Acid-Base Disorders Sharon Anderson, M.D. Division of Nephrology and Hypertension May 2003
Jan 11, 2016
Acid-Base Disorders
Sharon Anderson, M.D.
Division of Nephrology and Hypertension
May 2003
General Acid-Base Relationships
Henderson-Hasselbach equation:
pH = pK + log HCO3
_ /pCO2
H+ = 24 x pCO2/HCO3
_
0.1pH unit = 10 nm/L H+
Approach to Acid-Base Disorders
1. Consider the clinical setting! 2. Is the patient acidemic or alkalemic? 3. Is the primary process metabolic or
respiratory? 4. If metabolic acidosis, gap or non-gap? 5. Is compensation appropriate? 6. Is more than one disorder present?
Simple Acid-Base Disorders
Primary Compensatory
Disorder pH H+ Disorder Response
Metabolic acidosis HCO3
_ pCO2
Metabolic alkalosis HCO3
_ pCO2
Respiratory acidosis pCO2 HCO3
_
Respiratory alkalosis pCO2 HCO3
_
Metabolic Acidosis
Etiology: Inability of the kidney to excrete the dietary H+ load, or increase in the generation of H+ (due to addition of H+ or loss of HCO3
-)
Metabolic Acidosis: Elevated Anion Gap
AG = Na+ - (Cl- + HCO3-) = 12 ± 2
[Note: Diagnostic utility is best when AG > 25]
Causes: Ketoacidosis
Lactic acidosis
Intoxications
Renal failure
Rhabdomyolysis
Anion Gap in Hypoalbuminemia
The true anion gap is underestimated in hypoalbuminemia (= fall in unmeasured anions); AG must be adjusted
Formulas for adjusted AG:– For every 1.0 fall in albumin, increase AG by 2.5– Consider the patient’s “normal” AG to be (2 x alb) +
(0.5 x phosphate)– Adjusted AG = Observed AG + (2.5 x [normal alb -
adjusted alb]
Ketosis
Diabetes
Starvation
Alcoholic
Isopropyl alcohol*
* Ketosis with normal AG and HCO3
_
Ketosis: Points to Remember
-- Normal AG and HCO3_ = isopropyl alcohol
-- Beta-hydroxbutyrate not seen by ketotest
-- Acetoacetate spuriously Cr
-- False positive ketotest:
paraldehyde, disulfiram, captopril
Lactic Acidosis
Type A: Hypoxic
Lactate:pyruvate > 10:1
Type B: Glycolytic
Lactate:pyruvate = 10:1
Intoxications Causing High AG Acidosis
Aspirin - [high salicylate level; also primary respiratory alkalosis]
Methanol - [optic papillitis] Ethylene Glycol - [calcium oxalate crystals] Paraldehyde
Use of venous vs. arterial pH
As compared with arterial blood gasses:
pH 0.03-0.04
pCO2 7-8 mmHg
HCO3 2 mEq/L
The Delta/Delta: AG/ HC03
Rationale:
For each unit INCREASE in AG (above normal), HC03 should DECREASE one unit (below normal)
“Normal” values: AG = 12, HC03 = 24
Use of the Delta/Delta: Examples
AG HCO3 Diagnosis
18 ( 6) 18 ( 6) Appropriate; pure AG acidosis
18 ( 6) 22 ( 2) HCO3 has less than
predicted, so HCO3 is too high;
mixed AG acidosis AND met alk
18 ( 6) 12 ( 12) HCO3 has more than
predicted, so HCO3 is too low;
mixed AG AND non-AG acidosis
Causes of Low Anion Gap
Etiology: Fall in unmeasured anions
or rise in unmeasured cations
Hyperkalemia Lithium intoxication
Hypercalcemia Multiple myeloma
Hypermagnesemia
Artefactual: hypernatremia, bromide, hyperlipidemia
Osmolar Gap
Measured serum osmolality >
calculated serum osmolality by > 10 mOsm
Calc Sosm = (2 x Na) + BUN/2.8 + Glu/18
