This document provides a tutorial on interpreting arterial blood gases (ABGs). It discusses the key components of an ABG - pH, PaCO2, PaO2, HCO3-, base excess, and saturation. It explains how to assess for oxygenation, ventilation, and acid-base status issues. It details the causes and characteristics of respiratory and metabolic acidosis and alkalosis, including expected pH and HCO3- changes. Compensation mechanisms are also reviewed. The document is an in-depth resource for learning to interpret ABGs.

1. Interpretation of arterial
blood gases
Sarah Ramsay
Dept of Anaesthesia & Intensive Care
The Chinese University of Hong Kong
Prince of Wales Hospital
Modified by Charles Gomersall
Version 1.1
June 2004

2. Disclaimer
Although considerable care has
been taken in the preparation of
this tutorial, the author, the Prince
of Wales Hospital and The Chinese
University of Hong Kong take no
responsibility for any adverse event
resulting from its use.

3. Contents
Basic physiology
Interpretation of ABGs
Mixed disturbances
Examples

4. Acid-base
H20 + CO2 H2CO3 HCO3- + H+

5. H20 + CO2 H2CO3 HCO3- + H+
Normal [H+] = 40 nmol/l
pH = - log [H+] = 7.4

6. H20 + CO2 H2CO3 HCO3- + H+
Normal PaCO2 = 5.3 kPa

7. ALVEOLAR VENTILATION
H20 + CO2 H2CO3 HCO3- + H+
Normal PaCO2 = 5.3 kPa

8. Normal HCO3- = 22-26 mmol/l
H20 + CO2 H2CO3 HCO3- + H+

9. ALVEOLAR VENTILATION
Normal HCO3- = 22-26 mmol/l
H20 + CO2 H2CO3 HCO3- + H+
RENAL HCO3- HANDLING
Click here to continue tutorial

10. Interpretation of arterial
blood gases
pH
• Oxygenation
PaCO2
PaO2
• Ventilation
HCO3-
• Acid base status Base
excess
Saturation

11. Interpretation of arterial
blood gases
pH
• Oxygenation PaCO2
• Ventilation PaO 2
HCO3-
• Acid base status
Base
excess
Saturation

12. Interpretation of arterial
blood gases
pH
• Oxygenation PaCO2
• Ventilation
PaO2
HCO3-
• Acid base status
Base
excess
Saturation

13. Oxygenation
• What is the PaO2? pH
• Is this is adequate PaCO2
for the amount of PaO 2
inspired oxygen? HCO3-
• Does the ABG result Base
excess
agree with the Saturation
saturation probe?

14. 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
• A PaO2 of 13.3 kPa breathing 60% O2 is not
normal
• You need to know the FiO2 to interpret the ABG

15. Oxygenation
• Correlate the ABG result with the
saturation probe result
• If there is a discrepancy:
– Is there a problem with the probe (poor
perfusion? etc)
– Is there a problem with the blood gas (is
it a venous sample?)

16. 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

17. Oxygenation
The alveolar gas equation:
PAO2 = [94.8 x FIO2] – [PaCO2 x 1.25]
The alveolar-arterial oxygen difference
(A-a) PO2 = PAO2 - PaO2

18. Oxygenation
The alveolar gas equation:
PAO2 = [94.8 x FIO2] – [PaCO2 x 1.25]
The alveolar-arterial oxygen difference
(A-a) PO2 = PAO2 - PaO2

19. Oxygenation
The alveolar gas equation:
PAO2 = [94.8 x FIO2] – [PaCO2 x 1.25]
The alveolar-arterial oxygen difference
(A-a) PO2 = PAO2 - PaO2
In the normal state there is only a small gradient between the
alveolus and the arterial blood (1.33kPa). As CO2 accumulates in the
alveolus due to HYPOVENTILATION there is less room for oxygen. If
the lung is otherwise normal this oxygen can pass into blood as
normal. There just is not enough passing. If there are problems that
limit oxygen diffusion the gradient will get bigger.

20. Oxygenation
The alveolar gas equation:
PAO2 = [94.8 x FIO2] – [PaCO2 x 1.25]
The alveolar-arterial oxygen difference
(A-a) PO2 = PAO2 - PaO2
Continue tutorial
Examples

21. Acid base problems
Is there acidaemia or alkalaemia?
Normal pH = 7.38 – 7.42
Acidaemia < 7.38 Alkalaemia > 7.42

22. Acid base problems
Is the primary problem respiratory
or metabolic?
Look at the PaCO2
Normal PaCO2 = 5.3 kPa

23. Acid base problems
Is the primary problem respiratory
or metabolic?
Look at the [HCO3-]
Normal [HCO3-] = 24 mmol/l

24. Is there Is the PaCO2 Is the HCO3- It is
Acidaemia High Normal/high Respiratory
acidosis
Acidaemia Low Low Metabolic
acidosis
Alkalaemia Low Normal/low Respiratory
alkalosis
Alkalaemia High High Metabolic
alkalosis
Click to continue with tutorial

25. Is there Is the PaCO2 Is the HCO3- It is
Acidaemia High Normal/high Respiratory
( > 6 kPa) ( 24 mmol/l) acidosis
Acidaemia Low Low Metabolic
acidosis
Alkalaemia Low Normal/low Respiratory
alkalosis
Alkalaemia High High Metabolic
alkalosis
H20 + CO2 H2CO3 HCO3- + H+

26. Is there Is the PaCO2 Is the HCO3- It is
Acidaemia High Normal/high Respiratory
acidosis
Acidaemia Low Low Metabolic
( < 4.5 kPa) ( 23 mmol/l) acidosis
Alkalaemia Low Normal/low Respiratory
alkalosis
Alkalaemia High High Metabolic
alkalosis
H20 + CO2 H2CO3 HCO3- + H+

27. Is there Is the PaCO2 Is the HCO3- It is
Acidaemia High Normal/high Respiratory
acidosis
Acidaemia Low Low Metabolic
acidosis
Alkalaemia Low Normal/low Respiratory
( < 4.5 kPa) ( 23 mmol/l) alkalosis
Alkalaemia High High Metabolic
alkalosis
H20 + CO2 H2CO3 HCO3- + H+

28. Is there Is the PaCO2 Is the HCO3- It is
Acidaemia High Normal/high Respiratory
acidosis
Acidaemia Low Low Metabolic
acidosis
Alkalaemia Low Normal/low Respiratory
alkalosis
Alkalaemia High High Metabolic
( > 6 kPa) ( 24 mmol/l) alkalosis
H20 + CO2 H2CO3 HCO3- + H+

29. If there is a respiratory
problem…
• Is there an acidosis or an alkalosis?
• Is it acute or chronic?
• Is there renal compensation?
• Does the pH change as much as
expected?
• What is the bicarbonate?

