### Acid-base classification versus interpretation of blood gases

```TEN THINGS EVERYONE
BLOOD GASES
Doug Pursley, M.Ed., RRT
Program Director
Ozarks Technical Community College
Springfield, MO
1. There is a difference between
classification and interpretation of
blood gases
Classification vs. Interpretation
• Classification –
systematic
arrangement
according to
established criteria
Classification:
The High/Lo or Arrow System
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54 year old narcotic overdose:
pH
7.22
(7.40)
PaCO2 70
(40)
HCO3 27
(24)
• Partially compensated respiratory acidosis
according to the arrow system
Classification vs. Interpretation
• Interpretation – the
act of applying an
explanation to the
results
Interpretation
• Incorporates calculations, baseline values,
electrolytes (Na+/Cl-), and clinical picture into
the equation to come up with a more precise
explanation of the patient’s acid-base status
2. Hydrolysis happens
Hydrolysis
+
=
=
+
CO2 + H2O = H2CO3 = H+ + HCO3
• Starting at a PaCO2 of 40 mmHg, HCO3 will
increase from normal by 1 for every 10 acute
increase in PaCO2
• Starting at a PaCO2 of 40 mmHg, HCO3 will
decrease from normal by 2 for every 10 acute
decrease in PaCO2
• Simple physiochemical event and occurs
almost immediately
Correct interpretation when one
Starting at a PaCO2 of 40 mmHg, HCO3
will increase from normal by 1 for
every 10 increase in PaCO2
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•
54 year old narcotic overdose:
pH
7.22
PaCO2 70 (three tens above 40)
HCO3 27 (HCO3 expected to increase by 3)
Expected HCO3 same as Actual HCO3 so…
Acute respiratory acidosis
Base excess is zero in this case
3. Always evaluate the base
excess/deficit
• Normal BE/BD is -2 to +2
• With the exception of
• Values above +2 indicate compensation for
respiratory acidosis or metabolic alkalosis
• Values below -2 indicate compensation for
respiratory alkalosis or metabolic acidosis
• Normal value for base does not exclude
metabolic acidosis IF there is also a co-existing
and competing metabolic alkalosis and visa versa
General rule of thumb
• If the pH and PaCO2 are both out of whack in
opposite directions, and the base is normal,
start your interpretation out with ACUTE
RESPIRATORY…
• Possible exceptions:
– Co-existing Metabolic Acidosis AND Metabolic
Alkalosis
– Triple acid-base disturbances
4. Indirect metabolic assessment can
be performed by comparing the actual
pH with predicted respiratory pH
• Starting at pH of 7.4 and PaCO2 of 40…
• pH will increase by 0.1 for each 10 acute
decrease in PaCO2
• pH will decrease by 0.06 for each 10 acute
increase in PaCO2
• If PaCO2 suddenly changes from 40 to 30, pH
will change from 7.40 to 7.50
• If PaCO2 suddenly changes from 40 to 60, pH
will change from 7.40 to 7.28
• If actual pH is greater than the predicted
respiratory pH by more than 0.03, the patient
will have a base excess
• If actual pH is less than the predicted
respiratory pH by more than 0.03, there will
be a base deficit
• If both are within 0.03 of each other, then the
base will be normal most likely reflecting an
acute respiratory disturbance
5. The anion and bicarbonate gaps
are important parts of evaluating
acid-base disorders
Two types of metabolic acidosis
• High AG metabolic acidosis
– Accumulation of acid in the plasma
– Lactic acidosis, DKA, azotemic renal failure, toxins
• Normal AG metabolic acidosis
– Loss of base from the plasma
– Diarrhea
– Intestinal drainage tubes
– Renal tubular acidosis (RTA)
Anion Gap
• Represents the concentration of unmeasured
anions in the plasma
• Unmeasured anions also referred to as “acid
