Arterial Blood Gases

1
Arterial Blood Gases

• Richard Stretton
• Respiratory Registrar

2
Arterial Blood Gases

• Seen as complicated
• Misunderstood
• Important
• An easy way and a hard way

3
Objectives

• Develop an organised system for looking at blood
  gases
• Be able to comment on the arterial pO2 in
  relation to the FiO2
• Interpret acid base disturbance and its
  significance in the acutely unwell

4
What Are We Measuring?

• pH
• pO2
• pCO2
• HCO3
• Base Excess

5
Acid Base Balance

• pH is carefully controlled
• Enzymatic Function relies on pH control
• Buffers
• Haemoglobin
• BICARBONATE
• Ammonium
• Phosphate

6
Striking the Balance

• H HCO3- ? H2CO3 ? CO2 H2O
• When youve got too much H, lungs blow off CO2
• When you cant blow off CO2, kidneys try to get
  rid of H

7
5-step approach

• Assess Oxygenation
• Determine Acid-Base Deficit
• Determine the respiratory component
• Determine the metabolic component
• Which is primary and which is secondary

8
5-step approach

• Assess Oxygenation
• Determine Acid-Base Deficit
• Determine the respiratory component
• Determine the metabolic component
• Which is primary and which is secondary

9
5-step approach

• Assess Oxygenation
• pO2 10 -13 kPa on air
• Is the patient hypoxic?
• Is there a significant A-a Gradient?
• A-a Gradient is the difference in concentration
  of oxygen between the Alveolus (A) and the artery
  (a)
• Normal lt3
• A-a Gradient PAO2 (PaO2 PaCO2/0.8)

10
I shouldnt say this but

• v.v.v.v. rough guide
• Inspired O2 - (pO2 pCO2)
• Add together pO2 and pCO2 from your blood gas
• Take this away from the concentration of Oxygen
  the patient is breathing
• With an upper limit of normal of about 5

11
5-step approach

• Assess Oxygenation
• Determine Acid-Base Deficit
• Determine the respiratory component
• Determine the metabolic component
• Which is primary and which is secondary

12
5-step approach

• Determine Acid-Base Deficit
• pHgt7.45 alkalaemia
• pHlt7.35 acidaemia
• Acidosis - a process causing excess acid to be
  present in the blood. Acidosis does not
  necessarily produce acidaemia
• Alkalosis - a process causing excess base to be
  present in the blood. Alkalosis does not
  necessarily produce alkalaemia.

13
5-step approach

1. Assess Oxygenation
2. Determine Acid-Base Deficit
3. Determine the respiratory component
4. Determine the metabolic component
5. Which is primary and which is secondary

14
5-step approach

• Determine the respiratory component
• Does this explain the acid-base deficit?
• PaCO2 gt6.0 kPa - respiratory acidosis
• lt4.7kPa - respiratory alkalosis

15
5-step approach

1. Assess Oxygenation
2. Determine Acid-Base Deficit
3. Determine the respiratory component
4. Determine the metabolic component
5. Which is primary and which is secondary

16
5-step approach

• Determine the metabolic component.
• Does this explain the acid-base deficit?
• HCO3 lt22 mmols/l - metabolic acidosis
• gt26 mmols/l - metabolic alkalosis

17
Remember

• H HCO3- ? H2CO3 ? CO2 H2O
• When youve got too much H, lungs blow off CO2
• When you cant blow off CO2, kidneys try to get
  rid of H

18
5-step approach

1. Assess Oxygenation
2. Determine Acid-Base Deficit
3. Determine the respiratory component
4. Determine the metabolic component
5. Which is primary and which is secondary

19
5-step approach

• Which is primary and which is secondary?
• Remember
• Compensation doesnt always completely restore pH
  to the normal range
• A mixed picture may be present

20
5-step approach

1. Assess Oxygenation
2. Determine Acid-Base Deficit
3. Determine the respiratory component
4. Determine the metabolic component
5. Which is primary and which is secondary

21
Assumptions

• CO2 changes are related to respiratory changes
• HCO3 changes relate to metabolic changes
• Overcompensation does not occur
• Respiratory compensation is rapid
• Metabolic compensation is slow

22
Respiratory Acidosis

• Any cause of hypoventilation
• CNS depression
• Neuromuscular disease
• Acute or chronic lung disease
• Cardiac arrest
• Ventilator malfunction

23
Respiratory Alkalosis

• Any cause of hyperventilation
• Hypoxia
• Acute lung conditions
• Anxiety
• Fever
• Pregnancy
• Hepatic failure
• Some central CNS lesions

24
Metabolic Acidosis
Added Acid Loss of Bicarbonate
Renal failure Ketoacidosis Lactic acidosis Salicylate/Tricyclic overdose Renal tubular acidosis Diarrhoea Carbonic anhydrase inhibitors Ureteral diversion Chloride administration
25
Metabolic Alkalosis

• Loss of acid or gaining alkali
• Vomiting
• Diarrhoea
• Diuretics (and hypokalaemia generally)
• Ingestion of alkali

26
Reminder of normal values

• pH 7.35 7.45 (H 35 -45)
• pO2 10 - 13 kPa on air
• pCO2 4.6 - 6.0 kPa
• HCO3 25 - 35 mmols/l
• Base excess 2.0

27
Lets get going..

• Working out acidosis/alkalosis and compensation
  is usually the bit people struggle with
• So..