30. pH and HCO3- changes
pH [HCO 3-]
Acute respiratory Falls 0.06 Rises 0.8 mmol for every 1 kPa rise
acidosis (up to 30 mmol/l) in PaCO 2
Acute respiratory Rises 0.06 Falls 1.5 mmol for every 1 kPa fall in
alkalosis (down to 18 mmol/l) PaCO 2
Chronic respiratory Falls 0.02 Rises 3.0 mmol for every 1 kPa rise
acidosis (up to 36 mmol/l) in PaCO 2
Chronic respiratory Rises 0.02 Falls 3.8 mmol for every 1 kPa fall in
alkalosis (down to 18 mmol/l) PaCO 2

31. For acute respiratory
conditions
pH [HCO 3-]
Acute respiratory Falls 0.06 Rises 0.8 mmol for every 1 kPa rise
acidosis (up to 30 mmol/l) in PaCO 2
Acute respiratory Rises 0.06 Falls 1.5 mmol for every 1 kPa fall in
alkalosis (down to 18 mmol/l) PaCO 2
Chronic respiratory Falls 0.02 Rises 3.0 mmol for every 1 kPa rise
acidosis (up to 36 mmol/l) in PaCO 2
Chronic respiratory Rises 0.02 Falls 3.8 mmol for every 1 kPa fall in
alkalosis (down to 18 mmol/l) PaCO 2

32. Early renal compensation
for respiratory conditions
pH [HCO 3-]
Acute respiratory Falls 0.06 Rises 0.8 mmol for every 1 kPa rise
acidosis (up to 30 mmol/l) in PaCO 2
Acute respiratory Rises 0.06 Falls 1.5 mmol for every 1 kPa fall in
alkalosis (down to 18 mmol/l) PaCO 2
Chronic respiratory Falls 0.02 Rises 3.0 mmol for every 1 kPa rise
acidosis (up to 36 mmol/l) in PaCO 2
Chronic respiratory Rises 0.02 Falls 3.8 mmol for every 1 kPa fall in
alkalosis (down to 18 mmol/l) PaCO 2

33. pH [HCO 3-]
Acute respiratory Falls 0.06 Rises 0.8 mmol for every 1 kPa rise
acidosis (up to 30 mmol/l) in PaCO 2
Acute respiratory Rises 0.06 Falls 1.5 mmol for every 1 kPa fall in
alkalosis (down to 18 mmol/l) PaCO 2
Chronic respiratory Falls 0.02 Rises 3.0 mmol for every 1 kPa rise
acidosis (up to 36 mmol/l) in PaCO 2
Chronic respiratory Rises 0.02 Falls 3.8 mmol for every 1 kPa fall in
alkalosis (down to 18 mmol/l) PaCO 2
Late renal compensation
for respiratory conditions

34. pH and HCO3- changes
pH [HCO 3-]
Acute respiratory Falls 0.06 Rises 0.8 mmol for every 1 kPa rise
acidosis (up to 30 mmol/l) in PaCO 2
Acute respiratory Rises 0.06 Falls 1.5 mmol for every 1 kPa fall in
alkalosis (down to 18 mmol/l) PaCO 2
Chronic respiratory Falls 0.02 Rises 3.0 mmol for every 1 kPa rise
acidosis (up to 36 mmol/l) in PaCO 2
Chronic respiratory Rises 0.02 Falls 3.8 mmol for every 1 kPa fall in
alkalosis (down to 18 mmol/l) PaCO 2
Take time to review the table then click to continue

35. Causes of respiratory disturbances
RESPIRATORY ACIDOSIS
RESPIRATORY ALKALOSIS
Click to return to tutorial

36. Respiratory acidosis
Brainstem
Spinal cord
Airway Nerve root
Lung Nerve
Pleura
Neuromuscular
Chest wall junction
Respiratory
muscle

37. Respiratory acidosis
Brainstem
Spinal cord
Airway Nerve root
Lung Nerve
Pleura
Neuromuscular
Chest wall junction
Respiratory
muscle

38. Respiratory acidosis
Brainstem
Spinal cord
Airway Nerve root
Lung Nerve
Pleura
Neuromuscular
Chest wall junction
Respiratory
muscle

39. Respiratory acidosis
Brainstem
Spinal cord
Airway Nerve root
Lung Nerve
Pleura
Neuromuscular
Chest wall junction
Respiratory
muscle

40. Respiratory acidosis
Brainstem
Spinal cord
Airway Nerve root
Lung Nerve
Pleura
Neuromuscular
Chest wall junction
Respiratory
muscle

41. Respiratory acidosis
Brainstem
Spinal cord
Airway Nerve root
Lung Nerve
Pleura
Neuromuscular
Chest wall junction
Respiratory
muscle

42. Respiratory acidosis
Brainstem
Spinal cord
Airway Nerve root
Lung Nerve
Pleura
Neuromuscular
Chest wall junction
Respiratory
muscle

43. Respiratory acidosis
Brainstem
Spinal cord
Airway Nerve root
Lung Nerve
Pleura
Neuromuscular
Chest wall junction
Respiratory
muscle

44. Respiratory acidosis
Brainstem
Spinal cord
Airway Nerve root
Lung Nerve
Pleura
Neuromuscular
Chest wall junction
Respiratory
muscle

45. Respiratory acidosis
Brainstem
Spinal cord
Airway Nerve root
Lung Nerve
Pleura
Neuromuscular
Chest wall junction
Respiratory
muscle

46. Respiratory alkalosis
• Catastrophic CNS event (CNS
haemorrhage)
• Drugs (salicylates, progesterone)
• Pregnancy (especially the 3rd trimester)
• Decreased lung compliance (interstitial
lung disease)
• Liver cirrhosis
• Anxiety
Click to return to causes

47. If there is a metabolic
problem…
• Is it an acidosis or an alkalosis?
• What can we find out about a
metabolic acidosis?
• Information about base excess/deficit
• Is there respiratory compensation?
• Is there anything else going on?