anions” such as:
– Lactate (lactic acidosis)
– Acetoacetate (ketoacidosis)
– Sulphate, phosphate (azotemic renal failure)
Anion Gap
• Derived from subtracting measured cations
from measured anions
• Clinical equation is:
+
• Na - Cl - HCO3 = Anion Gap
Anion Gap
• Normal value is 12
• If value is increased, there is an accumulation
of acid anions in the plasma
• If AG is 20-29, there is 67% chance of high AG
metabolic acidosis
• If AG is >30, there is 100% chance of high AG
metabolic acidosis
Anion Gap
• If High AG acidosis is the only acid-base disorder,
then there should be a 1:1 correlation between
rise in the gap and fall in HCO3
• Example: If AG goes from 12 to 24, HCO3 should
go from 24 to 12
• A high AG with a normal or high HCO3 indicates
extra HCO3 on board from an additional
metabolic alkalosis or compensation for
respiratory acidosis
Normal AG Metabolic Acidosis
• Due to loss of base rather than accumulation
of acids
• Anion Gap will be normal
• Common causes are:
– Renal tubular acidosis
– Intestinal drainage tubes
– Diarrhea
Bicarbonate Gap
• Represents the change in AG from normal
minus the change in HCO3 from normal
• Normal value is zero, plus or minus 6
• Values greater than +6 indicate:
– Metabolic alkalosis or compensation for
respiratory acidosis
• Values less than -6 indicate:
– Normal AG gap metabolic acidosis or
compensation for respiratory alkalosis
Hard way to calculate BG
• ∆ AG - ∆ HCO3
• = [AG - 12] – [24 – HCO3]
• = [(Na – Cl – HCO3) – 12] – [24 – HCO3]
Easy way to calculate BG
•BG = Na – Cl - 36
• Comes from cancelling out the terms in the
equation below:
• BG = [(Na – Cl – HCO3) – 12] – [24 – HCO3]
6. Know the rules for
compensation
Compensation for respiratory
acidosis
• HCO3 will increase by 4 for every 10 chronic
increase in PaCO2 above 40
• Kidneys respond to chronic respiratory acidosis
by retaining HCO3
• Takes 3-4 days to reach a maximum value
Compensation for respiratory alkalosis
• HCO3 will decrease by 5 for every 10 chronic
decrease in PaCO2 below 40
• Kidneys respond to chronic respiratory
alkalosis by excreting HCO3
• Takes 3-4 days to reach a maximum value
Compensation for metabolic acidosis
• Expected PaCO2 in metabolic acidosis will follow
“One Point Five Plus Eight Rule” or Winter’s
Formula:
– PaCO2 expected = (1.5 x HCO3) + 8
• Compensation starts immediately and becomes
complete in several hours
• Limit of compensation is PaCO2 of about 8-10
Compensation for metabolic alkalosis
• Expected PaCO2 in metabolic alkalosis will
follow the “Point Seven Plus 20 Rule”
– PaCO2 expected = (0.7 x HCO3) + 20
• Compensation starts immediately and
becomes complete in several hours
• Limit of compensation traditional thought to
be PaCO2 of 60 but more recent studies
indicate CO2 may be linear with the HCO3
7. All acid-base values should
calculate out according to the
Henderson-Hasselbalch equation
H-H Equation
30 second accuracy check with a \$15
calculator
– Multiply PaCO2 by 0.03 (converts to H2CO3)
– Divide HCO3 by H2CO3
– Log the result
• Result should be within 0.03 units of the
measured pH
• If not, acid base is inconsistent (could be
HCO3 calculation error, sensor error, or
transcription error)
8. Total oxygen content (Cao2) is
the best index of oxygenation
CaO2
• CaO2 = (1.34 x Hb x SO2) + (PO2 x 0.003)
• Best index of blood oxygen because it is a
measure of total oxygen in blood
Other Oxygen indices
PaO2
• Measure of oxygen dissolved in plasma
• Indication of the amount of oxygen available to
combine with Hb
• Can be normal or high in anemia, COHb, and MetHb
• PaO2 of 60 is not bad if Hb and pH are normal
