Interpreting Blood Gases, Practical and easy approach

Interpreting
ABGs
Practical Approach
Muhammad Asim Rana
BSc, MBBS, MRCP, SF-CCM, FCCP, EDIC
Department of Adult Critical Care Medicine
KSMC, Riyadh

Arterial Blood Gases
• Written in following manner:

pH/PaCO2/PaO2/HCO3
– pH = arterial blood pH
– PaCO2 = arterial pressure of CO2
– PaO2 = arterial pressure of O2
– HCO3 = serum bicarbonate concentration

Acid-Base
• Acidosis or alkalosis:
– any disorder that causes an alteration in pH

• Acidemia or alkalemia:
– alteration in blood pH; may be result of one or
more disorders.

Some important concepts
• The determinants of extracellular fluid pH
indicate that tight control of the pH requires a
fairly constant PCO2/HCO3 ratio.
• Thus, a change in one of the determinants
(PCO2 or HCO3) must be accompanied by a
proportional change in the other determinant
to keep the PCO2/HCO3 ratio (and the pH)
constant.

Some important concepts
• Thus, an increase in PCO2 (respiratory
acidosis) must be accompanied by an increase
in HCO3 (metabolic alkalosis) to keep the pH
constant.
• This is how the control system for acid-base
balance operates.
• A respiratory disorder (change in PCO2) always
initiates a complementary metabolic response
(that alters the HCO3), and vice-versa

Primary Acid-Base Disorders and Associated Compensatory Changes

[H+] = 24 × PCO2/HCO3
Primary Disorder

Primary Change

Compensatory Change*

Respiratory acidosis

Increased PCO2

Increased HCO3

Respiratory alkalosis

Decreased PCO2

Decreased HCO3

Metabolic acidosis

Decreased HCO3

Decreased PCO2

Metabolic alkalosis

Increased HCO3

Increased PCO2

* Compensatory changes keep the PCO2/HCO3 ratio constant.

Check if data is consistent
{H} = 24 [ PaCO2/HCO3]
{H} = (7.8 – pH) x 100
Each 0.01 unit change in pH {H} will change by 1mEq/L
{H} = 40+(delta pH) (1mEq/L)/0.01
pH-------------- {H}
7.3---------------50
7.2---------------63
7.1---------------80
7.0--------------100
6.9--------------125
6.8--------------160

Check if data is consistent
{H} = 24 [PaCO2/HCO3]
{H} = (7.8 – pH) x 100
• The {H} in extracellular fluid normally varies less than
10 nEq/L
• The values of {H} should be within 10 for both
calculations !
• If it is beyond or more than 10 the blood gas analysis
is not interpretable.
• The reasons may include improper caliberation or
others

Here are some examples
• Written in following manner:

pH/PaCO2/PaO2/HCO3
• 7.8/36.6/76.4/55.4
• 7.7/35.5/80.3/50.6
• 7.54/53.1/63.7/44.6

24 x 36.6/55.4 = 15.85
7.8-7.8 x 100 = 0
The data is inconsistent

24 x 35.5/50.6 = 16.8
7.8-7.7 x 100 = 10
The data is inconsistent

24x53.1/44.6 = 28.57
7.8-7.54 x 100 = 26
The data is consistent

Case Study
• A 13 years old female presented in ER with
pain abdomen and drowsiness.
• Blood gas revealed
• 6.87/20.6/88/3.7
• Na 140.4, K 4.41, Cl 102

Step-wise Approach
1. Acedemia or Alkalemia

2. Metabolic or Respiratory (Primary Pathology)
3. For metabolic is it anion gap or non anion gap.
4. For AG acidosis, are there other disturbances.
5. Resp compensation for the metabolic disturbances.
6. For respiratory disturbances is it acute or chronic.