48. Metabolic acidosis
What is the anion gap?
What is the base excess/deficit
Is there any respiratory
compensation?
Click to return to tutorial

49. Anion Gap
Anion Gap = [Na+] – [Cl-] - [HCO3-]
• The anion gap is an artificial
difference between the commonly
measured anions and cations.
• In reality there is electrochemical
neutrality
[Na+] + [unmeasured cations] = [Cl-] + [HCO3-] + [unmeasured anions]
[unmeasured anions] - [unmeasured cations] = [Na+] - ([Cl-] + [HCO3-])

50. Anion Gap
Cations Anions
Na+ HCO3-
K+ Chloride-
Ca2+ Protein (albumin)
Mg2+ Organic acids
Phosphates
Sulphates
[unmeasured anions] - [unmeasured cations] = [Na+] - ( [Cl-] + [HCO3-] )
Anion Gap = [Na+] – ( [Cl-] + [HCO3-] )

51. Anion Gap
normal anion gap = 12 mmol/l
[Na+] – ( [Cl-] + [HCO3-] ) = Anion Gap Na+
144 – ( 108 + 24 ) = 12

52. Anion gap acidosis
If there is accumulation of an
organic acid not normally present in
serum (eg lactic acid, ketones etc)
these will replace HCO3-
A fall in [HCO3-] will widen the anion
gap
Link to causes of anion gap acidosis
Continue with tutorial

53. Normal anion gap acidosis
If there is loss of HCO3- (GI tract or
renal) there will be an increase in
[chloride] and the anion gap will not
change
If there is administration of
exogenous chloride, [HCO3-] will fall
but the anion gap will not change.
Link to causes of non-anion gap acidosis

54. Anion gap acidosis
• Lactic acidosis
– shock
– severe hypoxaemia
– generalized convulsions
– severe sepsis
• Ketoacidosis
– diabetic, alcoholic
• Alcohol poisons or drug intoxication
– methanol, ethylene glycol, paraldehyde, salicylates
• Renal failure (late stage)
Non-anion gap acidosis

55. Normal anion gap acidosis
• GI loss of HCO3-
– Diarrhoea
– Pancreatic/biliary drainage
– Urinary diversion
• Renal loss of HCO3-
- Compensation for respiratory alkalosis
- Renal tubular acidosis
- Renal hypoperfusion
- Carbonic anhydrase inhibitor (acetazolamide)
• Other causes: HCl or NH4Cl infusion, Cl gas
inhalation
Return to tutorial

56. Base excess / deficit
• Amount or acid or base that needs to be added
to 1 litre of blood to return pH to normal,
assuming standard conditions (temp 37oC,
PaCO2 = 5.3 kPa, pressure 1 atm)
– A measure of the metabolic component of a
disturbance
– Normal: -2 to +2 mmol/l
– BE is POSITIVE in metabolic alkalosis (or compensation
for a respiratory acidosis)
– BE is NEGATIVE in metabolic acidosis (or compensation
for a respiratory alkalosis)
– Useful to follow the TREND to assess treatment.
Return to tutorial

57. Metabolic acidosis
• Is there any respiratory
compensation?
– Occurs rapidly after the change in pH
– Predictable for metabolic acidosis by
Winter’s formula
– PaCO2 outside the predicted range
suggest additional respiratory
disturbances

58. Winter’s formula:
Expected PaCO2 = [ (1.5 x HCO3-) + (8 ± 2) ] x 0.133
Link to examples

59. Using Winter’s formula:
A patient with a metabolic acidosis has a [HCO3-] of 10 mmol/l.
Expected PaCO2 = [ (1.5 x HCO3-) + (8 ± 2) ] x 0.133
= [ (1.5 x 10) + (8 ± 2) ] x 0.133
= 2.8 – 3.3
A value outside this range suggests an additional respiratory disturbance
Click to continue

60. Using Winter’s formula:
A patient with a metabolic acidosis has a [HCO3-] of 10 mmol/l.
Expected PaCO2 = [ (1.5 x HCO3-) + (8 ± 2) ] x 0.133
= [ (1.5 x 10) + (8 ± 2) ] x 0.133
= 2.8 – 3.3
A value out side this range suggests an additional respiratory disturbance
• If the actual PaCO2 is
less than 2.8 kPa there
is also RESPIRATORY
ALKALOSIS
Click to continue

61. Using Winter’s formula:
A patient with a metabolic acidosis has a [HCO3-] of 10 mmol/l.
Expected PaCO2 = [ (1.5 x HCO3-) + (8 ± 2) ] x 0.133
= [ (1.5 x 10) + (8 ± 2) ] x 0.133
= 2.8 – 3.3
A value out side this range suggests an additional respiratory disturbance
• If the actual PaCO2 is
more than 3.3 kPa there
is also RESPIRATORY
ACIDOSIS
Return to tutorial

62. Metabolic alkalosis
• Is there respiratory compensation?
– Occurs rapidly after the change in pH
– 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-