Causes of High Osmolar Gap
Isotonic hyponatremia
Hyperlipidemia
Hyperproteinemia
Mannitol
Glycine infusion
Chronic renal failure
Ingestions
Ethanol, isopropyl alcohol, ethylene glycol, mannitol
Contrast Media
Relationship between AG and Osmolar Gap
AG Osm gap Comments
Ethylene glycol + + * Double gap
Methanol + + * Double gap
Renal failure + + * Double gap
Isopropyl alcohol - +
Ethanol - +
Lipids, proteins - +
Causes of Normal AG (Hyperchloremic) Metabolic Acidosis
High K+ Low K+
Adrenal insufficiency Diarrhea
Interstitial nephritis RTA
NH4Cl, Arg HCl Ureteral diversion
Use of the Urine Anion Gap (UAG) in Normal AG AcidosisBatlle et al. NEJM 318:594, 1988
Urine AG = (Na + K) - Cl
Negative UAG = Normal, or GI loss of HCO3
Positive UAG = altered distal renal acidification
Caveats: Less accurate in patients with volume depletion (low urinary Na); and in patients with increased excretion of unmeasured anions (e.g. ketoacidosis), where there is increased excretion of Na and K to maintain electroneutrality)
Use of the Urinary AG in Normal Gap AcidosisBatlle et al. NEJM 318:594, 1988
Plasma K UAG U pH Diagnosis
Normal - < 5.5 Normal
Normal-low - > 5.5 GI HCO3 loss
High + < 5.5 Aldo deficiency
High + > 5.5 Distal RTA
Normal-low + > 5.5 Proximal RTA
Use of the Urine Osmolal Gap
When UAG is positive, and it is unclear if increased cation excretion is responsible, urine NH4 concentration
can be estimated from urine osmolal gap Calc Uosm = (2 x [Na+K]) + urea nitrogen/2.8 + glu/18 The gap between the calculated and measured Uosm =
mostly ammonium In patients with metabolic acidosis, urine ammonium
should be > 20 mEq/L. Lower value = impaired acidification
Renal Tubular Acidosis
Type 1 (distal) Type 2 (proximal) Type 4
Defect distal acid. prox HCO3 reab aldo
HCO3 May be < 10 12-20 > 17
Urine pH > 5.3 Variable < 5.3
Plasma K Usually low Usually low High
Response Good Poor Fair
to HCO3 Rx
Calculation of Bicarbonate Deficit
Bicarb deficit = HCO3- space x HCO3
- deficit/liter
HCO3- space = 0.4 x lean body wt (kg)
HCO3- deficit/liter = [desired HCO3
-] - [measured HCO3-]
Osmolar GapAnionGap GI Fluid Loss?
UremiaLactateKetoacidsSalicylate
EthyleneglycolMethanol
Normal IncreasedDiarrheaIleostomyEnteric fistula
Urine pH
Distal RTA (Type 1)Serum K
Proximal RTA (Type 2)
Type 4RTA
High Normal
NoYes
> 5.5< 5.5
HighLow
Approach to Metabolic Acidosis
Metabolic Alkalosis
Etiology: Requires both generation of metabolic alkalosis (loss of H+ through GI tract or kidneys) and maintenance of alkalosis (impairment in renal HCO3 excretion)
Causes of metabolic alkalosis
Loss of hydrogen
Retention of bicarbonate
Contraction alkalosis Maintenance factors: Decrease in GFR, increase in HCO3
reabsorption
Use of Spot Urine Cl and K
Urine Chloride Very Low(< 10 mEq/L)
Vomiting, NG suctionPostdiuretic, posthypercapneicVillous adenoma, congenitalchloridorrhea, post- alkali
> 20 mEq/L
Urine Potassium
> 30 mEq/L
Diuretic phase of diuretic Rx, Bartter’s, Gitelman’s, primary aldo, Cushings, Liddle’s, secondary aldosteronism
Low (< 20 mEq/L)