Step 1: Acidemic or Alkalemic?
• Acidemic : PH < 7.35
• Alkalemic: PH > 7.45

An acid-base abnormality is present if either the PaCO2 or the
pH is outside the normal range.
(A normal pH or PaCO2 does not exclude the presence of an
acid-base abnormality)

Type of disturbance
pH 6.87
Acidemia

Primary Acid-Base Disorders
• A change in either the PCO2 or the HCO3 will cause a
change in the [H+] of extracellular fluid.
• When a change in PCO2 is responsible for a change in
[H+], the condition is called a respiratory acid-base
disorder
– an increase in PCO2 is a respiratory acidosis
– a decrease in PCO2 is a respiratory alkalosis.

• When a change in HCO3 is responsible for a change in
[H+], the condition is called a metabolic acid-base
disorder
– a decrease in HCO3 is a metabolic acidosis
– an increase in HCO3 is a metabolic alkalosis.

Step 2: Primary disturbance metabolic or Respiratory
If the pH and PaCO2 are both abnormal, compare the
directional change. If both change in the same
direction (both increase or decrease), the primary
acid-base disorder is metabolic, and if both change in
opposite directions, the primary acid-base disorder is
respiratory.
If either the pH or PaCO2 is normal, there is a mixed metabolic
and respiratory acid-base disorder (one is an acidosis and the
other is an alkalosis).
• If the pH is normal, the direction of change in PaCO2
identifies the respiratory disorder
• If the PaCO2 is normal, the direction of change in the pH
identifies the metabolic disorder.

Type of disturbance
pH 6.87
Acidemia
Metabolic

pH: 6.8
PaCO2: 14.5

Step 3 : What is the Anion Gap
• Anion gap measures the difference between
Anions(-) and Cations(+) present in blood
• AG = Na – (Cl + HCO3)
• Normal Anion gap is 12 mEq/L

Anion Gap
Unmeasured Anions

Unmeasured Cations

Proteins 15 mEq

Calcium 5 mEq

Organic acid 5 mEq

Potassium 4.5 mEq

Phosphate 2 mEq

Magnesium 1.5 mEq

Sulfates 1 mEq
Total 23 mEq

Total 11 mEq

Difference : 23 – 11 = 12

Extra for the experts
•
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Albumin carries negative charge .
Hypo-albuminemia causes falsely low AG.
To correct for that
AG adjusted = AG Observed + 0.25 × (4.5 – pt’s alb)

Other causes of low AG
Paraproteinemia,
Profound hypocalcemia,
Bromism,
hypomagnesemia
lithium toxicity,
hyponatremia

Extra for the experts
• In metabolic alkalosis AG can be high but it
could be due to unmeasured anions,
specifically the albumin.

Type of disturbance
pH 7.10
Acidemia

Anion Gap?
Na = 140.4
Cl = 104
HCO3 = 3.7

Metabolic
AG = 140.4 - 104 - 3.7 = 32.3
High AG

Causes Of Anion Gap Acidosis
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Methanol
Uremia
DKA
Paraldehyde
INH
Iron
Lactic Acidosis
Ethanol
Ethylene Glycol
Salicylic Acid

MUDPILES

Step 4: Is there other metabolic disturbances
coexisting with AG Acidosis
• In the presence of high AG metabolic acidosis, it is
possible that patient may have another metabolic
acid base disorder.
• A normal AG metabolic acidosis or a metabolic
alkalosis
• This can be discovered by comparing the AG excess
to the HCO3 deficit.

• Delta Anion Gap or ΔAG:
• Difference between measured and normal AG
– ΔAG = AG - 12

• Delta HCO3 or ΔHCO3:
– Difference between measured and normal HCO3
– ΔHCO3 = 24 – Measured HCO3
Delta Anion Gap or ΔAG is sometimes simply called Δgap

• If the disturbance is pure AG Acidosis
• Δ AG/Δ HCO3 = unity or 1

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In our example
HCO3= 3.7 so
ΔHCO3 = 24 – 3.7 = 20.3
Now Δ AG
AG = 32.3 so
Δ AG = 32.3 – 12 = 20.3
Δ AG /ΔHCO3 = 20.3/20.3 = 1.0
So this patient has pure high AG metabolic acidosis