63. Causes of metabolic disturbances
METABOLIC ACIDOSIS
METABOLIC ALKALOSIS
Click to continue tutorial

64. Anion gap acidosis
• Lactic acidosis
– shock
– severe hypoxaemia
– generalized convulsions
– severe sepsis
• Ketoacidosis
– diabetic, alcoholic
• Alcohol poisons or drug intoxication
– methanol, ethylene glycol, paraldehyde, salicylates
• Renal failure (late stage)
Non-anion gap acidosis

65. Normal anion gap acidosis
• GI loss of HCO3-
– Diarrhoea
– Pancreatic/biliary drainage
– Urinary diversion
• Renal loss of HCO3-
- Compensation for respiratory alkalosis
- Renal tubular acidosis
- Renal hypoperfusion
- Carbonic anhydrase inhibitor (acetazolamide)
• Other causes: HCl or NH4Cl infusion, Cl gas
inhalation
Return to causes

66. Metabolic alkalosis
• Volume contraction (vomiting,
overdiuresis, ascites)
• Hypokalemia
• Alkali ingestion (bicarbonate)
• Excess gluco- or mineralocorticoids
• Bartter's syndrome
Return to causes

67. Mixed disturbances
These are difficult to interpret
Expected corrections
Acid base nomogram
Examples

68. Primary 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 acidosis
Metabolic 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.

69. pH and HCO3- changes
pH [HCO 3-]
Acute respiratory Falls 0.06 Rises 0.8 mmol for every 1 kPa rise
acidosis (up to 30 mmol/l) in PaCO 2
Acute respiratory Rises 0.06 Falls 1.5 mmol for every 1 kPa fall in
alkalosis (down to 18 mmol/l) PaCO 2
Chronic respiratory Falls 0.02 Rises 3.0 mmol for every 1 kPa rise
acidosis (up to 36 mmol/l) in PaCO 2
Chronic respiratory Rises 0.02 Falls 3.8 mmol for every 1 kPa fall in
alkalosis (down to 18 mmol/l) PaCO 2
Return to expected corrections

70. Expected corrections
Primary 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 acidosis
Metabolic 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.

71. Expected corrections
Primary 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 acidosis
Metabolic 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.

72. Respiratory
compensation
• 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-
Return to expected corrections

73. Winter’s formula:
Expected PaCO2 = [ (1.5 x HCO3-) + (8 ± 2) ] x 0.133
Click to continue

74. Using Winter’s formula:
A patient with a metabolic acidosis has a [HCO3-] of 10 mmol/l.
Expected PaCO2 = [ (1.5 x HCO3-) + (8 ± 2) ] x 0.133
= [ (1.5 x 10) + (8 ± 2) ] x 0.133
= 2.8 – 3.3
A value out side this range suggests an additional respiratory disturbance
Click to continue

75. Using Winter’s formula:
A patient with a metabolic acidosis has a [HCO3-] of 10 mmol/l.
Expected PaCO2 = [ (1.5 x HCO3-) + (8 ± 2) ] x 0.133
= [ (1.5 x 10) + (8 ± 2) ] x 0.133
= 2.8 – 3.3
A value out side this range suggests an additional respiratory disturbance
• If the actual PaCO2 is
less than 2.8 kPa there
is also RESPIRATORY
ALKALOSIS
Click to continue

76. Using Winter’s formula:
A patient with a metabolic acidosis has a [HCO3-] of 10 mmol/l.
Expected PaCO2 = [ (1.5 x HCO3-) + (8 ± 2) ] x 0.133
= [ (1.5 x 10) + (8 ± 2) ] x 0.133
= 2.8 – 3.3
A value out side this range suggests an additional respiratory disturbance
• If the actual PaCO2 is
more than 3.3 kPa there
is also RESPIRATORY
ACIDOSIS
Click to continue

77. Corrected bicarbonate
• An anion gap acidosis may co-exist
with an non-anion gap acidosis or a
metabolic alkalosis.
• In a simple anion gap acidosis the
widened gap is due to absent
bicarbonate – adding the actual and
the missing bicarbonate adds up to
a normal value for bicarbonate
Click to continue

78. Corrected bicarbonate
Corrected [HCO3-] = measured [HCO3-] + (anion gap – 12)
• An patient with metabolic acidosis
and an anion gap of 26 mmol/l has
a serum [HCO3-] of 10 mmol/l.
• The corrected [HCO3-] = 24 mmol/l
Corrected [HCO3-] = 10 + (26 – 12)
• No other metabolic disturbance
exists
Click to continue

79. Corrected bicarbonate
Corrected [HCO3-] = measured [HCO3-] + (anion gap – 12)
• An patient with metabolic acidosis
and an anion gap of 26 mmol/l has
a serum [HCO3-] of 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

80. 7.0
100
6 9 12 15 18 21 24
90 27
HCO3 -(mmol/l)
80 30 7.1
33
70
36 7.2
60 39
42
H+ (nmol/l)
45
50 7.3
48
51
40 N 57 7.4
63
69
30 74 7.5
7.6
20 7.7
7.8
10 8.0
8.5
0
PCO2 (kPa)
Use two parameters to see if the result falls into expected values

81. Click for examples.
Example 1. Example 6.
Example 2. Example 7.
Example 3. Example 8.
Example 4. Example 9.
Example 5. Example 10.

