Acidbase disorders
Background
Determiners of acidbase status are:
 CO2
 Weak acids (primarily albumin)
 If albumin goes up more acidotic (since albumin is an acid)
 Strong ions
 Primarily NaCl
 Normal difference is ~38 (140102)
 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
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
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 = nongap 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 stepwise 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
 ↑PaCO2Respiratory acidosis
 Alkalemia
 ↑HCO3Metabolic 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 nongap 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 preexisting 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 acidbase 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 ~1215 mEq/L
Management
IV Fluids
 Normal SID (NaCl) 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 2428
 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
 Electrolyte Abnormalities (Main)
References
 http://emcrit.org/wpcontent/uploads/acid_base_sheet_22011.pdf
 Stewart Acid base http://www.acidbase.com/strongion.php
 Brandis K. Anesthesia MCQ. Rules for Metabolic AcidBase Disorders. http://www.anaesthesiamcq.com/AcidBaseBook/ab9_3.php
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