Remember !
• If Δ AG /ΔHCO3 < 1.0
• The decrease in the HCO3 is greater than the
increase in the AG and the ratio falls below 1
• It means there is accumulation of other acid
which does not affect the AG but causes a fall
in HCO3 i.e. NON-AG Metabolic Acidosis

Remember !
• If Δ AG /ΔHCO3 > 1.0
• When alkali is added in the presence of high
AG acidosis, the decrease in serum HCO3 is
less than the increase in the AG and the ratio
goes above 1
• Therefore, in the presence of high AG
metabolic acidosis a gap-gap ratio of greater
than 1 indicates co-existence of
– metabolic alkalosis

Concept of corrected HCO3
• Add rgap to measured HCO3
– If new value becomes normal (22-26)
– There is no other metabolic problems
– If it still stays < 22, then there is concomitant
metabolic acidosis, non AG metabolic acidosis
– If it goes > 26, then there is concomitant
metabolic alkalosis

Revision
rgap + HCO3 = N (Only one disorder i.e.↑AG Met Acid)
rgap + HCO3 = >N (↑AG Met Acid + Meta Alk)

rgap + HCO3 = <N (↑AG Met Acid + Nor AG Meta Acid)

Let us apply on our case
• Corrected HCO3 = HCO3 + Δ AG
• Corrected HCO3 = 3.7 + 20.3 = 24
• Perfect !!

In ↑AG metabolic acidosis
Extend your search further
To pin point the diagnosis

In case of high AG acidosis
• Always calculate Osmolar gap:
• Osm gap = measured Osm – Calc Osm
• Calc Osm =
(2 x Na+) + (glucose/18) + (BUN/2.8)

Normal Osm gap < 10 mOsm/kg
• In areas where alcohol is common
• Calc Osm =
(2 x Na+) + (glucose/18) +(BUN/2.8) + (EtOH/4.6)

In case of high AG acidosis
↑AG acidosis but N osmolar gap

↑AG acidosis and ↑osmolar gap

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• Ethanol
• Methanol
• Ehylene Glycol

DKA
Uremia
Lactic acidosis
Salisylates

Causes of non-Anion Gap Acidosis
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Hyper Alimentation
Acetazolamide
Renal Tubular Acidosis
Diarrhea
Ureterosigmoidostomy
Pancreatic Fistula
Primary Hyperparathyroidism

HARD-UP

Compensatory responses
• Compensatory responses are secondary
responses designed to limit the change in [H+]
produced by the primary acid-base disorder,
and this is accomplished by changing the
other component of the PaCO2/HCO3 ratio in
the same direction.

Secondary Responses
• If the primary problem is an increase in
PaCO2 (respiratory acidosis)
– The secondary response will involve an increase in
HCO3, and this will limit the change in [H+]
produced by the increase in PaCO2.

• Secondary responses should not be called
“compensatory responses” because they do
not completely correct the change in [H+]
produced by the primary acid-base disorder

Secondary/Compensatory responses

• If there is a primary metabolic acidosis or
alkalosis, use the measured HCO3 to identify the
expected PaCO2.
• If the measured and expected PaCO2 are
equivalent, the condition is fully compensated.
• If the measured PaCO2 is higher than the
expected PaCO2, there is a superimposed
respiratory acidosis.
• If the measured PCO2 is less than the expected
PCO2, there is a superimposed respiratory
alkalosis.
Metabolic Acidosis
Exp PaCO2 = 1.5 x HCO3 + 8 ± 2

Metabolic Alkalosis
Exp PaCO2 = 0.7 x HCO3 + 21 ± 2

Let’s see our case
pH 7.10
Acidemia
Metabolic
High AG

Compensated or ????
Winter’s Formula :
Expected PaCO2 = (1.5 x HCO3) + 8 ± 2

Applying Winter’s Formula :
Expected PaCO2 = (1.5 x 3.7) + 8 ± 2 = 13.5-15.5

So in our case it is :
Metabolic acidemia is compensated

Mixed Disorders
• If either the pH or PaCO2 is normal, there is a
mixed metabolic and respiratory acid-base
disorder
– (one is an acidosis and the other is an alkalosis).

