Acid- Base Disorders Interpretation and Problems

Acid- Base Disorders
Interpretation and Problems
Akram M Fayed, MD, ABIM
Lecturer, Department of Critical Care Medicine
Faculty of Medicine
University of Alexandria
Acid- Acid Base Interpretation
Any measurement that falls outside the following
ranges is considered to be abnormal:
pH= 7.36 to 7.44
PaCO2 = 36 to 44 mmHg
HCO HCO3 = 22 t to 26 mmHg
H

Rule 1:
Acid- Acid Base Interpretation
A primary metabolic acid- base disorder is present if
The pH is abnormal and the pH and PaCO 2 change in
the same direction
Rule 2:
Acid- Acid Base Interpretation
A superimposed respiratory acid- base disorder is present
if any a y of o the t e following o o g conditions co d t o s are a e satisfied sat s ed
1. The measured PaCO2 is normal.
2. The measured PaCO2 is higher than the expected PaCO2 ( denotes superimposed respiratory acidosis)
33. The measured PaCO PaCO2 is less than the expected PaCO PaCO2 ( denotes a superimposed respiratory alkalosis)

Rule 3:
Acid- Acid Base Interpretation
A primary respiratory acid- base disorder is present if
the PaCO 2 is abnormal and the PaCO 2 and pH change in
opposite directions
Rule 4:
Acid- Acid Base Interpretation
A mixed (acidosis and alkalosis) acid- base disorder is
present if f the h PaCO2 is abnormal b l and d the h pH is
unchanged or normal, or if the pH is abnormal and the
PaCO PaCO2 is unchanged or normal

Acidemia
If the pH is below 7.36, check the PaCO2 and proceed p as follows:
A low or normal PaCO2 indicates a primary
metabolic acidosis.
The difference between the measured and
expected PaCO 2 is then used to identify a
superimposed respiratory disorder
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Acidemia
If the pH is below 7.36, check the PaCO2 and proceed p as follows:
A high PaCO2 indicates a primary
respiratory i t acidosis id i
The change in pH is then used to
ddetermine t i whether h th the th disorder di d is i acute t
or chronic
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Alk Alkalemia l i
If the pH is above 7.44, check the PaCO2 and proceed as follows:
A normal or high PaCO2 indicates a
primary metabolic alkalosis.
A comparison of the measured and
expected PaCO 2 is then used to identify
an associated respiratory disorder
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Alk Alkalemia l i
If the pH is above 7.44, check the PaCO2 and proceed as follows:
A low PaCO PaCO2 indicates a primary
respiratory alkalosis.
The change in pH is then used to
determine whether the disorder is acute
or chronic, or whether a superimposed
metabolic disorder is present.
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Normal pH
If the arterial pH is unchanged or normal normal,
the PaCO2 should be checked:
A high PaCO 2 indicates a mixed
respiratory acidosis- metabolic alkalosis.
A low PaCO 2 indicates a mixed respiratory
alkalosis- metabolic acidosis.
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Normal pH
A normal pH combined with a normal
PaCO PaCO2 is not absolute evidence against an
acid- base disorder because a metabolic
acidosis coexisting with a metabolic
alkalosis lk l i can b be accompanied i d b by a normal l
pH and PaCO2. 9/28/2010 ASAIC 2010
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EExpected t d Ch Changes in i Acid-Base A id B Disorders Di d
Primary Disorder Expected Changes
MMetabolic t b li Acidosis A id i PPaCO2= CO2 1.5* 1 5* HCO3 HCO3+(8±2) (8±2)
Metabolic Alkalosis PaCO2= 0.7* HCO3+(21±2)
Acute Respiratory Acidosis ∆pH= 0.008 * (PaCO2-40)
Chronic Respiratory ∆pH= ∆pH 0.003 * (PaCO2-40)
(PaCO2 40)
Acidosis
Acute Respiratory ∆pH= 0.008 * (40- PaCO2)
Alkalosis
Chronic Respiratory ∆pH= 0.017 * (40- PaCO2)
Alkalosis
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Because a maximum of 3 disorders may exist at any one
time, it may be useful to calculate the anion gap first
rather than as the final step
In this way, a HAGMA will be immediately apparent or
excluded straight away
In the following sequence:
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Step 1: Calculate the anion gap
AG= Na- (Cl + HCO3); normal is roughly 9-16
mmol/ L. L
Since AG= Anion (unmeasured)- Cations
(unmeasured) (unmeasured), the AG can be elevated ( 17 17-20 20
mmol/L) without acidosis
If the AG> 20 20, then a HAGMA is most likely likel
present irrespective of HCO3 or pH
If the th AG>30 AG 30 th then a HAGMA iis present t
irrespective of HCO3 or pH
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Step p 1a: Calculate the delta (∆) ( )g gap p
(only in the presence of a significantly elevated AG)
∆ gap = AG-12
Add ∆ gap to the measured HCO3
If the resultant number is >26 mmol/L, then a coexisting
metabolic alkalosis is present
If the resultant number is < 24 mmol/L, then a coexisting g
normal- gap metabolic acidosis (NAGMA) is present
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Step 2: Go back to the most obvious disorder:
If the pH< 7.4, then acidemia
If the pH> 77.4, 4 then alkalemia
If the pH is low and the bicarbonate is low, then
metabolic acidosis
If the pH is low and PaCO2 is elevated, then
respiratory acidosis
If the pH is high and the bicarbonate is high,
th then metabolic t b li alkalosis lk l i
If the pH is high and PaCO2 is decreased, then
respiratory i t alkalosis lk l i
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Step p 3: Determine if the appropriate pp p compensation p exists;
if not, a second primary disorder is also present
Metabolic acidosis:
Metabolic alkalosis:
PaCO2= (1.5* HCO3+8)± 2
PaCO2= 40+ 0.7 * (HCO3measured- 24)
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Step p 3: Determine if the appropriate pp p compensation p exists;
if not, a second primary disorder is also present
Respiratory acidosis:
Acute:
HCO3 increases 1 mmol/L for every 10 mmHg increase
in PaCO2;
pH decreased 0.008 per 1 mmHg increase in PaCO2.
Chronic:
HCO3 increases 33.5 5 mmol/ L for every 10 mmHg
increase in PaCO3;
pH decreased 00.003 003 per 1 mmHg increase in PaCO2
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St Step 33: DDetermine t i if th the appropriate i t compensation ti exists; i t
if not, a second primary disorder is also present
Respiratory alkalosis:
AAcute: t
HCO3 decreases 2 mmol/L for every 10 mmHg decrease
iin PPaCO2; CO2
pH increased 0.008 per 1 mmHg decrease in PaCO2.
Chronic:
HCO3 decreases 5 mmol/ L for every 10 mmHg increase
in PaCO2;
pH decreased 00.003 003 per 1 mmHg increase in PaCO2
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A 41- year- old homeless man comes to the emergency department
because of intractable vomiting. Nausea and vomiting began 2 days
ago, making him unable to eat or to drink his routine daily bottle of
whiskey whiskey. Physical examination reveals pale, pale frail frail- appearing man in no
acute distress. Vital signs reveal normo-thermia, pulse is 105 beats/ min
supine and increased to 130 standing, and blood pressure is 108/62
mmHg without orthostatic change. The rest of the examination is
unremarkable
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Laboratory evaluation yielded the following values:
- PaO2= 105
- PaCO2= 28
-pH= 7.49
- Bicarobonate=24 mEq/L
-Sodium= Sodium 148 mEq/L
- Potassium= 3.8 mEq/L
- Chloride= Chloride 82 mEq/L
- Magnesium= 1.1 mg/dL
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Which of the following is the most likely cause for these laboratory
findings?
a. Metabolic acidosis and metabolic alkalosis
b. Respiratory alkalosis
c. Metabolic acidosis with respiratory compensation
d d. Metabolic acidosis acidosis, respiratory acidosis acidosis, and respiratory alkalosis
e. Metabolic acidosis, metabolic alkalosis and respiratory alkalosis
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CORRECT ANSWER:
e
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A 45-year-old with severe nephrotic syndrome is admitted with nausea, fever,
and hypotension. yp Her vital signs g are BP 88/50 mm Hg, g, HR 110/min, ,
RR 20/min, temperature 38.30C (1010F), pH H 77.35, 35 PPaco2 32 mm H Hg, sodium di 132 mmol/L, l/L potassium t i 44.0 0 mmoI/L, I/L
chloride 103 mmol/L, HCQ3 l7mmoI/L, albumin 1.5 g/dL, BUN 20 mg/dL,
and creatinine 11.4 4 mg/dL
Which one of the following acid-base disorders is present?
AA. Anion gap metabolic acidosis
B. Nonanion gap metabolic acidosis
C. Nonanion gap gpmetabolic acidosis and respiratory p yalkalosis
D. Anion and nonanion gap metabolic acidosis

