View source for 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 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 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 (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 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 DDX}}

==Diagnosis==
Diagnosis is based on clinical history as well as labs:
*VBG/ABG
*Lactate
*Albumin
*Acetone
*Chemistry
*Serum Osmolarity

==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>Stuart Acid base http://www.acid-base.com/strongion.php</ref>
===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 >45 then respiratory acidosis
*If <35 respiratory acidosis

===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
*'''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  [[Sepsis|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]]
**[[Alcohol ketoacidosis|AKA]]
**[[ASA]]
**[[Ethylene Glycol Toxicity|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]]
===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

{| {{table}}
| align="center" style="background:#f0f0f0;"|'''Delta Ratio'''
| align="center" style="background:#f0f0f0;"|'''Assessment Guideline'''
|-
| ||
|-
| < 0.4||Hyperchloraemic 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, or
|}

===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
*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)

==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]])

==See Also==
*[[Electrolyte Abnormalities (Main)]]

==References==
<references/>
[[Category:FEN]]
[[Category:Nephro]]
[[Category:Critical Care]]