• If the pH is normal, the direction of change in
PaCO2 identifies the respiratory disorder, and
if the PaCO2 is normal, the direction of change
in the pH identifies the metabolic disorder.

Mixed Disorders
• If there is a respiratory acidosis or alkalosis,
use the PaCO2 to calculate the expected pH
for respiratory acidosis or for respiratory
alkalosis.
• Compare the measured pH to the expected pH
to determine if the condition is acute, partially
compensated, or fully compensated.

Mixed Disorders
• For respiratory acidosis
– If the measured pH is lower than the expected pH
for the acute, uncompensated condition, there is a
superimposed metabolic acidosis
– If the measured pH is higher than the expected pH
for the chronic, compensated condition, there is a
superimposed metabolic alkalosis.

Mixed Disorders
• For respiratory alkalosis
– If the measured pH is higher than the expected pH
for the acute, uncompensated condition, there is a
superimposed metabolic alkalosis
– If the measured pH is below the expected pH for
the chronic, compensated condition, there is a
superimposed metabolic acidosis.

Formulae for secondary responses
Predicting Timing by pH change
Acute Respiratory Acidosis
Fall in pH or Δ pH = 0.008 x ΔPaCO2
Expected pH = 7.40 – [0.008 x ( PaCO2 – 40)]
Chronic Respiratory Acidosis
Fall in pH or Δ pH = 0.003 x ΔPaCO2
Expected pH = 7.40 – [0.003 x ( PaCO2 – 40)]
Acute Respiratory Alkalosis
Rise in pH or ΔpH = 0.008 x ΔPaCO2
Expected pH = 7.40 + [0.008 x ( 40 - PaCO2 )]
Chronic Respiratory Alkalosis
Rise in pH or Δ pH = 0.003 x ΔPaCO2
Expected pH = 7.40 + [0.003 x ( 40 - PaCO2 )]

Another way to cram compensatory responses
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Metabolic Acidosis HCO3 ↓----------------PaCO2 ↓
PaCO2 ↓ by 1.3 for each 1 mEq ↓in HCO3
Metabolic Alkalosis ↑in HCO3------------------ PaCO2 ↑
PaCO2 ↑ by 0.7 for each 1 mEq ↑in HCO3
Acute Respiratory Acidosis ↑in PaCO2---- -HCO3 ↑
HCO3 ↑ by 1 mEq for each 10 mmHg ↑in PaCO2
Acute Respiratory Alkalosis ↓in PaCO2----- HCO3 ↓
HCO3 ↓ by 2 mEq for each 10 mmHg ↓in PaCO2
Chronic Respiratory Acidosis ↑in PaCO2 ---HCO3 ↑
HCO3 ↑ by 3.5 mEq for each 10 mmHg ↑in PaCO2
Chronic Respiratory Alkalosis ↓in PaCO2---HCO3 ↓
HCO3 ↓ by 5mEq for each 10 mmHg ↓in PaCO2

Causes Of metabolic Alkalosis
• Volume contraction
– (Vomiting, diuresis, ascities)

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Hypokalemia
Alkali ingestion
Excess gluco-mineralocorticosteroids
Bartter’s Syndrome

Let’s Solve
PH 7.02/PaCO219/HCO32.8, Na 141, Cl 111
Status

Acedmia

Met/ Resp

Metabolic

Anion Gap

AG=141-111-3=27

Other disorder?

Compensation??

Corrected HCO3=
3 + (15) = 18

Additional
Non AG acidosis

PaCO2 = 1.5 × 3 + 8 ± 2
= 12.5 ± 2

Resp Acidosis

7.50/21.9/88.7/20.3/98.2%
Status

Alkalemia

Met/ Resp

Respiratory

Acute/Chronic

Acute Respiratory Alk

Compensation??

Exp HCO3=
24+0.2x40-21.9= 27.6

Acute Resp alkalosis with metabolic acidosis?
Acute Resp alkalosis not yet compensated?

