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Acid Base Disorders
Viyeka Sethi
PGY 4
Med-Peds
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Four Main Acid-Base Disorders
Suppression of acid
excretion; decrease in
reabsorption of HCO3
in plasmaHCO3
in plasmapCO2
Respiratory
Alkalosis
Increase in acid
excretion; increase in
reabsorption of HCO3
in plasma
HCO3
in plasma
pCO2
Respiratory
Acidosis
Hypoventilationincrease in
pCO2 in plasma
HCO3
Metabolic
Alkalosis
Hyperventilationin plasma
pCO2
in plasmaHCO3
Metabolic
Acidosis
Mechanism of
Response
Secondary
Response
Primary
Alteration
Disorder
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Normal Values
70pO2
>4040pCO2
2424HCO3
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Step by Step
1. Determine the primary disturbance:
Acidemia or Alkalemia: look at the pH
< 7.40 = acidemia> 7.40 = alkalemia
Respiratory or Metabolic: look at HCO3 and CO2
HCO3 = primary metabolic acidosis
pCO2 = primary respiratory acidosis
and vice versa for alkalosis
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2. Determine acute or chronic for Respiratory
Disturbance:
o Compensation attempts to normalize pH but canbe present with an abnormal pH
o Expected change in pCO2 best used for primary
metabolic disturbance and expected change in
HCO3 for primary respiratory disturbance
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Respiratory Disturbance
Compensation
30.0310Chronic Resp.Acidosis
10.0810Acute Resp.
Acidosis
in HCO3in pHpCO2
50.0310Chronic Resp.Alkalosis
20.0810Acute Resp.Alkalosis
in HCO3in pHpCO2
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3. Primary Metabolic Disturbance:
o Calculate anion gap : Na (Cl + HCO3)
o Normal = 12 +/- 2o If gap is >20 then there is primary metabolic
acidosis regardless of pH or bicarb.
o Helps narrow differential with a anion gap ornon-anion gap metabolic acidosis
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4. Assess appropriate respiratorycompensation for metabolic disorder:
o Respiratory compensation is fasto Winters formula:
Expected pCO2 = (1.5 * HCO3) + 8 (+/-2)
o If measured pCO2 is
< expected then co-existing resp. alkalosis
> expected then co-existing resp. acidosis
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5. Determine if other metabolic disturbances
co-exist with AG metabolic acidosis:
o Delta gap accounts for increase in anion gapand shows any variation in HCO3
o If no other disorder is present then the
calculation should be 24
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Delta Gap
Corrected HCO3 = measured HCO3 + (AG - 12)
o So if corrected HCO3
>24 then metabolic alkalosis co-exists
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Hints
Simple acid base compensatory response
always in direction of primary variable
Compensation is more pronounced withchronic disorders
Normal pH indicates two or more acid-base
disorders
If given electrolytes, use them!!!
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Pneumonics for pnuemonic lovers
CNS disease
Hypocapnia
Anxiety
Mech. Ventilation
Progesterone
Salicylates
Sepsis
Contraction
Licorice
Endocrine
(Conn/Cushing
/Bartters)
Vomiting
Excess alkali
Refeeding
Post-
hypercapnia
Diuretics
CNS depression
Airway
obstruction
Pulmonary
edema
Pneumonia
Hemo/Pneumo
thorax
Neuromuscular
Hyperalimentation
Acetazolamide
Renal Tubular
Acidosis
Diarrhea
Uretero-Pelvic
shunt
Post-hypocapnia
Spironolactone
Methanol
Uremia
DKA/Alcoholic
ketoacidosis
Paraldehyde
Isoniazid
Lactic acidosis
Ethanol
Renal
failure/Rhabdo
Salicylates
Respiratory
Alkalosis
CHAMPS
Metabolic
Alkalosis
CLEVERPD
Acute Resp.
Acidosis
anything
causing
hypoventilation
Metabolic
Acidosis Non-
Gap
HARDUPS
Metabolic
Acidosis Anion
Gap
MUDPILERS
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Case 1
3 yo boy with diarrhea is evaluated in the
ER. Initial ABG shows:
ph 7.23
HCO3 10
pCO2 23
AG - 13
Alkalemia or acidemia?Low pH = acidemia
Primary disturbance resp. or metabolic?
Low HCO3 = metabolic acidosis
Normal AG so no need for Delta gap equationBUT
Is there adequate respiratory compensation?
(Use Winters formula)
Expected pCO2 = (1.5 * HCO3) + 8 +/-2
So,Expected pCO2 = (1.5 * 10) + 8 +/-2
=(15) + 8 +/-2
=23 +/- 2
So, we have a metabolic acidosis with
respiratory compensation
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Case 2
5 yo boy presents to ED with dyspnea for 3
days. ABG shows the following:
pH 7.35
paCO2 60
paO2 57
HCO3 - 31
Acidemia or alkalemia?Low pH = Acidemia
Primary Resp. or Metabolic disturbance?
Respiratory
Check yourself.using rules for Primary Resp
disturbance
For every 10 increase in pCO2 -
pH decreases by .08 acutely or .03 chronically
HCO3 increases by 1 acutely or 3 chronically
So, this is a chronic resp. acidosis
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Case 3
15 yo with 4 days of vomiting and fever.
pH- 7.50 Na- 138
pCO2- 42 Cl- 80
pO2- 80 HCO3- 34
Acidemia or alkalemia?
High ph = alkalemia
Respiratory or Metabolic?
High HCO3 = metabolic
Is Resp. compensation appropriate?
Exp. pCO2 = (1.5 * HCO3) + 8 +/-2=(51) + 8 +/-2
=59 +/- 2
So, NO the CO2 is lower so we have a
Co-existent resp.alkalosis
Anion Gap?
138 (80 + 34) = 24
Checking the Delta gap only usefulin a primary metabolic acidosis.
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Case 4