2-2011.pdf
- ↑ Stewart Acid base http://www.acid-base.com/strongion.php
- ↑ Brandis K. Anesthesia MCQ. Rules for Metabolic Acid-Base Disorders. http://www.anaesthesiamcq.com/AcidBaseBook/ab9_3.php