82. Example 1.
A 33 male patient
with SARS has a pH 7.43
saturation of 91% PaCO2 4.76
on Fi02 0.4 PaO2 8.1
HCO3- 23
1. Is he hypoxic? Base -0.6
excess
2. Is there an acid Saturation 90%
base or ventilation
problem?
Click to continue

83. Example 1.
Is he hypoxic?
pH 7.43
PaCO2 4.76
YES. PaO2 8.1
The SpO2 and HCO3- 23
calculated Base
excess
-0.6
saturation agree Saturation 90%
Click to continue

84. Example 1.
Is he hypoxic?
YES.
pH 7.43
PaCO2 4.76
(A-a) PO2 = 23.9 kPa PaO2 8.1
HCO3- 23
There is major Base -0.6
problem with oxygen excess
transfer into the lung Saturation 90%
To calculate (A-a) PO2 Click to continue

85. Example 1.
Is there an acid base
or ventilation pH 7.43
problem? PaCO2 4.76
NO. PaO2 8.1
pH, PaCO2 and PaCO2
PaCO2 23
are normal
Base -0.6
excess
This is pure Saturation 90%
hypoxaemic
respiratory failure
Return to examples

86. Example 2.
A patient with in the
recovery room has pH 7.08
been found to be PaCO2 10.6
cyanosed, with PaO2 4.9
shallow breathing. HCO3- 26
This is the ABG result Base +2
on room air. excess
Saturation 86%
Click to continue

87. Example 2.
Is the patient hypoxic
due simply because of pH 7.08
hypoventilation as a
PaCO2 10.6
result of residual
PaO2 4.9
anaesthetic agents or
have they also aspirated HCO3- 26
and developed lung Base +3
excess
parenchymal problems?
Saturation 86%
Click to continue

88. Example 2.
Calculate the A-a gradient:
pH 7.08
PAO2 = [94.8 x 0.21] – [10.6 x 1.25]
= 6.65 kPa PaCO2 10.6
PaO2 4.9
(A-a) PO2 = 6.65 – 4.9 HCO3- 26
= 1.75 kPa
Base +3
excess
This is a near normal A-a gradient,
Saturation 86%
and hypoventilation alone can
explain the hypoxaemia. Increased
ventilation will improve
hypercapnia and oxygenation too.
To calculate (A-a) PO2 Click to continue

89. Example 2.
Is there an acid
base or pH 7.08
ventilation PaCO2 10.6
problem? PaO2 4.9
HCO3- 26
Base +2
excess
YES. Saturation 86%
Click to continue

90. Example 2.
There is:
• Acidosis pH 7.08
• PaCO2 is elevated
PaCO2 10.6
PaO2 4.9
RESPIRATORY ACIDOSIS HCO3- 28
Base +2
excess
Saturation 86%
Diagnose disturbance Click to continue

91. Example 2.
There is:
• HCO3- = 28 pH 7.08
• Expected HCO3-
PaCO2 10.6
• = 24 + [(10.6 – 5.3) x 0.8] = 28.2 PaO2 4.9
This is the expected [HCO3- ] if HCO3- 28
there has only been a small
Base +2
amount of renal compensation excess
Saturation 86%
ACUTE RESPIRATORY ACIDOSIS
Click to continue

92. Example 2.
There is:
• pH change: pH 7.08
[10.6 – 5.3] x 0.06 = 0.32
PaCO2 10.6
pH = [7.4 – 0.32] = 7.08 PaO2 4.9
HCO3- 28
CONSISTENT WITH SIMPLE Base +2
ACUTE RESPIRATORY ACIDOSIS; excess
NO ADDITIONAL DISTURBANCE Saturation 86%
Renal compensation Return to examples

93. Example 3.
A patient has been
brought to A&E after pH 7.23
a head injury; he is PaCO2 8.1
deeply unconscious. PaO2 4.9
This is the ABG on HCO3- 26
room air. Base +3
excess
Saturation 86%
Clearly he is very
hypoxic
Click to continue

94. Example 3.
Is the patient hypoxic
due simply because pH 7.23
of hypoventilation as PaCO2 8.1
a result of CNS PaO2 4.9
depression or have HCO3- 26
they also aspirated Base +3
and developed lung excess
parenchymal Saturation 86%
problems?
Click to continue

95. Example 3.
Calculate the A-a gradient:
pH 7.23
PAO2 = [94.8 x 0.21] – [8.1 x 1.25]
= 10.1 kPa PaCO2 8.1
PaO2 4.9
(A-a) PO2 = 10.1 – 4.9 HCO3- 26
= 5.2 kPa
Base +3
excess
The A-a gradient is increased
Saturation 86%
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
To calculate (A-a) PO2 Click to continue

96. Example 3.
Is there an acid
base or pH 7.23
ventilation PaCO2 8.1
problem? PaO2 4.9
HCO3- 26
Base +3
excess
YES. Saturation 86%
Click to continue

97. Example 3.
There is
• Acidosis pH 7.23
• PaO2 is elevated PaCO2 8.1
RESPIRATORY
PaO2 4.9
ACIDOSIS
HCO3- 26
Base +3
excess
Saturation 86%
Diagnose disturbance Click to continue

98. Example 3.
There is:
• HCO3- = 26 pH 7.23
• Expected HCO3-
PaCO2 8.1
• = 24 + [(8.1 – 5.3) x 0.8] = 26.2 PaO2 4.9
This is the expected [HCO3- ] if HCO3- 26
there has only been a small
Base +3
amount of renal compensation excess
Saturation 86%
ACUTE RESPIRATORY ACIDOSIS
Click to continue

99. Example 3.
There is:
• pH change: pH 7.23
[8.1 – 5.3] x 0.06 = 0.32
PaCO2 8.1
pH = [7.4 – 0.17] = 7.23 PaO2 4.9
HCO3- 26
CONSISTENT WITH SIMPLE Base +3
ACUTE RESPIRATORY ACIDOSIS; excess
NO ADDITIONAL DISTURBANCE Saturation 86%
Renal compensation Return to examples

100. Example 4.
A patient with
COPD has a ABG pH 7.34
taken in out- PaCO2 8.0
patient clinic to PaO2 7.5
HCO3- 32.1
assess his need Base +8
for home oxygen. excess
He is breathing
Saturation 86%
room air.
Click to continue

101. Example 4.
Is he hypoxic?
YES. pH 7.34
PaCO2 8.0
The (A-a) PO2 = 2.4 kPa PaO2 7.5
The (A-a) gradient is HCO3- 32.1
increased, and home Base +8
excess
oxygen might be Saturation 86%
appropriate
To calculate (A-a) PO2 Click to continue

102. Example 4.
Is there an acid base
or ventilation pH 7.34
problem? PaCO2 8.0
PaO2 7.5
HCO3- 32.1
YES. Base
excess
+8
Saturation 86%
Click to continue

