Interpretation of arterial blood gases & compensation calculation

Interpretation of arterial blood gases &

compensation calculation

West China HospitalSichuan University

Nov. 19th, 2003

Oxygen cascadeDry atmospheric gas: 21 kPa

Humidified tracheal gas: 19.8 kPa Alveolar gas: 14 kPa

Arterial blood: 13.3 kPa

Capillary blood: 6-7 kPa Mitochondria: 1-5 kPa

Venous blood: 5.3 kPaClick for acid base physiology

+H20 CO2

+HCO3- H+H2CO

3

Acid-base

Normal [H+] = 40 nmol/lpH = - log [H+] = 7.4

+H20 CO2

+HCO3- H+H2CO

3

ALVEOLAR VENTILATION

Normal PaCO2 = 40 mmHg

+H20 CO2 +HCO3- H+H2CO3

ALVEOLAR VENTILATION

RENAL HCO3- HANDLING

Normal HCO3- = 22-27

mmol/l

Click here to continue tutorial

Interpretation of arterialblood gases

• Oxygenation• Ventilation• Acid base status

pH

PaCO2

PaO2

HCO3-

Base excess

Saturation

Interpretation of arterialblood gases

• Oxygenation• Ventilation• Acid base status

pH

PaCO2

PaO2

HCO3-

Base excessSaturation

Oxygenation

• What is the PaO2? - Partial O2 pressure

that is physically dissolved in plasma

pH

PaCO2

PaO2

HCO3-

Base excess

Saturation

Oxygen dissociation curve

50

75

100

Sat u

r atio

n %

PO2 kPa3.5 5.3 13.3

90

8.0

Oxygenation

• Normal PaO2 breathing air (FiO2 = 21%) is 12-13.3 kPa ; small reduction with age

• Lower values constitute hypoxaemia• PaO2 <6.7 kPa on room air = respiratory failure• PaO2 should go up with increasing FiO2

• PaO2 of 13.3 kPa breathing 60% O2 is not adequate

• You need to know the FiO2 to interpret the ABG

Oxygenation

Correlate ABG result with the SpO2

- reasons for discrepancy:• problem with the probe

(poor perfusion)• problem with the blood gas

(venous sample)

Oxygenation

• Is the PO2 is lower than expected?• Calculate the A-a gradient to assess

if the low PO2 is due to:• low alveolar PO2

• Structural lung problems causing failure of oxygen transfer

Oxygenation

PAO2 = [94.8 x FIO2] – [PaCO2 x 1.25]

The alveolar gas equation:

The alveolar-arterial oxygen difference

(A-a) PO2 = PAO2 - PaO2

Oxygenation

normally only a small gradient (1.33kPa). As CO2 accumulates in the alveolus due to HYPOVENTILATION there is less room for oxygen. If the lung is normal this oxygen can pass into blood as normal. If there are problems that limit oxygen diffusion the gradient will get bigger.

Acid base problems

Is there acidaemia or alkalaemia?

Normal pH = 7.35 – 7.45

Acidaemia < 7.35 Alkalaemia > 7.45

Acid base problems

Is the primary problem respiratory or metabolic?

Look at PaCO2

Normal PaCO2 = 5.3 kPa

Acid base problems

Is the primary problem respiratory or metabolic?

Look at [HCO3-]

Normal [HCO3-] = 24 mmol/l

Is there Is the PaCO2 Is the HCO3- It is

Acidaemia High( > 6 kPa)

Normal/high( 24 mmol/l)

Respiratory acidosis

Acidaemia Low Low Metabolic acidosis

Alkalaemia Low Normal/low Respiratory alkalosis

Alkalaemia High High Metabolic alkalosis

+H20 CO2

+HCO3- H+H2CO3


Acidaemia High Normal/high Respiratory acidosis

Acidaemia Low( < 4.5 kPa)

Low( 22 mmol/l)

Metabolic acidosis







Alkalaemia Low( < 4.5 kPa)

Normal/low( 23 mmol/l)