• Direct correlation with SaO2
– Absent dyshemoglobinemia, if a patient’s PaO2 is 60, his
SaO2 will be 90% assuming a normally shaped O2-Hb
curve
SaO2calc
• An estimation of the oxygen saturation from
the O2Hb curve
• Plots pH with PO2 to determine saturation
• Not accurate in COHb and MetHb
SaO2CO-ox
• The actual, fractionalized value of oxygen
saturation measure by a CO-oximeter
• Most accurate SaO2
SpO2
• Uses red and infrared wavelengths of light to
determine saturation
• Measures “functional” saturation i.e. assumes
only oxygen is attached to Hb
• Overestimates saturation in COHb
• Migrates towards 85% in MetHb
All saturations
• In severe anemia i.e. Hb 4 g/dl, SaO2calc ,
SaO2CO-ox , and SpO2 can all be high and the
patient could still be have tissue hypoxia
a/A ratio
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Index of shunting
PaO2 ÷ PAO2
Normal value is 0.9 (i.e. PaO2 90 ÷ PAO2 100)
Lower limit of normal is 0.75
The lower the number, the worse the shunt
9. The P/F ratio is a good index of
shunting, but it does have a
limitation
P/F ratio
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•
•
•
Index of shunting
Simplified version of a/A ratio
PaO2 ÷ FIO2
Normal value is 400-500
– PaO2 90 ÷ FIO2 0.21 = 429
• The lower the number, the worse the shunt
• Not accurate in severe hypercarbia
10. The alveolar air equation has
actual clinical application
Alveolar Air Equation
• PAO2 = (PB – 47 x FIO2) – (PaCO2 x 1.2)
• Needed to calculate PaO2/PAO2 ratio and
• A modification of the formula can be used to
estimate device FIO2 in normal subjects
Case 1
• A adult male patient recently brought to the ER
has the following ABG on a nasal cannula at 2
l/m:
• 3:49 am
• pH 6.83
• PaCO2 139
• PaO2 138
• HCO3 23
• Base -13
What is the interpretation?
• A second ABG is taken 23 minutes later while
the patient is being manually ventilated:
• 4:12 am
• pH 7.16
• PaCO2 47
• PaO2 92
• HCO3 17
• Base -11
Why did the HCO3 drop from 23 to
17 in just twenty-three minutes?
Case 2
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A patient is admitted with the following ABG:
pH
7.47
PaCO2 51
HCO3 35.5
Base
+12
Classify the acid-base
What if I told you…
• 78 year old end stage CO2 retainer admitted
to the ETC with exacerbation of his COPD.
Pneumococcal pneumonia is suspected.
• pH
7.47
• PaCO2 51
• HCO3 35.5
• Base
+12
Change your mind? Or stick with
the original assessment.
Case 3
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34 year old aspirin overdose
pH
7.45
PaCO2 9 mmHg
Base
-16.4
PaO2
115 mmHg breathing room air
Na+ 140 mEQ/L
Cl- 104 mEQ/L
HCO3 6 mEQ/L
Calculate the anion and bicarbonate
gaps.
What’s the interpretation?
Is the acid-base status accurate?
pH 7.45, PaCO2 9, HCO3 6
– Multiply PaCO2 by
0.03 (converts to
H2CO3)
– Divide HCO3 by
H2CO3
– Log the result
Case 4
• 44 year old female, previously healthy,
narcotic overdose
• pH 7.19
• PaCO2 60
• HCO3 22
Interpretation?
Case 5
• 28 year-old in her third trimester of pregnancy
is admitted after having severe vomiting for
several days.
pH 7.58
PaC02 31
HC03 28
Base +6
Interpretation?
Case 6
• The following ABG and electrolytes were taken
from a 55 year old male in the ICU with liver
disease:
• pH 7.40
• PaCO2 20
• HCO3 12
• Base -12
• Na 146
• Cl 106
Calculate the anion and bicarbonate
gaps and interpret the ABG
Case 7
• 17 year old female seen in the ETC with
dizziness and shortness of breath:
• pH 7.60
• PaC02 20
• HC03 20
• Base -1
Interpret the ABG
Case 8
• A 23 year old male has just returned to the US
from overseas and is having acute abdominal
pain with severe diarrhea.