28
Outcome codes
Outcome Code Outcome Code
pH High Alkali Low Acid
pCO2 High Acid Low Alkali
HCO3 High Alkali Low Acid
29
Translate
Value Code Translate Opinion
pH 7.1 Low Acid Acidaemia
pCO2 5.3 Normal Normal Normal
HCO3 16 Low Acid Primary
Uncompensated Metabolic Acidosis
30
Translate
Value Code Translate Opinion
pH 7.1 Low Acid Acidaemia
pCO2 8.3 High Acid Primary
HCO3 26 Normal Normal Normal
Uncompensated Respiratory Acidosis
31
Translate
Value Code Translate Opinion
pH 7.56 High Alkali Alkalaemia
pCO2 2.3 Low Alkali Primary
HCO3 25 Normal Normal Normal
Uncompensated Respiratory Alkalosis
32
Translate
Value Code Translate Opinion
pH 7.37 Normal Normal Normal
pCO2 2.1 Low Alkali ????
HCO3 14 Low Acid ????
Compensated Metabolic Acidosis or Compensated
Respiratory Alkalosis
33
Translate
Value Code Translate Opinion
pH 7.40 Normal Normal Normal
pCO2 8 High Acid ????
HCO3 35 HIgh Alkali ????
Compensated Respiratory Acidosis or Compensated
Metabolic Alkalosis
34
Translate
Value Code Translate Opinion
pH 7.21 Low Acid Acidaemia
pCO2 12 High Acid Primary
HCO3 32 High Alkali Secondary
Decompensated Respiratory Acidosis
35
What Now?

• Now you can determine any acid base pattern
• Convert the numbers into high/low/normal
• Convert that into acid/alkali
• What is primary, what is compensation?
• Distinguish between uncompensated, compensated,
  and decompensated

36
Nomenclature

• Uncompensated Respiratory Acidosis
• Acute Type 2 Respiratory Failure
• Compensated Respiratory Acidosis
• Chronic Type 2 Respiratory Failure
• Decompensated Respiratory Acidosis
• Acute on Chronic Type 2 Respiratory Failure

37
Case 1

• Young female admitted with overdose of unknown
  tablets and smelling of alcohol
• pO2 12 kPa on air
• pH 7.24
• PaCO2 2.5
• HCO3 8
• Metabolic Acidosis with respiratory compensation

38
Case 2

• Elderly male admitted from nursing home with one
  week history of fever and vomiting
• pO2 12 kPa on 4l by mask
• pH 7.49
• PaCO2 6.3
• HCO3 35
• Metabolic alkalosis with respiratory
  compensation

39
Case 3a

• Middle aged man admitted with cough sputum and
  haemoptysis. Life-long smoker
• pO2 4 on air
• pH 7.19
• PaCO2 9.7
• HCO3 28
• Acute respiratory acidosis with no time for
  metabolic compensation

40
Case 3b

• Middle aged man admitted with cough sputum and
  haemoptysis. Life-long smoker
• pO2 6 on air SpO2 92
• pH 7.32
• PaCO2 10.0
• HCO3 39
• Acute respiratory acidosis with no time for
  metabolic compensation

41
Case 4

• Middle aged man post cardiac arrest. Breathing
  spontaneously on endotracheal tube
• pO2 35 on 15l via reservoir mask
• pH 6.9
• PaCO2 8.9
• HCO3 13
• Mixed metabolic and respiratory acidosis

42
Case 5

• Elderly lady with congestive cardiac failure
• pO2 9 on 40 oxygen
• pH 7.64
• PaCO2 3.5
• HCO3 29
• Respiratory alkalosis secondary to pulmonary
  oedema.
• Acute as no metabolic compensation

43
Case 6

• Young diabetic male admitted with chest
  infection, vomiting and drowsiness
• pO2 12 on air
• pH 7.31
• PaCO2 1.6
• HCO3 6.0
• Acute metabolic acidosis with respiratory
  compensation

44
Case 7

• 54 yr-old lady post MI. Acutely unwell, cold,
  clammy, hypotensive and oliguric
• pO2 10 on 60 oxygen
• pH 6.99
• PaCO2 7.8
• HCO3 14
• Mixed pattern of respiratory and metabolic
  acidosis

45
Case 8

• 50 yr-old man admitted with exacerbation of
  long-standing bronchial asthma. Respiratory rate
  of 18
• pO2 5.1 on 60 oxygen
• pH 7.39
• PaCO2 5.8
• HCO3 26
• Severe type I respiratory failure

46
Questions

• ?