103. Example 4.
There is:
• Mild acidosis pH 7.34
• PaCO2 is elevated
PaCO2 8.0
PaO2 7.5
RESPIRATORY ACIDOSIS HCO3- 32.1
Base +8
excess
Saturation 86%
Diagnose disturbance Click to continue

104. Example 4.
There is:
• HCO3- = 32.1 pH 7.34
• Expected HCO3-
PaCO2 8.0
• = 24 + [(8.0 – 5.3) x 3.0] = 33.9 PaO2 7.5
This is the expected [HCO3- ] if HCO3- 32.1
there has been significant renal
Base +8
compensation over a long period; excess
in addition the base excess has Saturation 86%
increased.
CHRONIC RESPIRATORY
ACIDOSIS
Click to continue

105. Example 4.
There is:
• pH change: pH 7.34
[8.0 – 5.3] x 0.02 = 0.054
PaCO2 8.0
pH = [7.4 – 0.054] = 7.35 PaO2 7.5
HCO3- 32.1
CONSISTENT WITH SIMPLE Base +8
CHRONIC RESPIRATORY ACIDOSIS; excess
NO ADDITIONAL DISTURBANCE Saturation 86%
Renal compensation Return to examples

106. Example 5.
A 35 year old
woman with a pH 7.54
history of anxiety PaCO2 2.9
attacks presents PaO2 12.1
HCO3- 22
to A&E with Base +2
palpitations. excess
Saturation 100%
Click to continue

107. Example 5.
Is she hypoxic?
pH 7.54
PaCO2 2.9
NO. PaO2 12.1
This is a normal HCO3- 22
PaO2 for room air Base
excess
+2
Saturation 100%
Click to continue

108. Example 5.
Is there an acid
base or pH 7.54
ventilation PaCO2 2.9
problem? PaO2 12.1
HCO3- 22
Base +2
excess
YES. Saturation 100%
Click to continue

109. Example 5.
There is:
• Alkalosis pH 7.54
• PaCO2 is decreased PaCO2 2.9
PaO2 12.1
RESPIRATORY ALKALOSIS HCO3- 22
Base +2
excess
Saturation 100%
Diagnose disturbance Click to continue

110. Example 5.
There is:
• HCO3- = 20
pH 7.54
• Expected HCO3-
PaCO2 2.9
• = 24 - [(5.3 – 2.9) x 1.5] = 20.4
This is the expected [HCO3- ] if PaO2 12.1
there has only been a small HCO3- 20
amount of renal Base +2
excess
compensation
Saturation 100%
ACUTE RESPIRATORY
ALKALOSIS
Click to continue

111. Example 5.
There is:
• pH change: pH 7.54
[5.3-2.9] x 0.06 = 0.144
PaCO2 2.9
pH = [7.4 + 0.144] = 7.54 PaO2 12.1
HCO3- 22
CONSISTENT WITH SIMPLE Base +2
ACUTE RESPIRATORY ALKALOSIS; excess
NO ADDITIONAL DISTURBANCE Saturation 100%
Renal compensation Return to examples

112. Example 6.
A 42 year old
diabetic woman pH 7.23
present with UTI PaCO2 3.3
PaO2 29.9
symptoms; she HCO3- 12
has deep sighing Base -10
respiration. This is excess
Saturation 100%
the ABG on FiO2
0.4
Click to continue

113. Example 6.
Is she hypoxic?
pH 7.23
PaCO2 3.3
NO. PaO2 29.9
This PaO2 is HCO3- 12
adequate for an
Base -10
excess
FiO2 of 0.4 Saturation 100%
Click to continue

114. Example 6.
Is there an acid
base or ventilation pH 7.23
problem? PaCO2 3.3
PaO2 29.9
HCO3- 12
YES. Base
excess
-10
Saturation 100%
Click to continue

115. Example 6.
There is:
• Acidosis pH 7.23
• PaCO2 is decreased PaCO2 3.3
• NOT respiratory acidosis PaO2 29.9
HCO3- 12
Look at [HCO3-] Base -10
excess
• [HCO3-] is reduced
Saturation 100%
• Base excess is negative
METABOLIC ACIDOSIS
Diagnose disturbance Click to continue

116. Example 6.
Using Winter’s formula:
Expected PaCO2 pH 7.23
= [ (1.5 x 12) + (8 ± 2) ] x 0.133 PaCO2 3.3
PaO2 29.9
= 3.2 – 3.7 kPa
HCO3- 12
The PaCO2 falls within this range Base -10
excess
SIMPLE METABOLIC ACIDOSIS
Saturation 100%
Respiratory compensation Click to continue

117. Example 6.
What is the anion gap?
= [Na+] – ( [Cl-] + [HCO3-] ) pH 7.23
= [135] – ( 99 + 12 ) Na PaCO2 3.3
PaO2 29.9
= 24 mmol/l
HCO3- 12
Base -10
• There is an anion gap excess
acidosis due to
Na+ 135
accumulation of organic Cl- 99
acids caused by diabetic
ketoacidosis
Click to continue

118. Example 6.
Corrected bicarbonate
= 24 mmol/l pH 7.23
PaCO2 3.3
The PaCO2 falls within the PaO2 29.9
expected range HCO3- 12
Base -10
excess
SIMPLE METABOLIC ACIDOSIS; Na+ 135
NO OTHER DISTURBANCE
Cl- 99
More on metabolic acidosis Return to examples

119. Example 7.
A 70 year old man
presents with a 3 pH 7.5
day history of PaCO2 6.2
severe vomiting. PaO2 10.6
HCO3- 38
Here is his ABG on Base +8
room air.
excess
Saturation 96%
Click to continue

120. Example 7.
Is he hypoxic?
pH 7.5
PaCO2 6.2
NO. This is a PaO2 10.6
normal PaO2 for a HCO3- 38
patient this age Base
excess
+8
breathing room air Saturation 96%
Click to continue

