Acid-base disorders: Difference between revisions
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**If difference shrinks (i.e. more Cl) more acidotic | **If difference shrinks (i.e. more Cl) more acidotic | ||
**Principle of electrical neutrality requires more H+ to offset the additional Cl | **Principle of electrical neutrality requires more H+ to offset the additional Cl | ||
**If difference increases (i.e. more Na) more | **If difference increases (i.e. more Na) more alkalotic | ||
**Principle of electrical neutrality requires more bicarb to offset the additional Na | **Principle of electrical neutrality requires more bicarb to offset the additional Na | ||
===Strong ion gap (SIG)=== | ===Strong ion gap (SIG)=== | ||
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*Base excess of -6 = base deficit of 6 | *Base excess of -6 = base deficit of 6 | ||
*Normal = -2 to +2 | *Normal = -2 to +2 | ||
*If base deficit is normal but | *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 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 | *If no BD is available 24.2 – serum bicarb can be used as okay substitute | ||
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{{Acid-base disorders DDX}} | {{Acid-base disorders DDX}} | ||
== | ==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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==Stuart Step Wise Approach== | ==Stuart Step Wise Approach== | ||
*''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> | *''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 >45 then respiratory acidosis | *If pCO2 >45 then [[respiratory acidosis]] | ||
*If <35 respiratory | *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 | **[[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 | **↓HCO3 -[[Metabolic acidosis]] | ||
**↑PaCO2-Respiratory | **↑PaCO2-[[Respiratory acidosis]] | ||
*Alkalemia | *Alkalemia | ||
**↑HCO3-Metabolic | **↑HCO3-[[Metabolic alkalosis]] | ||
**↓PaCO2 - Respiratory | **↓PaCO2 - [[Respiratory alkalosis]] | ||
===Calculate the [[Anion gap]]=== | ===Calculate the [[Anion gap]]=== | ||
Anion gap = [Na+]– [HCO3-] – [Cl-] | Anion gap = [Na+]– [HCO3-] – [Cl-] | ||
===Calculate the delta gap=== | ===Calculate the delta gap=== | ||
*∆gap = anion gap - 12 | *∆gap = anion gap - 12 | ||
*This is to determine a coexistent metabolic alkalosis or non-gap acidosis | *This is to determine a coexistent metabolic alkalosis or non-gap acidosis | ||
{| {{table}} | |||
| align="center" style="background:#f0f0f0;"|'''Delta Ratio''' | |||
| align="center" style="background:#f0f0f0;"|'''Assessment Guideline''' | |||
|- | |||
|< 0.4 | |||
|Hyperchloremic [[non anion gap acidosis|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 [[anion gap acidosis|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=== | ===Calculate the starting bicarbonate=== | ||
*∆gap + (HCO3) = “starting bicarbonate” | *∆gap + (HCO3) = “starting bicarbonate” | ||
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==Determinants of compensation== | ==Determinants of compensation== | ||
'''Metabolic acidosis:''' | '''Metabolic acidosis:''' | ||
*PaCO2 = 1.5 (HCO3) + 8 | *PaCO2 = 1.5 (HCO3) + 8 ± 2 | ||
*PaCO2 = last two digits of pH | *PaCO2 = last two digits of pH | ||
*PaCO2= ↓ 1.0–1.5per ↓ 1mEq/L HCO3 | *PaCO2= ↓ 1.0–1.5per ↓ 1mEq/L HCO3 | ||
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'''Estimate of baseline PCO2 in patients with Acute Respiratory Acidosis:''' | '''Estimate of baseline PCO2 in patients with Acute Respiratory Acidosis:''' | ||
*Estimated baseline PCO2 = 2.4 (admission measured HCO3 – 22) | *Estimated baseline PCO2 = 2.4 (admission measured HCO3 – 22) | ||
'''Chronic respiratory acidosis'''<ref>Brandis K. Anesthesia MCQ. Rules for Metabolic Acid-Base Disorders. http://www.anaesthesiamcq.com/AcidBaseBook/ab9_3.php</ref> | |||
*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== | ==Management== | ||
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*Normal SID (Na-Cl) is 38 | *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 of 38 would be basic b/c it would dilute out the albumin (weak acid) | ||
**Fluid that has SID identical to | **Fluid that has SID identical to patient's serum bicarb is pH neutral | ||
***If SID of fluid is greater than | ***If SID of fluid is greater than patient's bicarb level then it is alkalotic | ||
***If SID of fluid is less than | ***If SID of fluid is less than patient's bicarb level then it is acidotic | ||
====Examples==== | ====Examples==== | ||
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**SID is 892 (very alkalotic) is 8.4% | **SID is 892 (very alkalotic) is 8.4% | ||
^Consider balanced solution (LR) in | ^Consider balanced solution (LR) in patients with low pH (e.g. [[DKA]]) | ||
==See Also== | ==See Also== | ||
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<references/> | <references/> | ||
[[Category:FEN]] | [[Category:FEN]] | ||
[[Category: | [[Category:Renal]] | ||
[[Category:Critical Care]] | [[Category:Critical Care]] |
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
- 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