Respiratory alkalosis


+H20 CO2

+HCO3- H+H2CO3





Alkalaemia High( > 6 kPa)

High( 27 mmol/l)

Metabolic alkalosis


pH [HCO3- ]

Acute respiratory acidosis

Falls 0.06 Rises 0.8 mmol (up to 30 mmol/ l)

for every 1 kPa rise in PaCO2

Acute respiratory alkalosis

Rises 0.06 Falls 1.5 mmol (down to 18 mmol/ l)

for every 1 kPa fall in PaCO2

Chronic respiratory acidosis



Chronic respiratory alkalosis



For acute respiratory conditions

pH [HCO3- ]













Early renal compensation for respiratory conditions

pH [HCO3- ]













Late renal compensation for respiratory conditions

Respiratory acidosis• Central nervous system depression

– sedatives, CNS disease, obesity, hypoventilation syndrome

• Pleural Disease– pneumothorax

• Lung Disease – COPD, severe pneumonia, late stage ARDS

• Musculoskelatal disorders– kyphoscoliosis, Guillain-Barre, myasthenia

gravis, polio

Respiratory alkalosis• Catastrophic CNS event (CNS

hemorrhage) • Infection, fever • Pregnancy (especially the 3rd trimester) • Decreased lung compliance (interstitial

lung disease) • Liver cirrhosis • Anxiety

Metabolic acidosis

• Anion gap acidosis• Non anion gap acidosis • Any respiratory compensation?

Anion GapThe anion gap is an artificial

difference between the commonly unmeasured anions and cations.

In reality there is electrochemical neutrality

Anion GapCations AnionsNa+ HCO3

-

K+ Chloride-

Ca2+ Protein (albumin)Mg2+ Organic acids

PhosphatesSulphates

Anion Gapnormal anion gap = 12 mmol/l

[Na+] + [unmeasured cations] = [Cl-] + [HCO3-] +

[unmeasured anions]

[unmeasured anions] - [unmeasured cations] = [Na+] - ([Cl-] + [HCO3

-])

144 – ( 108 + 24 ) = 12 [Na+] – ( [Cl-] + [HCO3

-] ) = Anion Gap Na+

Anion gap acidosisAccumulation of organic acid not

normally present in serum (lactic acid, ketones), replace HCO3

-

Fall in [HCO3-] will widen AG

Link to causes of anion gap acidosis

Non anion gap acidosisLoss of HCO3

- (GI tract or renal) increase in [chloride], AG not

changeAdministration of exogenous

chloride, [HCO3-] falls without AG

change.

Link to causes of non-anion gap acidosis

Anion gap acidosis• Uremia • Ketoacidosis

– diabetic hyperglycemia, • Alcohol poisons or drug intoxication

– Methanol• Lactic acidosis

– Sepsis, left ventricular failure

Non-anion gap acidosis

Non-anion gap acidosis• GI loss of HCO3

- (diarrhea) • Renal loss of HCO3

- - Compensation for respiratory alkalosis - Carbonic anhydrase inhibitor - Renal tubular acidosis

• Other causes: HCl or NH4Cl infusion, Cl gas inhalation, hyperalimentation

Return to tutorial

Metabolic acidosis

• Respiratory compensation?• Occurs rapidly after pH change• Predictable for metabolic acidosis

by Winter’s formula• PaCO2 outside the predicted range

suggest additional respiratory disturbances

Winter’s formula

Expected PaCO2 = [ (1.5 x HCO3-) + (8 ± 2) ] x 0.133

Link to examples

Using Winter’s formula

A patient with a metabolic acidosis has a [HCO3-] of 10

mmol/l. By Winter’s formula expected PaCO2 should be 2.8 – 3.3 kPa

Expected PaCO2 = [ (1.5 x 10) + (8 ± 2) ] x 0.133

A value out side this range suggests an additional respiratory disturbance

Click to continue

Using Winter’s formula

• If the actual PaCO2 is less than 2.8 kPa there is also RESPIRATORY ALKALOSIS

A patient with a metabolic acidosis has a [HCO3-] of 10 mmol/l.