Laxative abuseOther profound K depletion
Treatment of Metabolic Alkalosis
1. Remove offending culprits.
2. Chloride (saline) responsive alkalosis: Replete volume with NaCl.
3. Chloride non-responsive (saline resistant) alkalosis:
Acetazolamide (CA inhibitor)
Hydrochloric acid infusion
Correct hypokalemia if present
Calculation of Bicarbonate Excess
Bicarb excess = HCO3- space x HCO3
- excess/liter
HCO3- space = 0.5 x lean body wt (kg)
HCO3- excess/liter = [measured HCO3
-] - [desired HCO3-]
Respiratory Acidosis
Causes of Respiratory Acidosis
Inhibition of medullary respiratory center
Disorders of respiratory muscles and chest wall
Upper airway obstruction
Disorders affecting gas exchange across pulmonary capillaries
Mechanical ventilation
Respiratory Alkalosis
Causes of Respiratory Alkalosis
Hypoxemia
Pulmonary disease
Stimulation of medullary respiratory center
Mechanical ventilation
Mixed Acid-Base Disorders: Clues
-- Degree of compensation for primary
disorder is inappropriate
-- Delta AG/delta HCO3
_ = too high or too low
-- Clinical history
Problem 1
A 30-yo man with DM presents with a week of polyuria, polydipsia, fever to 102, nausea, and abdominal pain. He is orthostatic on admission.
130 I 94 I 75 I 906 pH 7.14
6.1 I 6 I 2.3 pCO2 18
pO2 102
Problem 1, cont.
1. Anticipate the disorder
DKA (with anion gap acidosis)
2. Acidemic or alkalemic? 3. Metabolic or respiratory?
pH = acidemic; must be metabolic (low HCO3, low pCO2)
4. If metabolic acidosis: gap or non-gap?
AG = 30; + anion gap metabolic acidosis
5. Is compensation appropriate?
pCO2 should = last 2 digits of pH [18] or (1.5 x HCO3) + 8 [17]
6. Mixed disorder?
AG = 30 (18); HCO3 = 618); thus simple AG met acidosis
130 I 94 I 75 I 906 7.14/18/102
6.1 I 6 I 2.3
Problem 2
A 30-yo man with DM presents with a week of polyuria, polydipsia, fever to 102, and vomiting for four days.
135 I 89 I 50 I 1181 pH 7.26
6.1 I 10 I 2.3 pCO2 23
pO2 88
Problem 2, cont.1. Anticipate the disorder
DKA (AG acidosis); met alk from vomiting
2. Acidemic or alkalemic? 3. Metabolic or respiratory?
pH = acidemic; must be metabolic (low HCO3, low pCO2)
4. If metabolic acidosis: gap or non-gap?
AG = 36; + anion gap metabolic acidosis
5. Is compensation appropriate?
pCO2 should = last 2 digits of pH [26] or (1.5 x HCO3) + 8 [23]
6. Mixed disorder?
AG = 36 (24); HCO3 = 1014); HCO3 is too high; mixed AG metabolic acidosis and
metabolic alkalosis
135 I 89 I 50 I 1181 7.26/23/88
6.1 I 10 I 2.3
Problem 3
A 30-yo man with DM presents with a week of polyuria, polydipsia, fever to 102, and diarrhea.
138 I 111I 49 I 650 pH 7.26
5.5 I 8I 1.4 pCO2 23
pO2 88
Problem 3, cont.1. Anticipate the disorder
DKA (AG acidosis); nongap met acidosis from diarrhea
2. Acidemic or alkalemic? 3. Metabolic or respiratory?
pH = acidemic; must be metabolic (low HCO3, low pCO2)
4. If metabolic acidosis: gap or non-gap?
AG = 19; + anion gap metabolic acidosis
5. Is compensation appropriate?
pCO2 should = last 2 digits of pH [26] or (1.5 x HCO3) + 8 [23]
6. Mixed disorder?
AG = 19 (7); HCO3 = 816); HCO3 is too low; mixed AG metabolic acidosis and
metabolic acidosis (nongap)
138 I 111 I 49 I 650 7.26/23/88
5.51 I 8 I 1.4
Problem 4
A 30-yo man with DM presents with a week of polyuria, polydipsia, fever, cough, and prurulent sputum.
140 I 104 I 75 I 1008 pH 6.95
7.0 I 7 I 2.6 pCO2 33
pO2 60
Problem 4, cont.
