1. Interpretation of Arterial Blood Gases
James Wigfull
Critical Care and Anaesthesia

2. What do you get? pH pO2 (FiO2) pCO2 Bicarbonate Base Excess Sodium
Potassium
Chloride
Calcium
Lactate
Haemoglobin
COHb/MetHb
SaO2
Strong Ion Diff
Strong Ion Gap
Anion Gap

3. What do you get? pH pO2 (FiO2) pCO2 Bicarbonate Base Excess Sodium
Potassium
Chloride
Calcium
Lactate
Haemoglobin
COHb/MetHb
SaO2
Strong Ion Diff
Strong Ion Gap
Anion Gap

4. Oxygen content

5. Variable performance device30
Flow
In a variable performance device the oxygen concentration the patient receives will depend on the pattern of ventilation. In this example, where the subject is receiving oxygen at 6l/min, the inspired oxygen concentration is 100% while the inspiratory flow rate is <6 l/min as the inspiratory requirement can be met entirely by the fresh gas flow through the mask 6
Time

6. Hydrogen ion homoeostasis
Extracellular
H+ = 40nmol/l
Intracellular
Varies in different subcellular compartments (7.17 to 6.69)
Mean pH varies between cell types
Skeletal muscle 7.07
Proximal renal tubule 7.13

7. pH is a logarithmic scale
pH [H+] nmol/l

8. Importance of [H+]
H+ ions have a greater affinity for negatively charged proteins than Na+ or K+
Any change to [H+] causes net loss or gain of H+ ions bound to proteins
Because H+ ions are substantially smaller than Na+ or K+ such changes affect the charge distribution and subsequently tertiary protein structure

9. Importance of [H+] Example: Glycolysis
Enzyme function is inversely proportional to [H+] BUT different enzymes have different sensitivities to pH changes
Acidosis inhibits glycolysis overall but the aerobic pathway is inhibited more than the anaerobic pathway
Lactate rises proportionately to [H+].

10. Strong vs Weak
Strong ions & acids are those that completely dissociate in water
Weak ions and acids are those that incompletely dissociate in water

11. Buffers
A weak acid or base that can donate or accept hydrogen ions in relationship to the concentration of free hydrogen ions in solution, allowing for relatively large changes in total hydrogen ion content to take place with relatively little change in free (ionized) hydrogen ion concentration.

12. Buffers H2CO3 H+ + HCO3- Extracellular buffers are all weak acids
Bicarbonate system
Phosphates
Proteins (especially albumin)
N.B. Haemoglobin is an intracellular buffer which has a significant extracellular effect, due to it’s relative mass, that cannot be quantified simply

13. Total Carbon Dioxide
CO2 + H2O H2CO3 H+ + HCO3-
Carbonic Anhydrase

14. Bicarbonate Actual bicarbonate is calculated from pH and pCO2
pH = log[HCO3-]/(0.23xpCO2)
Standard Bicarbonate (removes respiratory influence)
the concentration of HCO3- that would be present if:
SaO2 = 100%
temp = 37o
pCO2 = 5.33kPa

15. Base Excess
BE is the amount of acid or base required to return 1litre of blood to ‘normal’ mean standard bicarbonate of 22.9 mEq/l
BE = 1.2 (sHCO )

16. Compensation Mechanisms
Respiratory
Chemoreceptors located on the ventral surface of the medulla
Respond to changes in pH of CSF
Lower pH stimulates respiration
Metabolic (Renal)
Control of SID by removal of Cl- from extracellular fluid in response to acidosis
Initially by transcellular pumps
Subsequently by urinary loss
Concomitant increase in HCO3- is due to maintenance of electrical neutrality

17. How to interpret ABGs if biology was simple
A survival guide
How to interpret ABGs if biology was simple
Principles to remember:
Increased CO2 reduces pH
Increased HCO3 increases pH
Respiratory compensation alters CO2 in response to primary metabolic change
Renal compensation alters HCO3 in response to primary respiratory change
No compensatory mechanism can overshoot (iatrogenic intervention can)

18. Metabolic Acid-Base AbnormalitiespH
pCO2
HCO3 BE
Metabolic Acidosis (acute) N -
Metabolic Acidosis (subacute)
Metabolic Acidosis (chronic)
Metabolic Alkalosis (acute) +
Metabolic Alkalosis (subacute)
Metabolic Alkalosis (chronic)

