Difference between revisions of "Acid-base disorders"

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{{Acid-base disorders DDX}}
 
{{Acid-base disorders DDX}}
  
==Diagnosis==
+
==Evaluation==
 
Diagnosis is based on clinical history as well as labs:
 
Diagnosis is based on clinical history as well as labs:
*VBG/ABG
+
*[[VBG]]/[[ABG]]
*Lactate
+
*[[Lactate]]
 
*Albumin
 
*Albumin
 
*Acetone
 
*Acetone
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*''Based on a stepwise approach taught about by Dr. Weingart based on the Stewart's Strong Ion Difference<ref>http://emcrit.org/wp-content/uploads/acid_base_sheet_2-2011.pdf</ref><ref>Stewart Acid base http://www.acid-base.com/strongion.php</ref>
 
*''Based on a stepwise approach taught about by Dr. Weingart based on the Stewart's Strong Ion Difference<ref>http://emcrit.org/wp-content/uploads/acid_base_sheet_2-2011.pdf</ref><ref>Stewart Acid base http://www.acid-base.com/strongion.php</ref>
 
===Determine pH===
 
===Determine pH===
*If pH >7.45 then patient's primary problem is alkalosis
+
*If pH >7.45 then patient's primary problem is [[alkalosis]]
*If pH <7.35 the patient's primary problem is acidosis
+
*If pH <7.35 the patient's primary problem is [[acidosis]]
 
*The body never over-corrects any acid-base disorder!
 
*The body never over-corrects any acid-base disorder!
 
===Evaluate blood gas===
 
===Evaluate blood gas===
*If pCO2 >45 then respiratory acidosis
+
*If pCO2 >45 then [[respiratory acidosis]]
*If pCO2 <35 respiratory alkalosis
+
*If pCO2 <35 [[respiratory alkalosis]]
  
 
===Calculate Strong Ion Difference (SID)===
 
===Calculate Strong Ion Difference (SID)===
 
''SID = Na - Cl''
 
''SID = Na - Cl''
 
*'''Low SID is <38 and indicates a strong ion acidosis = hyperchloremic acidosis = non-gap acidosis and causes include'''
 
*'''Low SID is <38 and indicates a strong ion acidosis = hyperchloremic acidosis = non-gap acidosis and causes include'''
**Fluid administration
+
**[[IVF|Fluid administration]]
***Any fluid that has SID of <24 can cause acidosis (e.g. NS, 1/2NS, D5W)
+
***Any fluid that has SID of <24 can cause acidosis (e.g. [[NS]], 1/2NS, D5W)
**Renal Tubular Acidosis
+
**[[Renal tubular acidosis]]
 
***Calculate Urine Anion Gap: (Urine Na + K – Cl); if negative, not RTA
 
***Calculate Urine Anion Gap: (Urine Na + K – Cl); if negative, not RTA
 
**#Type I: Urine pH <5.55
 
**#Type I: Urine pH <5.55
 
**#Type II: Urine pH >5.55
 
**#Type II: Urine pH >5.55
**#Type IV: Hyperkalemic; from aldosterone deficiency, diabetes
+
**#Type IV: [[hyperkalemia|Hyperkalemic]]; from aldosterone deficiency, diabetes
**Diarrhea
+
**[[Diarrhea]]
 
*'''High SID is >38 and indicates a metabolic alkalosis and causes include:'''
 
*'''High SID is >38 and indicates a metabolic alkalosis and causes include:'''
 
**Nasogastric suction
 
**Nasogastric suction
**Diuretics
+
**[[Diuretics]]
 
**Hyperaldosteronism
 
**Hyperaldosteronism
**Volume depletion
+
**[[hypovolemia|Volume depletion]]
  
 
===Evaluate the Lactate===
 
===Evaluate the Lactate===
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**[[Ethylene Glycol Toxicity|Ethylene Glycol]], methanol, propylene glycol
 
**[[Ethylene Glycol Toxicity|Ethylene Glycol]], methanol, propylene glycol
 
**[[Iron Toxicity]]
 
**[[Iron Toxicity]]
**INH
+
**[[INH toxicity]]
 
**Paraldehyde
 
**Paraldehyde
 
**[[Lactic Acidosis]] (from short gut/blind loop - will not show on lactate assay)
 
**[[Lactic Acidosis]] (from short gut/blind loop - will not show on lactate assay)
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*Osm Gap = Measured Osmal – (2 Na + Gluc/18 + BUN/2.8 + ETOH/3.7)
 