• He is admitted to a NY hospital with the
following values:
• pH 7.30, PaCO2 28, HCO3 13, Base -11.7,
Na 144, Cl 119
Calculate the anion and bicarbonate
gaps and interpret the ABG
Case 9
• A 42 year old man is admitted to the hospital
with dehydration and hypotension. No ABG is
obtained. Venous electrolytes show:
+
• Na 165
+
• K 4.0
• Total CO2 32
• Cl 112
Calculate the anion and bicarbonate
gaps and make a statement about the
patient’s acid-base status
Note 1. Use Total CO2 as a substitute for
HCO3 on a lab report
Note 2: When calculating BG from venous
electrolytes, the formula becomes:
BG = Na – Cl – 39, since normal value for
TCO2 is 27 compared with the HCO3
normal of 24
Case 10
• A 19 year old pregnant insulin dependent
diabetic female was admitted to a San Diego
hospital (PB = 760 mmHg) with a history of
polyuria and thirst.
• She has a history of poor compliance with
medical therapy. On examination, she was
afebrile, chest was clear, peripheral circulation
Blood gases while breathing room air:
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Na 136, Cl 100
pH 7.17
PaCO2 17
PaO2 115
HCO3 6
Base -20
Calculate the anion and bicarbonate
gaps and interpret the ABG
Case 11 – Oxygen Content
Which patient has better overall
oxygenation status?
Mr. Jones
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pH 7.48
PaCO2 34
PaO2 85
SaO2 95%
Hb 7 g/dl
Mr. Burton
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pH 7.32
PaCO2 74
PaO2 55
SaO2 85%
Hb 17 g/dl
Case 12 – O2-Hb curve
Which patient will have a higher SaO2?
Mrs. Roberts
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pH 7.04
PaCO2 100
PaO2 80
Hb 15 g/dl
Mrs. Nelson
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pH 7.50
PaCO2 30
PaO2 80
Hb 15 g/dl
Case 13 – Alveolar Air
Assuming PB is 760 mmHg, by the laws
of physics, which patient must be
breathing supplemental O2?
Patient A
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pH 7.04
PaCO2 100
PaO2 80
Hb 15 g/dl
Patient B
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pH 7.50
PaCO2 30
PaO2 80
Hb 15 g/dl
Case 14 - P/F ratio
A narcotic overdose patient is admitted to
the hospital (PB 730) with the following
blood gases while breathing room air:
• pH 7.16, PaCO2 80, HCO3 28,
PaO2 42, SaO2 63%, Base 0
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Calculations:
PAO2 47
a/A ratio 0.89
P/F ratio 200
Make a statement regarding this
patient’s oxygenation?
Case 15 - Carboxyhemoglobinemia
• A 30 year old man, previously healthy, is
brought to the ED after suffering smoke
inhalation. He is placed on a NRBM at 15 l/m.
COHb is measured at 32%, MetHb 1%, and
Hb is 15 g/dl. ABG’s are:
• pH 7.32, PaCO2 32, PaO2 380
All of the following statements are
true of this patient EXCEPT:
1. Conventional pulse oximetry will
be high
2. SaO2cal will be high
3. SaO2CO-ox will be roughly 65-67%
4. It is impossible for the PaO2 to
be 380
5. There is a metabolic acidosis
If we place this patient on 100% O2,
how long will it take for the COHb level
to drop to 4%?
Summary
• Always take in to account hydrolysis when
evaluating acid-base disorders
• There are rules that govern how pH, PaCO2, and
HCO3 change
• Anion and Bicarbonate gaps can be useful in
determining proper acid-base interpretation
• Use Winter’s formula to determine the patient’s
degree of respiratory compensation in metabolic
acidosis
Summary
• There’s a big difference between classification
and interpretation of blood gases
• If the base is normal and the pH and PaCO2 are
out of whack in opposite directions, you have an
acute respiratory alkalosis or acidosis in the vast
majority of cases
• There are two kinds of metabolic acidosis: High
anion gap and normal anion gap
• All acid-base values should calculate out
according to the H-H equation
Summary
• Hemoglobin and cardiac output are the keys to
tissue oxygenation
• The alveolar air equation has actual clinical
application
• The P/F ratio is a good index of shunting except
that it does not take into account changes in
PaCO2
• PaO2 can be normal or high in COHb and MetHb
• Oxygen saturations (SaO2calc , SaO2CO-ox , and
SpO2) are not created equal
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