Interpreting Blood Gases, Practical and easy approach

Interpreting ABGs

Practical Approach

Muhammad Asim RanaBSc, MBBS, MRCP, SF-CCM, FCCP, EDIC

Department of Adult Critical Care MedicineKSMC, Riyadh

Venous Arterial

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

Part 1

Acid-Base Disorders

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 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

Primary Acid-Base Disorders and Associated Compensatory Changes

[H+] = 24 × PCO2/HCO3

* 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---------------507.2---------------637.1---------------807.0--------------1006.9--------------1256.8--------------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 = 15.857.8-7.8 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 = 28.577.8-7.54 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: 6.8PaCO2: 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

• 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,

Bromism, lithium toxicity,

Profound hypocalcemia, hypomagnesemia

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

Metabolic

High AG

Anion Gap?Na = 140.4

Cl = 104

HCO3 = 3.7

AG = 140.4 - 104 - 3.7 = 32.3

Causes Of Anion Gap Acidosis

• 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.

Step 4: Is there other metabolic disturbances coexisting with AG Acidosis

• 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

Step 4: Is there other metabolic disturbances coexisting with AG Acidosis

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

• 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

• DKA

• Uremia

• Lactic acidosis

• Salisylates

↑AG acidosis and ↑osmolar gap

• Ethanol

• Methanol

• Ehylene Glycol

Causes of non-Anion Gap Acidosis

• 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 AcidosisExp PaCO2 = 1.5 x HCO3 + 8 ± 2

Metabolic AlkalosisExp 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.

• 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.

Mixed Disorders

Formulae for secondary responsesPredicting Timing by pH change

Acute Respiratory AcidosisFall in pH or Δ pH = 0.008 x ΔPaCO2

Expected pH = 7.40 – [0.008 x ( PaCO2 – 40)]Chronic Respiratory AcidosisFall in pH or Δ pH = 0.003 x ΔPaCO2

Expected pH = 7.40 – [0.003 x ( PaCO2 – 40)]Acute Respiratory AlkalosisRise in pH or ΔpH = 0.008 x ΔPaCO2

Expected pH = 7.40 + [0.008 x ( 40 - PaCO2 )]Chronic Respiratory AlkalosisRise in pH or Δ pH = 0.003 x ΔPaCO2

Expected pH = 7.40 + [0.003 x ( 40 - PaCO2 )]

Predicting Timing by response

Another way to cram compensatory responses

• 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)

• Hypokalemia

• Alkali ingestion

• Excess gluco-mineralocorticosteroids

• Bartter’s Syndrome

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

Acedmia

AG=141-111-3=27

Corrected HCO3=3 + (15) = 18

AdditionalNon AG acidosis

PaCO2 = 1.5 × 3 + 8 ± 2= 12.5 ± 2

Resp Acidosis

Metabolic

Status

Met/ Resp

Anion Gap Other disorder?

Compensation??

7.50/21.9/88.7/20.3/98.2%

Alkalemia

Acute Respiratory Alk

Exp HCO3=24+0.2x40-21.9= 27.6

Respiratory

Status

Met/ Resp

Acute/Chronic

Compensation??

Acute Respiratory AlkalosisRise in pH or ΔpH = 0.008 x ΔPaCO2

Expected pH = 7.40 + [0.008 x ( 40 - PaCO2 )]Chronic Respiratory AlkalosisRise 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)]

Acute Resp alkalosis with metabolic acidosis?Acute Resp alkalosis not yet compensated? PaCO2 = 0.7 x HCO3 + 21 ± 2

7.23/58/96/24

Acidosis

Respiratory

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

Δ PH=0.003 x 18 = 0.054

PH=7.326PH=7.236

ChronicAcute

Compensated or? Exp HCO3 = 24 + [0.1(PaCO2-40)]

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

Acute resp acidosis not yet fully compensated

7.35/48/69/29

Mixed Disorder

Respiratory

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

7.40 – 0.064 =

Δ PH= 0.003 x (48-40)=0.003 x 8 = 0.024

7.40 – 0.024 =

PH=7.37PH=7.336

ChronicAcute

Compensated or? Exp HCO3 = 24 + [0.4(PaCO2-40)]

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

Chronic compensated resp acidosisWith metabolic alkalosis

More Examples

7.27/87.4/83.5/40.1

• 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

– Expected HCO3 for acute 24 - 3.6 = 20.4 – Expected HCO3 for chronic 24 - 9= 15

Acute: HCO3 ↓by 2 for each 10 mmHg ↓ in CO2

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

Although the last calculation is not required !!

Thank you

See you in part 2

Oxygenation Status