47
The 6th step

• If an acidosis is present work out the anion gap
  to help determine cause.
• Anion Gap is the difference between the measured
  positive and negatively charged ions.
• It gives an estimate of the unmeasured ions in
  the serum
• Unmeasured proteins, sulphates

48
Anion Gap

• Anion Gap NaK CLHCO3
• Normal anion gap 10-18

49
Metabolic Acidosis

• Increased anion gap (added acid)
• Renal failure
• Ketoacidosis
• Lactic acidosis
• Salicylate/Tricyclic overdose

50
Metabolic Acidosis

• Decreased anion gap (loss of bicarbonate)
• Renal tubular acidosis
• Diarrhoea
• Carbonic anhydrase inhibitors
• Ureteral diversion
• Chloride administration

51
High Anion Gap

• A
• M
• U
• D
• P
• I
• L
• E
• S

52
High Anion Gap

• Alcohol (Alcohol dissociates to become a week
  acid)
• M
• U
• D
• P
• I
• L
• E
• S

53
High Anion Gap

• Alcohol (Alcohol dissociates to become a week
  acid)
• Methanol (See alcohol. Causes blindness)
• U
• D
• P
• I
• L
• E
• S

54
High Anion Gap

• Alcohol (Alcohol dissociates to become a week
  acid)
• Methanol (See alcohol. Causes blindness)
• Uraemia (Failure to reabsorb HCO3- and excrete
  H)
• D
• P
• I
• L
• E
• S

55
High Anion Gap

• Alcohol (Alcohol dissociates to become a weak
  acid)
• Methanol (See alcohol. Causes blindness)
• Uraemia (Failure to reabsorb HCO3- and excrete
  H)
• DKA (Ketones are dehydrogenated alcohols, and
  dissociate to acid)
• P
• I
• L
• E
• S

56
High Anion Gap

• Alcohol (Alcohol dissociates to become a weak
  acid)
• Methanol (See alcohol. Causes blindness)
• Uraemia (Failure to reabsorb HCO3- and excrete
  H)
• DKA (Ketones are dehydrogenated alcohols, and
  dissociate to acid)
• Paraquat (Very nasty poison, universally lethal)
• I
• L
• E
• S

57
High Anion Gap

• Alcohol (Alcohol dissociates to become a weak
  acid)
• Methanol (See alcohol. Causes blindness)
• Uraemia (Failure to reabsorb HCO3- and excrete
  H)
• DKA (Ketones are dehydrogenated alcohols, and
  dissociate to acid)
• Paraquat (Very nasty poison, universally lethal)
• Infection (Commonest cause. Localised tissue
  hypoxia leads to...)
• L
• E
• S

58
High Anion Gap

• Alcohol (Alcohol dissociates to become a weak
  acid)
• Methanol (See alcohol. Causes blindness)
• Uraemia (Failure to reabsorb HCO3- and excrete
  H)
• DKA (Ketones are dehydrogenated alcohols, and
  dissociate to acid)
• Paraquat (Very nasty poison, universally lethal)
• Infection (Commonest cause. Localised tissue
  hypoxia leads to...)
• Lactic Acid (Product of anaerobic respiration,
  and tissue necrosis)
• E
• S

59
High Anion Gap

• Alcohol (Alcohol dissociates to become a weak
  acid)
• Methanol (See alcohol. Causes blindness)
• Uraemia (Failure to reabsorb HCO3- and excrete
  H)
• DKA (Ketones are dehydrogenated alcohols, and
  dissociate to acid)
• Paraquat (Very nasty poison, universally lethal)
• Infection (Commonest cause. Localised tissue
  hypoxia leads to...)
• Lactic Acid (Product of anaerobic respiration,
  and tissue necrosis)
• Ethylene Gylcol (Antifreeze. Quite a potent
  acid, no longer sold in UK)
• S

60
High Anion Gap

• Alcohol (Alcohol dissociates to become a weak
  acid)
• Methanol (See alcohol. Causes blindness)
• Uraemia (Failure to reabsorb HCO3- and excrete
  H)
• DKA (Ketones are dehydrogenated alcohols, and
  dissociate to acid)
• Paraquat (Very nasty poison, universally lethal)
• Infection (Commonest cause. Localised tissue
  hypoxia leads to...)
• Lactic Acid (Product of anaerobic respiration,
  and tissue necrosis)
• Ethylene Gylcol (Antifreeze. Quite a potent
  acid, no longer sold in UK)
• Salicylates (Aspirin causes resp alkalosis, then
  metabolic acidosis)

61
Normal Anion Gap

• Addisons Disease
• High Output Fistulas
• RTA I, II, IV
• Acetazolamide Therapy
• Diarrhoea

62
Any more Questions?