CORRECT ANSWER:
Influence of Albumin:
A
Another source of error in the interpretation of the AG occurs when the
contribution of albumin is overlooked
Albumin is major j source of unmeasured anions, and a 50% reduction
in the albumin concentration will result in a 75% reduction in the
anion gap
Since hypoalbuminemia is common in ICU patients, the influence of
albumin on the AG must be considered in all ICU patients

Determinants of the anion gap
Causes of anion gap acidosis include all of the following except:
A. Salicylate poisoning
B B. Isopropyl alcohol ingestion
C. Uremia
D. Seizures

CORRECT ANSWER:
B
An elevation anion gap and an elevation of the osmolar gap may be seen
in all of the following except:
A A. Uremia
B. Ethanol intoxication
C. Methanol poisoning
D. Diabetic Ketoacidosis

CORRECT ANSWER:
B
A 78- year- old African – American male is brought to an urban ED by EMS
after friends found him minimally responsive on the floor of his apartment.
On arrival, arrival his airway is patent, patent his respiration is rapid and deep deep, and you note
his breath has a fruity odor. He is hemodynamically stable but somewhat
tachycardic, y , with slight g decrease in ppulse
volume.
In addition, you are told the patient is a smoker who is noncompliant with his
“puffers”.
The following laboratory values are obtained:
Sodium 130 mmol/L mmol/L,
Potassium 3.8 mmol/L,
Chloride 92 mmol/L,
Bicarbonate 6 mmol/L
Glucose 17.2 mmol/L

His initial arterial blood gas measurements while receiving room air
are as follows:
pH 6.95,
PaO2 28 mmHg,
PaO2 70 mmHg,
Bicarbonate 6 mmol/L
Which of the following acid- base disturbances is now present in this patient?
A. High- anion gap metabolic acidosis.
B B. High- High anion gap metabolic acidosis acidosis, normal normal- gap metabolic acidosis acidosis,
and respiratory alkalosis.
C. High- High anion gap metabolic acidosis, respiratory alkalosis.
D. High- anion gap metabolic acidosis, respiratory acidosis.
E. High- anion gap metabolic acidosis, normal- gap metabolic acidosis, and
respiratory acidosis.

CORRECT ANSWER:
e