1. Anticipate the disorder
DKA (AG acidosis); resp alk or resp acidosis from hypoxemia/pneumonia
2. Acidemic or alkalemic? 3. Metabolic or respiratory?
pH = acidemic; must be metabolic (low HCO3, low pCO2)
4. If metabolic acidosis: gap or non-gap?
AG = 29; + anion gap metabolic acidosis
140 I 104 I 75 I 1008 6.95/33/60
7.0 I 7 I 2.6
Problem 4, cont.
5. Is compensation appropriate?
pCO2 should = last 2 digits of pH [95!!] or (1.5 x
HCO3) + 8 [18]; pCO2 is too high so he has a
superimposed respiratory acidosis
6. Mixed disorder?
AG = 29 (17); HCO3 = 717); so metabolic acidosis is pure AG acidosis. Thus, mixed AG metabolic acidosis and respiratory acidosis
140 I 104 I 75 I 1008 6.95/33/60
7.0 I 7 I 2.6
Problem 5
A 31-yo woman who is 33 weeks pregnant presents with a 2-day history of vomiting.
140 I 104 I 8 I 85 pH 7.64
3.0 I 26 I 0.6 pCO2 25
pO2 93
Problem 5, cont.
1. Anticipate the disorder
Pregnancy: resp alk Vomiting: met alk
2. Acidemic or alkalemic?
pH = alkalemic
3. Metabolic or respiratory?
If resp, HCO3 should be low; if metabolic, then pCO2 should be
high; must have both
4. If metabolic acidosis: gap or non-gap?
N/A; no acidosis; no AG
140 I 104 I 8 I 85 7.64/25/93
3.0 I 26 I 0.6
Problem 5, cont.
5. Is compensation appropriate?
NO (by eyeball, for reasons listed above)
6. Mixed disorder?
Yes, mixed metabolic and respiratory alkalosis. No acidosis component.
140 I 104 I 8 I 85 7.64/25/93
3.0 I 26 I 0.6
Problem 6
A 60-yo man has crushing chest pain, SOB and diaphoresis. He has HTN, for which he takes HCTZ. Exam shows BP 88/60, bilateral crackles, S3. EKG shows ischemia; CXR = pulmonary edema.
140 I 94 I 45 I 300 pH 7.14
5.9 I 20 I 1.9 pCO2 60
pO2 52
Problem 6, cont.1. Anticipate the disorder
Pulm edema -> resp alk or resp acidosis; shock -> metabolic acidosis; HCTZ -> metabolic alkalosis
2. Acidemic or alkalemic?
pH = acidemic
3. Metabolic or respiratory?
If resp, HCO3 should be > 24 in compensation; if metabolic, then pCO2
should < 40; must have both respiratory and metabolic acidoses
4. If metabolic acidosis: gap or non-gap?
AG = 26; + anion gap metabolic acidosis
140 I 94 I 45 I 300 7.14/60/52
5.9 I 20 I 1.9
Problem 6, cont.
5. Is compensation appropriate?
NO (by eyeball, for reasons listed above)
6. Mixed disorder? Anything else?
AG = 26 (14); HCO3 = 20); so HCO3 is
too high; must have a superimposed metabolic alkalosis.
Thus, triple disorder: respiratory acidosis, anion gap metabolic acidosis, and metabolic alkalosis
140 I 94 I 45 I 300 7.14/60/52
5.9 I 20 I 1.9
Problem 7
A 55-yo woman with a history of a CVA presents to clinic complaining of shortness of breath.
140 I 100 I 30 I 115 pH 7.36
3.9 I 30 I 1.5 pCO2 38
pO2 91
Problem 7, cont.
1. Anticipate the disorder
Resp alk due to CNS disorder or acute pulmonary process
2. Acidemic or alkalemic?
pH = acidemic
3. Metabolic or respiratory? 4. If metabolic acidosis: AG?
HCO3 is high (not metabolic acidosis); pCO2 is < 40 (not
respiratory acidosis); AG is normal (10), so what’s going on??
140 I 100 I 30 I 115 7.36/38/91
3.9 I 30 I 1.5
Problem 7, cont.