By Winter’s formula expected PaCO2 should be 2.8 – 3.3 kPa

Click to continue

Using Winter’s formulaA patient with a metabolic acidosis has a [HCO3

-] of 10 mmol/l.

By Winter’s formula expected PaCO2 should be 2.8 – 3.3 kPa

Click to continue

•If the actual PaCO2 is more than 3.3 kPa there is also RESPIRATORY ACIDOSIS

Metabolic alkalosis

• Respiratory compensation• Occurs rapidly after pH change• Not complete or easily predictable for

metabolic alkalosis • Rarely achieve PaCO2 > 7 kPa• A suggested formula:

Expected PaCO2 = 0.8 kPa per 10 mmol/l in HCO3-

Metabolic alkalosis

• Volume contraction (vomiting, overdiuresis, ascites)

• Hypokalemia • Alkali ingestion (bicarbonate)

Return to causes

Mixed disturbances

Difficult to interpret

Expected corrections

Respiratory compensation

For metabolic acidosis Winter’s formula:

For metabolic alkalosis:Expected PaCO2 = 0.8 kPa per 10 mmol/l in HCO3-

Expected PaCO2 = [ (1.5 x HCO3-) + (8 ± 2) ] x 0.133

Return to example

Expected correctionsPrimary change Compensatory

change

Respiratory acidosis Rise in PaCO2 Rise in [HCO3-] 1. pH change

consistent with PaCO2

2. Calculate expected rise in [HCO3

-]Respiratory alkalosis Fall in PaCO2 Fall in [HCO3

-] 1. pH change consistent with PaCO2

2. Calculate expected fall in [HCO3

-]Metabolic acidosis Fall in [HCO3

-] Fall in PaCO2 1. Winter’s formula for expected PaCO2

2. Corrected [HCO3-]

in anion-gap acidosisMetabolic alkalosis Rise in [HCO3

-] Rise in PaCO2 1. Difficult to predict, use suggested formula

If the correction is NOT as expected there is another disturbance.

Metabolic acidosis

Corrected [HCO3-] = measured [HCO3

-] + (anion gap – 12)

Correcting bicarbonate

Anion Gap = [Na+] – [Cl-] - [HCO3-]

Anion Gap

Return to example

Corrected bicarbonate

• Anion gap acidosis may co-exist non-anion gap acidosis or metabolic alkalosis

• Simple anion gap acidosis: widened gap is due to absent bicarbonate

Click to continue

Corrected bicarbonate

• An patient with metabolic acidosis • AG 26 mmol/l [HCO3

-] 10 mmol/l

• Corrected [HCO3-] = 24 mmol/l

Corrected [HCO3-] = 10 + (26 – 12)

• No other metabolic disturbance exists

Corrected [HCO3-] = measured [HCO3

-] + (anion gap – 12)

Click to continue

Corrected bicarbonate• An patient with metabolic acidosis AG 26 mmol/l HCO3

- 15 mmol/l

• The corrected [HCO3-] = 29 mmol/l

Corrected [HCO3-] = 15 + (26 – 12)

• There is extra bicarbonate in the system and a metabolic alkalosis co-exists

Return to expected corrections

Example 1.A 33 male patient with SARS has a saturation of 91% on Fi02 0.4

1. Is he hypoxic?2. Is there an acid base

or ventilation problem?

pH 7.43PaCO2 4.76PaO2 8.1HCO3

- 23Base excess

-0.6Saturation

90%

Click to continue

Example 1.1. Is he hypoxic?