121. Example 7.
Is there an acid
base or ventilation pH 7.5
problem? PaCO2 6.2
PaO2 10.6
HCO3- 38
YES. Base
excess
+8
Saturation 96%
Click to continue

122. Example 7.
There is:
• Alkalosis pH 7.5
• PaCO2 is elevated
PaCO2 6.2
• NOT respiratory PaO2 10.6
alkalosis
HCO3- 38
Base +8
Look at [HCO3-] excess
• [HCO3-] is increased Saturation 96%
• Base excess is positive
METABOLIC ALKALOSIS
Diagnose disturbance Click to continue

123. Example 7.
Is there respiratory
compensation? pH 7.5
PaCO2 6.3
Expected PaCO2 PaO2 10.6
= 0.8 kPa per 10 mmol/l in HCO3- 38
HCO3- Base +8
excess
= 5.3 + (0.8 x ([ 38 – 24 ]/10)) Saturation 96%
= 6.4
CONSISTENT WITH SIMPLE
METABOLIC ALKALOSIS
Respiratory compensation Return to examples

124. Example 8.
A 54 year old woman
has multiple organ pH 7.07
failure due to intra- PaCO2 8.63
abdominal sepsis. She PaO2 11.8
has ARDS, renal HCO3- 17.9
failure and requires Base
excess
-6.5
inotropic support. Saturation 95%
This is her ABG on
FiO2 1.0
Click to continue

125. Example 8.
Is she hypoxic?
pH 7.07
PaCO2 8.63
YES. This PaO2 is PaO2 11.8
very low for an HCO3- 17.9
FiO2 of 1.0 Base
excess
-6.5
Saturation 95%
Click to continue

126. Example 8.
Is there an acid
base or pH 7.07
ventilation PaCO2 8.63
problem?
PaO2 11.8
HCO3- 17.9
Base -6.5
excess
YES. Saturation 95%
Click to continue

127. Example 8.
There is
• Acidosis pH 7.07
• PaO2 is elevated PaCO2 8.63
RESPIRATORY
PaO2 11.8
ACIDOSIS
HCO3- 17.9
Base -6.5
excess
Saturation 95%
Diagnose disturbance Click to continue

128. Example 8.
Expected pH
= 7.4 – ([8.63-5.3] x 0.03) pH 7.07
= 7.2 PaCO2 8.63
Observed pH is lower
PaO2 11.8
HCO3- 17.9
Expected bicarbonate Base -6.5
= 24 + ([8.63-5.3] x 0.8) excess
Saturation 95%
= 26.7 mmol/l
Observed bicarbonate is
too low
Renal compensation Click to continue

129. Example 8.
Lower pH
Lower bicarbonate pH 7.07
Base deficit negative PaCO2 8.63
ADDITIONAL METABOLIC PaO2 11.8
ACIDOSIS
HCO3- 17.9
Base -6.5
excess
Severe ARDS leads to
Saturation 95%
hypoxia & hypercapnia
with respiratory
acidosis; renal failure
and poor perfusion lead
to metabolic acidosis
Return to examples

130. Example 9.
A 77 year old man
presents with a 3 pH 7.23
day history of PaCO2 3.3
severe diarrhoea. PaO2 10.6
HCO3- 8
Here is his ABG on Base -10
room air.
excess
Saturation 96%
Click to continue

131. Example 9.
Is he hypoxic?
pH
pH 7.5
7.23
PPCO2 2
a aCO 6.2
3.3
NO. This is a PPO2 2
a aO 10.6
10.6
normal PaO2 for a HCO3- -
HCO3 38
8
patient this age Base
Base
excess
excess
+8
-10
breathing room air Saturation 96%
Saturation 96%
Click to continue

132. Example 9.
Is there an acid
base or ventilation pH 7.23
problem? PaCO2 3.3
PaO2 10.6
HCO3- 8
YES. Base
excess
-10
Saturation 96%
Click to continue

133. Example 9.
There is:
• Acidosis pH 7.23
• PaCO2 is decreased PaCO2 3.3
• NOT respiratory acidosis PaO2 10.6
29.9
HCO3- 8
12
Look at [HCO3-] Base -10
excess
• [HCO3-] is reduced
Saturation 96%
100%
METABOLIC ACIDOSIS
Diagnose disturbance Click to continue

134. Example 9.
Using Winter’s formula:
Expected PaCO2 pH 7.23
= [ (1.5 x 12) + (8 ± 2) ] x 0.133 PaCO2 3.3
PaO2 10.6
= 3.2 – 3.7 kPa
HCO3- 8
The PaCO2 falls within this range Base -10
excess
SIMPLE METABOLIC ACIDOSIS
Saturation 96%
Respiratory compensation Click to continue

135. Example 9.
What is the anion gap?
= [Na+] – ( [Cl-] + [HCO3-] ) pH 7.23
= [135] – ( 115 + 8 ) Na PaCO2 3.3
PaO2 10.6
= 12 mmol/l
HCO3- 8
Base -10
• There is an non-anion excess
gap acidosis due to loss
Na+ 135
of bicarbonate in Cl- 115
diarrhoea. Dehydration
concentrates [Cl-]
Return to examples

136. Example 10.
A 43 year old man
presents with an pH 7.37
overdose of PaCO2 2.3
aspirin. This is his PaO2 12
HCO3- 10
ABG on air. Base -7.4
excess
Saturation 97%
Click to continue

137. Example 10.
Is he hypoxic?
pH 7.37
PaCO2 2.3
NO. This is a PaO2 12
normal PaO2 for a HCO3- 10
patient this age Base
excess
-7.4
breathing room Saturation 97%
air
Click to continue

138. Example 10.
Is there an acid
base or ventilation pH 7.37
problem? PaCO2 2.3
PaO2 12
HCO3- 10
NO. Or is there? Base
excess
-7.4
Saturation 97%
Click to continue