Acute Respiratory Alkalosis
Rise in pH or ΔpH
= 0.008 x ΔPaCO2
Expected pH =
7.40 + [0.008 x ( 40 - PaCO2 )]
Chronic Respiratory Alkalosis
Rise in pH or Δ pH =
0.003 x 40 – ΔPaCO2
Expected pH =
7.40 + [0.003 x ( 40 - PaCO2 )]

ΔHCO3 = 0.2 x ΔPaCO2
Exp HCO3 = 24 + [ 0.2 x (40 – PaCO2)]
PaCO2 = 0.7 x HCO3 + 21 ± 2

7.23/58/96/24
Acidosis

Chronic

Respiratory

Acute

Δ PH= 0.008 x (58-40)
=0.08 x 1.8 = 0.144

Δ PH=0.003 x 18 = 0.054

PH=7.236
Compensated or?

Exp HCO3 = 24 + [0.1(PaCO2-40)]
Exp HCO3 = 24 + [0.1(58-40)]
Answer = 25.8

PH=7.326

Acute resp acidosis
not yet fully compensated

7.35/48/69/29
Mixed Disorder

Acute

Respiratory

Δ PH= 0.008 x (48-40)
=0.008 x 8 = 0.064
7.40 – 0.064 =

PH=7.336

Compensated or?

Exp HCO3 = 24 + [0.4(PaCO2-40)]
Exp HCO3 = 24 + [0.4(48-40)]
Answer = 27.2

Chronic
Δ PH= 0.003 x (48-40)
=0.003 x 8 = 0.024
7.40 – 0.024 =
PH=7.37

Chronic compensated resp acidosis
With metabolic alkalosis

7.27/87.4/83.5/40.1
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•

Acidemia
Respiratory
Acute or Chronic ?
ΔpH:
–
–
–
–

Acute: 0.008 x ΔPaCO2 = 0.008 x 47 = 0.379
Expected pH = 7.40 – 0.379 = 7.021
Chronic: 0.003 x ΔPaCO2 = 0.003 x 47 = 0.142
Expected pH = 7.40 – 0.142 = 7.258

• Chronic Respiratory Acidosis
• Compensation:
– 3.5 x 47 / 10 = 16.59
– Expected HCO3 = 24 + 16.59 = 40.59

For each 10 mmHg CO2 rise
HCO3 rises by 3.5

7.24/62/58/22
•
•
•
•

Acidemia
Primary …. Respiratory acidosis as PaCO2↑
Acute or Chronic ?
ΔpH:
–
–
–
–

Acute: 0.008 x ΔPaCO2 = 0.008 x 22= 0.176
Expected pH = 7.40 - 0.176 = 7.224
Chronic: 0.003 x ΔPaCO2 = 0.003 x 22 = 0.066
Expected pH = 7.40 + 0.066 = 7.334

• So it is Acute Respiratory Acidosis
• Compensation for acute resp acidosis
– Expected ↓ in HCO3 = 22/10 = 2.2
– Expected HCO3 = 24 – 2.2 = 21.8

HCO3 will fall by 1 with each
10 mmHg rise in CO2

7.365/22/110/12.3
• Mixed Disorder ….. pH (N) and CO2 ↓
• Respiratory alkalosis as PaCO2↓
• ΔpH:
–
–
–
–

Acute: 0.008 x ΔPaCO2 = 0.008 x 18 = 0.114
Expected pH = 7.40 + 0.114 = 7.514
Chronic: 0.003 x ΔPaCO2 = 0.003 x 18 = 0.054
Expected pH = 7.40 + 0.054 = 7.454

• In both cases the pH should be higher than what we have !!
• So there is concomitant metabolic acidosis !!
• Expected HCO3 for respiratory alkalosis Acute: HCO3 ↓by 2 for each
– Expected HCO3 for acute 24 - 3.6 = 20.4
– Expected HCO3 for chronic 24 - 9= 15

Although the last calculation is not required !!

10 mmHg ↓ in CO2
Chronic: HCO3 ↓by 5 for
each 10 mmHg ↓ in CO2

Thank you
See you in part 2
Oxygenation Status

Interpreting Blood Gases, Practical and easy approach

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