YES.The SpO2 and calculated saturation agree


- 23Base excess

-0.6Saturation

90%

Click to continue

Example 1.1. Is he hypoxic?YES.(A-a) PO2 = 23.9 kPa There is major problem with oxygen transfer into the lung


- 23Base excess

-0.6Saturation

90%

To calculate (A-a) PO2 Click to continue

Example 1.2. Is there an acid base or ventilation problem?NO.pH, PaCO2 and PaCO2 are normalThis is pure respiratory failure

pH 7.43PaCO2 4.76PaO2 8.1PaCO2 23Base excess

-0.6Saturation

90%

Return to examples

Example 2.A patient with in the recovery room has been found to be cyanosed, with shallow breathing. This is the ABG result on room air.


- 26Base excess

+2Saturation

86%

Click to continue

Example 2.1. Is the patient hypoxic? 2. due simply to

hypoventilation as a result of residual anaesthetic agents

3. or have they also aspirated and developed lung parenchymal problems?


- 26Base excess

+3Saturation

86%

Click to continue

Example 2.Calculate the A-a gradient:

PAO2 = [94.8 x 0.21] – [10.6 x 1.25]= 6.65 kPa

(A-a) PO2 = 6.65 – 4.9= 1.75 kPa

This is a near normal A-a gradient, and hypoventilation alone can explain the hypoxaemia. Increased ventilation will improve hypercapnia and oxygenation too.


- 26Base excess

+3Saturation

86%

Click to continue

Example 2.4. Is there an acid base


YES.


- 26Base excess

+2Saturation

86%

Click to continue

Example 2.There is:• Acidosis• PaCO2 is elevated

RESPIRATORY ACIDOSIS


- 28Base excess

+2Saturation

86%

Click to continue

Example 2.There is:• HCO3

- = 28• Expected HCO3

- • = 24 + [(10.6 – 5.3) x 0.8] =

28.2This is the expected [HCO3

- ] if there has only been a small amount of renal compensation ACUTE RESPIRATORY ACIDOSIS


- 28Base excess

+2Saturation

86%

Click to continue

Example 2.There is:• pH change:

[10.6 – 5.3] x 0.06 = 0.32pH = [7.4 – 0.32] = 7.08

CONSISTENT WITH SIMPLE ACUTE RESPIRATORY ACIDOSIS;

NO ADDITIONAL DISTURBANCE


- 28Base excess

+2Saturation

86%

Return to examplesRenal compensation

Example 3.A patient has been brought to ER after a head injury; he is deeply unconscious. This is the ABG on room air.

Clearly he is very hypoxic


- 26Base excess

+3Saturation

86%

Click to continue

Example 3.1. Is the patient hypoxic

due simply because of hypoventilation as a result of CNS depression

1. or has he also aspirated and developed lung parenchymal problems?


- 26Base excess

+3Saturation

86%

Click to continue

Example 3.Calculate the A-a gradient:

PAO2 = [94.8 x 0.21] – [8.1 x 1.25]= 10.1 kPa

(A-a) PO2 = 10.1 – 4.9= 5.2 kPaThe A-a gradient is increased suggesting that less of the O2 available in the alveolus is able to get into the arterial blood. There is a lung problem; possibly aspiration


- 26Base excess

+3Saturation

86%

Click to continue

Example 3.


- 26Base excess

+3Saturation

86%

3. Is there an acid base or ventilation problem?

YES.

Click to continue

Example 3.


- 26Base excess

+3Saturation

86%

There is• Acidosis • PaCO2 is elevated


Click to continue

Example 3.


- 26Base excess

+3Saturation

86%

There is:• HCO3


- • = 24 + [(8.1 – 5.3) x 0.8] =


- ] if there has only been a small amount of renal compensation

ACUTE RESPIRATORY ACIDOSIS

Click to continue

Example 3.


- 26Base excess

+3Saturation

86%

There is:• pH change:

[8.1 – 5.3] x 0.06 = 0.17pH = [7.4 – 0.17] = 7.23

CONSISTENT WITH SIMPLE ACUTE RESPIRATORY ACIDOSIS; NO ADDITIONAL DISTURBANCE

Return to examples

Example 4.A patient with COPD has a ABG taken in out-patient clinic to assess his need for home oxygen. He is breathing room air.