19. Respiratory Acid-Base AbnormalitiespH
pCO2
HCO3 BE
Respiratory Acidosis (acute) N
Respiratory Acidosis (subacute) +
Respiratory Acidosis (chronic)
Respiratory Alkalosis (acute)
Respiratory Alkalosis (subacute) -
Respiratory Alkalosis (chronic)

20. 23yo asthmatic, 3 day cough Normal Range
7.35 – 7.45
10.6 – 13.3
4.67 – 6.0
20 – 30
0 – 2
pH 7.33 pO2 7.6kPa on 60% O2 pCO2 7.0kPa Bic 24mmol/l BE 0.1mmol/l Lactate 0.9mmol/l

21. 23yo asthmatic, 3 day cough Normal Range
7.35 – 7.45
10.6 – 13.3
4.67 – 6.0
20 – 30
0 – 2
pH 7.29 pO2 7.6kPa on 60% O2 pCO2 7.0kPa Bic 18mmol/l BE -2.7mmol/l Lactate 3mmol/l

22. 73yo asthmatic, 3 day cough Normal Range
7.35 – 7.45
>12.5 on air
4.67 – 6.0
20 – 30
0 – 2
pH 7.31 pO2 7.6kPa on 60% O2 pCO2 7.5kPa Bic 33mmol/l BE 3.7mmol/l Lactate 0.9mmol/l

23. 23yo diabetic, 3 day cough Normal Range
7.35 – 7.45
>12.5 on air
4.67 – 6.0
20 – 30
0 – 2
pH 7.11 pO2 30.6kPa on 60% O2 pCO2 2.9kPa Bic 11mmol/l BE -13.7mmol/l Lactate 3.9mmol/l

24. This approach simply says:
If there is more base than acid, pH >7.4
If there is more acid than base, pH <7.4
This is a simplistic method that ignores the complexities of biology and frequently fails to explain what is going on in real patients

25. What is the source of H+ ions in the body?
H2O ⇆ H+ + OH-
Kw = [H+] . [OH-]
If [H+] increases, [OH-] decreases and vice verca
BUT
Electrical neutrality must be maintained

26. The Law of Mass Action
The velocity of a reaction is proportional to the product of the concentrations of the reactants
H2O  H+ + OH-
Velocity to the right: V1 = k1.[H2O]
Velocity to the left: V2 = k2.[H+].[OH-]
At equilibrium, V1 = V2
k1.[H2O] = k2.[H+].[OH-]
K* = k1/k2 = [H+].[OH-] / [H2O]
BUT [H2O] is so large it is effectively constant
K*.[H2O] = Kw = [H+].[OH-]
Therefore as [H+] increases [OH-] must decrease and vice verca

27. ECF is a complex solution of:
Electolytes and proteins
Measured and unmeasured compounds
Buffers
ECF must obey laws of physics
Electrical neutrality
Dissociation equillibria for water, weak acids, bicarbonate and carbonate ions

28. Weak acids and bicarbonate are negatively charged
To maintain electrical neutrality there must be more strong cations than strong anions
[Na]+[K]+[Ca]+[Mg] = [Cl]+[HCO3]+[A]
[Na]+[K]+[Ca]+[Mg]-[Cl] = 44Meq = SID

29. Cat An

30. If [OH-] goes up, [H+] goes down
If the difference between any negatively charged group and strong cations change there will be a “force” on other equillibria to restore electrical neutrality
Including water
H2O OH- + H+

31. If any negatively charged group increases wrt strong anions there will be a depression of [OH-] to restore electrical neutrality.
If [OH-].[H+] =Kw, then [H+] must increase

32. [H+] is only influenced by SID, pCO2 and [Atot]
Other variables [H+], [HCO3], [OH-] are dependant variable and cannot influence acid-base status

33. A patient with pneumonia has been critically ill on ITU for 10 days
pH 7.42
pCO
HCO3 24
BE +1.0
Na+ 130
Cl- 105
Lactate 5.2
Albumin 10

34. Has fluid resuscitation been sufficient?
1st set: Hypovolaemic following RTA 2nd set: following splenectomy and fluid resuscitation pH
pCO
HCO BE
Has fluid resuscitation been sufficient? Na Cl
Albumin
Lactate

35. Conclusion
Acid-Base regulation is horribly complicated and not fully understood by anyone
Use the survival guide as a first line method. If this doesn’t work tell your consultant that you suspect an imbalance of strong ions and weak acids with reference to the Stewart-Fencl theory and you recommend referral to a Biochemist/Renal Physician/Intensivist.
N.B. This will either shut them up or make them really cross. Good luck!