LAB ERROR!
By Henderson-Hasselbach
H+ = 24 x pCO2/HCO3 = 24 x (38/30) = 30
pH should be 7.50
140 I 100 I 30 I 115 7.36/38/91
3.9 I 30 I 1.5
Problem 8
You are in the ER, and are aware that the lab has been having intermittent problems with the chemistry autoanalyzer. A 30-yo diabetic man, well known to you from previous visits, comes in with severe nausea and vomiting. His blood alcohol level is very high. The ER attending advises you to check his labs and send him home if they are OK.
140 I 84I 28 I 160 pH 7.40
3.0 I 24I 1.3 pCO2 40
pO2 88
Problem 8, cont.
1. Anticipate the disorder
Vomiting -> met alk; if unconscious, resp acidosis
2. Acidemic or alkalemic? 3. Metabolic or respiratory?
pH, pCO2 and HC03 are all normal --> no apparent
disorder
4. Lab error? Check H-H equations.
H+ = 24 x (pCO2/HCO3) = 24 x (40/24) = 40, so pH = 7.40
5. Do you send him home?
140 I 84 I 28 I 160 7.40/40/88
3.0 I 24 I 1.3
Problem 8, cont.
5. Do you send him home?
AG = 32; + anion gap acidosis
AG = 32 (20); HCO3 = 240); so HCO3 is too high;
must have a superimposed metabolic alkalosis.
Thus, mixed AG acidosis and metabolic alkalosis
140 I 84 I 28 I 160 7.40/40/88
3.0 I 24 I 1.3
Problem 9
A 58-yo man with cirrhosis and Type 2 DM presents with fever, abdominal pain, SOB, and vomiting.
159 I 112 I 55 I 160 pH 7.31
3.3 I 12 I 2.8 pCO2 19
pO2 77
Problem 9, cont.
1. Anticipate the disorder
Renal dis --> acidosis; dead gut --> lactic acidosis; vomiting --> met alk; pain --> resp alk; liver disease --> resp alk
2. Acidemic or alkalemic? 3. Metabolic or respiratory?
pH = acidemic; must be metabolic (low HCO3, low pCO2)
4. If metabolic acidosis: gap or non-gap?
AG = 35; + anion gap metabolic acidosis
159 I 112 I 55 I 160 7.31/19/77
3.3 I 12 I 2.8
Problem 9, cont.
5. Is compensation appropriate?
pCO2 should = last 2 digits of pH [31; not] or (1.5 x
HCO3) + 8 [26]; pCO2 is too low so he has a
superimposed respiratory alkalosis
6. Mixed disorder?
AG = 35 (23); HCO3 = 1212); so HCO3 is too high,
so there must be a metabolic alkalosis.
Thus, triple disorder: AG metabolic acidosis, respiratory alkalosis, and metabolic alkalosis
159 I 112 I 55 I 160 7.31/19/77
3.3 I 12 I 2.8
Problem 10
A 70-yo man presents with vomiting and abdominal pain, for which he has been taking Rolaids. He is hypotensive and has a tender abdomen.
140 I 69 I 40 I 118 pH 7.74
3.4 I 40 I 1.5 pCO2 30
pO2 105
Problem 10, cont.
1. Anticipate the disorder
Dead gut --> lactic acidosis; vomiting or Rolaids --> met alk; pain --> resp alk
2. Acidemic or alkalemic? 3. Metabolic or respiratory?
pH = alkalemic; must be both metabolic (high HCO3) and
respiratory (low pCO2)
4. If metabolic acidosis: gap or non-gap?
AG = 31; + anion gap metabolic acidosis
140 I 69 I 40 I 118 7.74/30/105
3.4 I 40 I 1.5
Problem 10, cont.
5. Is compensation appropriate?
Cannot compute, too many disorders
6. Mixed disorder?
AG = 31 (19); HCO3 = 40, not down; so HCO3 is
too high, so there must be a metabolic alkalosis.
Thus, triple disorder: AG metabolic acidosis, respiratory alkalosis, and metabolic alkalosis
140 I 69 I 40 I 118 7.74/30/105
3.4 I 40 I 1.5