- 32.1Base excess

+8Saturation

86%

Click to continueClick to continue

Example 4.


- 32.1Base excess

+8Saturation

86%

1. Is he hypoxic?YES.

The (A-a) PO2 = 2.4 kPa The (A-a) gradient is

increased, and home oxygen might be appropriate

Click to continue

Example 4.


- 32.1Base excess

+8Saturation

86%


YES.

Click to continue

Example 4.There is:• Mild acidosis• PaCO2 is elevated



- 32.1Base excess

+8Saturation

86%

Diagnose disturbance Click to continue

Example 4.There is:• HCO3

- = 32.1• Expected HCO3

- • = 24 + [(8.0 – 5.3) x 3.0] = 32.1

This is the expected [HCO3- ] if

there has been significant renal compensation over a long period; in addition the base excess has increased.

CHRONIC RESPIRATORY ACIDOSIS


- 32.1Base excess

+8Saturation

86%

Click to continue

Example 4.There is:• pH change:

[8.0 – 5.3] x 0.02 = 0.054pH = [7.4 – 0.054] =

7.346

CONSISTENT WITH SIMPLE CHRONIC RESPIRATORY ACIDOSIS; NO ADDITIONAL DISTURBANCE


- 32.1Base excess

+8Saturation

86%

Return to examples

Example 5.A 35 year old woman with a history of anxiety attacks presents to ER with alpitations.


- 22Base excess

+2Saturation

100%

Click to continue

Example 5.1. Is she hypoxic?

NO.

This is a normal PaO2 for room air


- 22Base excess

+2Saturation

100%

Click to continue

Example 5.2. Is there an acid

base or ventilation problem?

YES.


- 22Base excess

+2Saturation

100%

Click to continue

Example 5.There is:

• Alkalosis• PaCO2 is decreased

RESPIRATORY ALKALOSIS


- 22Base excess

+2Saturation

100%

Diagnose disturbance Click to continue

Example 5.


- 20Base excess

+2Saturation

100%

There is:• HCO3


- • = 24 - [(5.3 – 2.9) x 1.5] =


- ] if there has only been a small amount of renal compensation

ACUTE RESPIRATORY ALKALOSIS

Click to continue

Example 5.


- 22Base excess

+2Saturation

100%

There is:• pH change:

[5.3-2.9] x 0.06 = 0.144pH = [7.4 + 0.144] =

7.54

CONSISTENT WITH SIMPLE ACUTE RESPIRATORY ALKALOSIS; NO ADDITIONAL DISTURBANCE

Return to examples

Example 6.


- 12Base excess

-10Saturation

100%

A 42 year old diabetic woman present with UTI symptoms; she has deep sighing respiration. This is the ABG on FiO2 0.4

Click to continue

Example 6.


- 12Base excess

-10Saturation

100%

1. Is she hypoxic?

NO.

This PaO2 is adequate for an FiO2 of 0.4

Click to continue

Example 6.


- 12Base excess

-10Saturation

100%


YES.

Click to continue

Example 6.


- 12Base excess

-10Saturation

100%

There is:• Acidosis• PaCO2 is decreased NOT respiratory acidosis• Look at [HCO3

-]• [HCO3

-] is reduced• Base excess is negative

METABOLIC ACIDOSISClick to continue

Example 6.


- 12Base excess

-10Saturation

100%

Using Winter’s formula:Expected PaCO2 = [ (1.5 x 12) + (8 ± 2) ] x

0.133= 3.2 – 3.7 kPaThe PaCO2 falls within this

range SIMPLE METABOLIC

ACIDOSISClick to continue

Example 6.


- 12Base excess

-10Na+ 135Cl- 99

What is the anion gap?= [Na+] – ( [Cl-] + [HCO3

-] )

= [135] – ( 99 + 12 ) Na= 24 mmol/l

• There is an anion gap acidosis due to accumulation of organic acids caused by diabetic ketoacidosis

Click to continue

Example 6.


