Simple Approach to Acid-Base Disorders
5 Steps to evaluation of an acid-base disturbance
Before you begin, you must have an ABG and Chem-7 drawn at the same time.
The HCO3 “bicarb” in these calculations comes from the Chem-7, NOT the ABG.
Here are the 5 steps described below:
1. Look at the pH to determine if the patient has an acid-emia or alkal-emia.
2. Determine the primary type of disturbance.
3. Evaluate for compensation for the primary disturbance
If there is an acidosis…
4. Determine if there is an anion gap.
5. If an elevated anion gap metabolic acidosis is present, check for the delta-delta.
1. Look at the pH to determine if the patient has an acid-emia or alkal-emia.
This is different from an acidosis or alkalosis, which refers to the disorders.
Determining if the patient has an alkalemia or academia is simple. If the pH is
>7.42, they are alkalemic and the primary process is an alkalosis. If the pH is
<7.38, the have an academia and the primary process is an acidosis.
2. After determining acidemia or alkalemia, determine the primary type of
disturbance.
Type of Disorder
pH
Metabolic Acidosis
Decreased
Metabolic Alkalosis
Increased
Respiratory Acidosis
Decreased
Respiratory Alkalosis
Increased
*Change is due to compensation
PaCO2
Decreased*
Increased*
Increased
Decreased
HCO3
Decreased
Increased
Increased*
Decreased*
3. Evaluate compensation for the primary disturbance
Disorder
Metabolic
Acidosis
Primary Change
Decrease in HCO3
Metabolic
Alkalosis
Increase in HCO3
Acute Respiratory
Acidosis
Increase in PaCO2
Chronic
Respiratory
Acidosis
Increase in PaCO2
Compensation
Winter’s equation: PaCO2=1.5[HCO3] + 8
-ORFor every decrease of HCO3 by 10, PaCO2
decreases by 12.
For every increase in HCO3 by 10, the
PaCO2 increases by 6.
-ORIncr PaCO2 = 0.75 x change in HCO3
For every increase in PaCO2 by 10, the
HCO3 increases by 1
-ORIncr HCO3 = 0.1 x change in PaCO2
For every increase in PaCO2 by 10, the
HCO3 increases by 4.
-ORIncr HCO3 = 0.4 x change in PaCO2
Acute Respiratory
Alkalosis
Decrease in PaCO2
Chronic
Respiratory
Alkalosis
Decrease in PaCO2
For every decrease in PaCO2 by 10, the
HCO3 decreases by 2.
-ORDecr HCO3 = 0.2 x change in PaCO2
For every decrease in PaCO2 by 10, the
HCO3 decreases by 5.
-ORDecr HCO3 = 0.4 x change in PaCO2
Remember, appropriate compensation is NOT a secondary acidosis or alkalosis. But,
too much or not enough compensation does indicate a second disorder.
4. Calculate the anion gap. This helps differentiate between AG met acidosis and
non-AG met acidosis. Always calculate the AG. This may expose a hidden
metabolic disturbance.
AG = Na – (Cl + HCO3)
Normal AG is around 12.
5. If an elevated anion gap metabolic acidosis is present, check for the “deltadelta” ratio. This is found only in patients with an AG met acidosis. What this
rule says is that for every increase in AG, there should an equal decrease in
the serum bicarbonate level. For example, if the AG is 20 (8 above normal), the
expected HCO3 should be 16, (8 below a normal value of 24).
If the expected bicarb is higher than 16, there is too much bicarb and another disturbance
that is a metabolic alkalosis is present. If it is less than 16, there is another disturbance
that is a NON-AG metabolic acidosis.
Brief Diffential List:
AG Met Acidosis – Methanol, Urea, DKA, Paraldahyde, Inorganic Phosphates, Lactic
Acidosis, Ethylene Glycol/Ethyl Alcohol, Salicylates.
Non-AG Met Acidosis – Diarrhea, Ureteroenteric Fistual, RTA, Hyperalimentation,
Acetazolamine/Addison’s, Miscellaneous (toluene glue sniffing)
Resp. Alkalosis – Hypoxia, sepsis, liver disease, salicylates, pain.
Resp. Acidosis – Lung disease, CNS hypoventilation, sedatives, neuromuscular weakness
Met Alkalosis – Chloride responsive (Cl is low) – diuretics, hypovolemia, vomiting, NG
suction. Chloride unresponsive (Cl is nml or high) – renal decreased HCO3 excretion or
endocrine – mineralcorticoid excess.
Additonal Pearls:
Anion Gap = unmeasured anions in blood. Albumin, phosphate, and sulfate are unmeasured anions. If albumin is low,
then the “normal range” of AG should be lower.
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Thus, for every decrease in albumin of 1, the AG should decrease 2.5
Salicylates cause an AG acidosis + a respiratory alkalosis
Ketones for ketoacidosis – alcohol, diabetic, starvation
Tox screen
Lactate for lactic acisosis – hypoperfusion, ischemia, sepsis
Osmolal gap – for other “unmeasured anions” like ethylene glycol
Urine anion gap – for workup of NAG acidosis
Sample Questions
1. 25 y.o. AAF with history of DM I presents with nausea, vomiting and polyuria.
Na 128 K 6.5 Cl 90 Bicarb 10 BUN 40 Cr 2.0 Gluc 500 Ca 8.2 Mg 1.8 Phos 2.5
ABG – pH 7.00, pCO2 25, pO2 80, Bicarb 8, O2 Sat 98% on RA
2. 72 y.o. WM with 50 pack year history of tobacco use presents with AMS and
shortness of breath.
Na 145 K 4.5 Cl 100 Bicarb 35 BUN 15 Cr 1.0 Gluc 100
ABG – pH 7.20, pCO2 90, pO2 60, Bicarb 33, O2 Sat 88% on RA
3. 88 y.o. WF with dementia and recent antibiotic treatment for bronchitis comes in
with profuse bloody diarrhea.
Na 130 K 4.0 Cl 105 Bicarb 15 BUN 50 Cr 2.0 Gluc 100
ABG – pH 7.20, pCO2 30, pO2 80, Bicarb 13, O2 Sat 98% on RA
4. 24 y.o. WF with asthma presents with acute onset wheezing and shortness of
breath not relieved by albuterol prn.
Vitals – Temp 98.6, HR 110, RR 34, BP 100/60
ABG – pH 7.48, pCO2 20, pO2 80, Bicarb 20, O2 Sat 92% on RA
Treatment?
Despite therapy she continues to worsen, you know to recheck vitals and an ABG.
Vitals – Temp 98.6, HR 110, RR 20, BP 100/60
ABG – pH 7.40, pCO2 40, pO2 80, Bicarb 24, O2 Sat 92% on RA
What do you think is going on?
Treatment?
5. After eating at Golden Corral a 30 y.o. WM develops acute onset nausea and
vomiting 2 hours later.
Vitals – Temp 99.6, HR 110, RR 14, BP 100/60
Na 145 K 4.5 Cl 90 Bicarb 44 BUN 10 Cr 1.0 Gluc 100
ABG – pH 7.50, pCO2 52, pO2 80, Bicarb 40, O2 Sat 98% on RA
6.
A patient with long standing alcohol use and cirrhosis presents with altered
mental status. Pt disheveled, breath smells like alcohol.
Na 130, K 3.2, Cl 100, HCO3 20, BUN 18, Cr 1.1, Glu 60
INR 2.1, Albumin 2.0.
ABG – 7.26/30/85
What is the corrected AG?
What is going on? Work up?
Treatment?
JCH/acc
6/3/09