*Osm Gap = Measured Osmal – (2 Na + Gluc/18 + BUN/2.8 + ETOH/3.7)
 
*Positive if osm gap >10 and differential includes:
 
*Positive if osm gap >10 and differential includes:
**Toxic alcohols (if Osm gap >50)
+
**[[Toxic alcohols]] (if Osm gap >50)
 
**[[Methanol]]
 
**[[Methanol]]
 
**[[Ethylene glycol]]
 
**[[Ethylene glycol]]
**Mannitol
+
**[[Mannitol]]
**Isopropanol (isopropyl alcohol)
+
**[[isopropanol Toxicity|Isopropanol]] (isopropyl alcohol)
 
**Propylene glycol
 
**Propylene glycol
 
**[[Lithium]]
 
**[[Lithium]]
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===Determine the Primary Diagnosis===
 
===Determine the Primary Diagnosis===
 
*Acidemia
 
*Acidemia
**↓HCO3 -Metabolic Acidosis
+
**↓HCO3 -[[Metabolic acidosis]]
**↑PaCO2-Respiratory Acidosis
+
**↑PaCO2-[[Respiratory acidosis]]
 
*Alkalemia
 
*Alkalemia
**↑HCO3-Metabolic Alkalosis
+
**↑HCO3-[[Metabolic alkalosis]]
**↓PaCO2 - Respiratory Alkalosis
+
**↓PaCO2 - [[Respiratory alkalosis]]
 
===Calculate the [[Anion gap]]===
 
===Calculate the [[Anion gap]]===
 
Anion gap = [Na+]– [HCO3-] – [Cl-]
 
Anion gap = [Na+]– [HCO3-] – [Cl-]
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|-
 
|-
 
|< 0.4
 
|< 0.4
|Hyperchloraemic normal anion gap acidosis
+
|Hyperchloremic [[non anion gap acidosis|normal anion gap acidosis]]
 
|-
 
|-
 
|0.4 - 0.8
 
|0.4 - 0.8
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|1 to 2
 
|1 to 2
 
|
 
|
*Usual for uncomplicated high-AG acidosis.  
+
*Usual for uncomplicated [[anion gap acidosis|high-AG acidosis]].  
*Lactic acidosis: average value 1.6
+
*[[Lactic acidosis]]: average value 1.6
*DKA more likely to have a ratio closer to 1 due to urine ketone loss (esp if patient not dehydrated)
+
*[[DKA]] more likely to have a ratio closer to 1 due to urine ketone loss (esp if patient not dehydrated)
 
|-
 
|-
 
| > 2
 
| > 2
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*HCO3 increases by 4 for every 10 mmHg ↑ in pCO2 above 40
 
*HCO3 increases by 4 for every 10 mmHg ↑ in pCO2 above 40
 
*∆H+=0.4 (∆PaCO2)
 
*∆H+=0.4 (∆PaCO2)
*In chronic resp acidosis, kidneys retain HCO3, which takes a few days
+
*In chronic respiratory acidosis, kidneys retain HCO3, which takes a few days
 
'''Chronic respiratory alkalosis'''
 
'''Chronic respiratory alkalosis'''
 
*HCO3 decreases by 5 for every 10 mmHg decrease in pCO2 below 40
 
*HCO3 decreases by 5 for every 10 mmHg decrease in pCO2 below 40

Latest revision as of 16:09, 15 October 2019

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

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
  • High SID is >38 and indicates a metabolic alkalosis and causes include:

Evaluate the Lactate

Calculate the osmolar gap

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

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
  • Usual for uncomplicated high-AG acidosis.
  • Lactic acidosis: average value 1.6
  • DKA more likely to have a ratio closer to 1 due to urine ketone loss (esp if patient not dehydrated)
> 2

Suggests a pre-existing elevated HCO3 level so consider:

  • a concurrent metabolic alkalosis
  • a pre-existing compensated respiratory acidosis

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