139. Example 10.
PaCO2 is low
• Respiratory alkalosis? pH 7.37
• Metabolic acidosis? PaCO2 2.3
HCO3- is low PaO2 12
HCO3- 10
Negative base deficit Base -7.4
• Metabolic acidosis? excess
Saturation 97%
Diagnose disturbance Click to continue

140. Example 10.
Expected PaCO2 by Winter’s
formula pH 7.37
=2.8 – 3.3 kPa PaCO2 2.3
Observed PaCO2 is out of PaO2 12
this range HCO3- 10
Base -7.4
MIXED DISTURBANCE:
excess
Saturation 97%
RESPIRATORY ALKALOSIS
AND
METABOLIC ACIDOSIS
Respiratory compensation Click to continue

141. Example 10.
Aspirin overdose
characteristically causes a
pH 7.37
metabolic acidosis due
the effect of salicylic acid PaCO2 2.3
and a respiratory alkalosis PaO2 12
due to hyperventilation HCO3- 10
Base -7.4
excess
Saturation 97%
Return to examples

142. A patient with in the
recovery room has pH 7.08
been found to be PaCO2 10.6
cyanosed, with PaO2 4.9
shallow breathing. HCO3- 26
This is the ABG result Base +3
on room air. excess
Saturation 86%
Click to continue

143. Is the patient hypoxic
due simply because pH 7.08
of hypoventilation as PaCO2 10.6
a result of residual PaO2 4.9
anaesthetic agents or HCO3- 26
have they also Base +3
aspirated and excess
developed lung Saturation 86%
parenchymal
problems?
Click to continue

144. Calculate the A-a gradient:
pH 7.08
PAO2 = [94.8 x 0.21] – [10.6 x 1.25]
= 6.65 kPa PaCO2 10.6
PaO2 4.9
(A-a) PO2 = 6.65 – 4.9 HCO3- 26
= 1.75 kPa
Base +3
excess
This is a near normal A-a gradient,
Saturation 86%
and hypoventilation alone can
explain the hypoxaemia. Increased
ventilation will improve
hypercapnia and oxygenation too.
Click to continue

145. A patient has been
brought to A&E after pH 7.23
a head injury; they PaCO2 8.1
are deeply PaO2 4.9
unconscious. This is HCO3- 26
the ABG on room air Base +3
excess
Saturation 86%
Click to continue

146. Is the patient hypoxic
due simply because pH 7.23
of hypoventilation as PaCO2 8.1
a result of CNS PaO2 4.9
depression or have HCO3- 26
they also aspirated Base +3
and developed lung excess
parenchymal Saturation 86%
problems?
Click to continue

147. Calculate the A-a gradient:
pH 7.23
PAO2 = [94.8 x 0.21] – [8.1 x 1.25]
= 10.1 kPa PaCO2 8.1
PaO2 4.9
(A-a) PO2 = 10.1 – 4.9 HCO3- 26
= 5.2 kPa
Base +3
excess
The A-a gradient is increased
Saturation 86%
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
Return to tutorial

148. Oxygenation
The alveolar gas equation:
PAO2 = [94.8 x FIO2] – [PaCO2 x 1.25]
The alveolar-arterial oxygen difference
(A-a) PO2 = PAO2 - PaO2
As CO2 accumulates in the alveolus due to HYPOVENTILATION there
is less room for oxygen. If the lung is otherwise normal this oxygen
can pass into blood as normal. There just is not enough passing. In
the normal state there is only a small gradient between the alveolus
and the arterial blood (1.33kPa). If there are problems that limit
oxygen diffusion the gradient will get bigger.
Return to example

149. Oxygenation
The alveolar gas equation:
PAO2 = [94.8 x FIO2] – [PaCO2 x 1.25]
The alveolar-arterial oxygen difference
(A-a) PO2 = PAO2 - PaO2
As CO2 accumulates in the alveolus due to HYPOVENTILATION there
is less room for oxygen. If the lung is otherwise normal this oxygen
can pass into blood as normal. There just is not enough passing. In
the normal state there is only a small gradient between the alveolus
and the arterial blood (1.33kPa). If there are problems that limit
oxygen diffusion the gradient will get bigger.
Return to example

150. Respiratory
compensation
For metabolic acidosis Winter’s formula:
Expected PaCO2 = [ (1.5 x HCO3-) + (8 ± 2) ] x 0.133
For metabolic alkalosis:
Expected PaCO2 = 0.8 kPa per 10 mmol/l in HCO3-
Return to example

151. Metabolic acidosis
Anion Gap
Anion Gap = [Na+] – [Cl-] - [HCO3-]
Correcting bicarbonate
Corrected [HCO3-] = measured [HCO3-] + (anion gap – 12)
Return to example

152. pH and HCO3- changes
pH [HCO 3-]
Acute respiratory Falls 0.06 Rises 0.8 mmol for every 1 kPa rise
acidosis (up to 30 mmol/l) in PaCO 2
Acute respiratory Rises 0.06 Falls 1.5 mmol for every 1 kPa fall in
alkalosis (down to 18 mmol/l) PaCO 2
Chronic respiratory Falls 0.02 Rises 3.0 mmol for every 1 kPa rise
acidosis (up to 36 mmol/l) in PaCO 2
Chronic respiratory Rises 0.02 Falls 3.8 mmol for every 1 kPa fall in
alkalosis (down to 18 mmol/l) PaCO 2
Return to example

153. Is there Is the PaCO2 Is the HCO3- It is
Acidosis High Normal/high Respiratory
acidosis
Acidosis Low Low Metabolic
acidosis
Alkalosis Low Normal/low Respiratory
alkalosis
Alkalosis High High Metabolic
alkalosis
Return to example

154. Oxygen dissociation curve
100
50
3.5 13.3 PO2 kPa

155. Oxygen dissociation curve
100
75
50
3.5 5.3 13.3 PO2 kPa

156. Oxygen dissociation curve
100
88
75
50
3.5 5.3 6.7 13.3 PO2 kPa

157. Oxygen cascade
Dry 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
Click for acid base
physiology Venous blood: 5.3 kPa