- 12Base excess

-10Na+ 135Cl- 99

Corrected bicarbonate = 24 mmol/l

The PaCO2 falls within the expected range

SIMPLE METABOLIC ACIDOSIS; NO OTHER DISTURBANCE

Return to examples

Example 7.A 70 year old man presents with a 3 day history of severe vomiting.Here is his ABG on room air.


- 38Base excess

+8Saturation

96%

Click to continue

Example 7.1.Is he hypoxic?

NO.

This is a normal PaO2 for a patient this age breathing room air


- 38Base excess

+8Saturation

96%

Click to continue

Example 7.2. Is there an acid base


YES.


- 38Base excess

+8Saturation

96%

Click to continue

Example 7.There is:• Alkalosis• PaCO2 is elevated NOT respiratory

alkalosis

Look at [HCO3-]

• [HCO3-] is increased

• Base excess is positive METABOLIC ALKALOSIS


- 38Base excess

+8Saturation

96%

Click to continue

Example 7.3. Is there respiratory

compensation?

Expected PaCO2

= 0.8 kPa per 10 mmol/l in HCO3

-

= 5.3 + (0.8 x ([ 38 – 24 ]/10))

= 6.4 CONSISTENT WITH SIMPLE

METABOLIC ALKALOSIS


- 38Base excess

+8Saturation

96%

Return to examples

Example 8.A 54 year old woman has multiple organ failure due to intra-abdominal sepsis. She has ARDS, renal failure. This is her ABG on FiO2 1.0


- 17.9Base excess

-6.5Saturation

95%

Click to continue

Example 8.1.Is she hypoxic?

YES. This PaO2 is very

low for an FiO2 of 1.0


- 17.9Base excess

-6.5Saturation

95%

Click to continue

Example 8.


YES.


- 17.9Base excess

-6.5Saturation

95%

Click to continue

Example 8.There is• Acidosis • PaCO2 is elevated



- 17.9Base excess

-6.5Saturation

95%

Click to continueDiagnose disturbance

Example 8.Expected pH= 7.4 – ([8.63-5.3] x 0.06)= 7.2 Observed pH is lowerExpected bicarbonate = 24 + ([8.63-5.3] x 0.8)= 26.7 mmol/l Observed bicarbonate is

too low


- 17.9Base excess

-6.5Saturation

95%

Click to continue

Example 8.Lower pHLower bicarbonateBase deficit negative ADDITIONAL METABOLIC

ACIDOSIS


- 17.9Base excess

-6.5Saturation

95%Severe ARDS leads to hypoxia & hypercapnia with respiratory acidosis; renal failure and poor perfusion lead to metabolic acidosis

Return to examples

Example 9.

A 43 year old man presents with an overdose of aspirin. This is his ABG on air.

pH 7.37PaCO2 2.3PaO2 12HCO3

- 10Base excess

-7.4Saturation

97%

Click to continue

Example 9.1. Is he hypoxic?

NO.

This is a normal PaO2 for a patient this age breathing room air


- 10Base excess

-7.4Saturation

97%

Click to continue

Example 9.


- 10Base excess

-7.4Saturation

97%


NO? Or is there?

Click to continue

Example 9.


- 10Base excess

-7.4Saturation

97%

PaCO2 is low• Respiratory alkalosis?• Metabolic acidosis?

HCO3- is lowNegative base deficit• Metabolic acidosis?

Click to continueDiagnose disturbance

Example 9.


- 10Base excess

-7.4Saturation

97%

Expected PaCO2 by Winter’s formula=2.8 – 3.3 kPa

Observed PaCO2 is out of this range

MIXED DISTURBANCE: RESPIRATORY ALKALOSIS

AND METABOLIC ACIDOSIS

Click to continueRespiratory compensation

Example 9.


- 10Base excess

-7.4Saturation

97%

Aspirin overdose characteristically causes a metabolic acidosis due the effect of salicylic acidand a respiratory alkalosis due to hyperventilation

Return to examples

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Interpretation of arterial blood gases & compensation calculation

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