1. Arterial Blood Gas Analysis
By Mohamed hamdy
Assistant lecturer of Anesthesia
Ain-Shams University
Egyptian Resuscitation Council (ERC)Instructor

2. Interpretation of ABG Analyses
Systematic Approch
ABG Abnormalities

3. Why we do Arterial Blood Gas Analysis?
Oxygenation
Represented by PaO2
Ventilation
Represented by Pa Co2
Acid Base Status
Represented by pH, HCO3 and base deficit.
Hb, Hct, oxygen saturation
Electrolyte e.g. Na+, K+.

4. Defense against the change in pH. WHY?!!

5. Because H+ react highly with cellular proteins resulting in alteration in their function therefore avoiding acidemia and alkalemia by tightly regulation H+ which is essential for normal cellular function.

6. Calculation of Alveolar Gas Equation and A-a Gradient:
PAO2 =
FiO2×(Bp-pH2O)-PaCO2/R. =
21×(760-47)-40/0.8 =
100 mmHg.
A-a Gradient is alvealo-arterial O2 gradient.
A-a Gradient = PAO2 -PaO2
It is normally = Age/4+4.
It’s Value: concise D.D of hypoxemia.
e.g.:
normal A-a Gradient
Decrease FiO2
Hypoventilation
Ventilation perfusion mismatch
Rt to Lt shunting
increase A-a Gradient
Diffusion abnormality

7. The DIFFERENCE between:
What is an A - a gradient ?
The DIFFERENCE between:
Oxygen Content in arterial blood
(equivalent to that leaving lungs)
Oxygen Content in
Alveolus Gas
(measured during exhalation)
In a healthy person, what would you expect the A - a to be?
No difference, greater than 0, or less than 0
Normal: A – a, up to ~ 10 mm Hg, varies with age

8. Factors contributing to A - a Gradient
Blood Shunts
Matching

9. Alveolar SPACE Blood Mixing SIMPLE CONCEPT OF A SHUNT AIR FLOW CO2 O2
arterial vessel
BLOOD FLOW
Blood
Mixing
Lowered O2/l00 ml
No Gas Exchange = SHUNT

10. Matching Blood to Air Flow
Matching What?
Blood to Air Flow
Total Ventilation
Exchange
Oxygen
Total Perfusion, Q
If the volumes used for exchange are aligned
– We might consider the system to be “ideally matched”

11. Matching Dead Air Space (Airways) Alveolar Ventilation (VA) Oxygen
Exchange
Oxygen
Arterial Perfusion (Qc)
Slide or Misalign the distribution volumes
Shunt (Qs)
(Bronchial
Artery)
Some Volumes are wasted,
Matching Ratio = VA/Qc = 0.8
Normal Case; Small Shunt, low volume Dead Space

12. = Lung Disease with a Large A – a gradient
Severe Mismatch
Dead Air Space
Alveolar Ventilation VA
Exchange
Oxygen
Arterial Perfusion Q
blood mixing
Shunt
= Lung Disease with a Large A – a gradient

13. Approach To Hypoxemia N N A-a Do2 pCO2 FIO2 Alv. Hypo. 100% O2
PaO2
A-a Do2
pCO2
FIO2
Alv. Hypo.
100% O2
Corrects
V/Q Mis.
Diffusion
No Correction
Shunt

15. 1) Arterial/alveolar ratio(a/A)
PaO2/PAO2
Normal value for the a/A ratio is 0.8, meaning that 80% of the alveolar oxygen is reaching the arterial system
2) PaO2/ FIO2 ratio
Normal ratio is 550 (a person breathing FIO2 of 1.0 at sea level should have a PaO2 of 550 to 600 mmHg)
3) A-a gradient (on 100% oxygen)
PAO2 - PaO2
Where PAO2 is calculated by the alveolar air equation previously presented

16. PO2 and PCO2 in Blood

17. Arterial PCO2 - Alveolar PCO2
4) Arterial-alveolar PCO2 Gradient (a-A PCO2)
Arterial PCO2 - Alveolar PCO2
Where Alveolar PCO2 is measured by means of end–tidal PCO2
Normal gradient is an alveolar PCO2 2 mmHg less than arterial,
Acute increase reflects increase in physiologic dead space

18. Oxygen Transport Whole blood oxygen content based on:
hemoglobin content and,
dissolved O2
Described by the equation:
CaO2 = (1.34 x Hb x SaO2) + (0.003 x PaO2)

19. Oxygen Content Assuming 15 g/100ml Hb concentration O2 sat of 99%
Hb O2 = 1.34 x 15 x 0.99 = 19.9 ml/dL
For a PaO2 of 100
Dissolved O2 = x 100 = 0.3 ml/dL

20. Oxygen Content Thus, most of blood O2 content is contained in the Hb
PO2 is only important if there is an accompanying change in O2 sat.
Therefore O2 sat more reliable than PO2 for assessment of arterial oxygenation

21. Oxygen Delivery O2 delivery = DO2 = CO x CaO2
Usually = ml/min/m2

22. Oxygen Uptake A function of: Cardiac output
Difference in oxygen content b/w arterial and venous blood
VO2 = CO x 1.34 x Hb (SaO2 – SvO2) 10

23. Oxygen Extraction Ratio
VO2/DO2 x 100
Ratio of oxygen uptake to delivery
Usually 20-30%
Uptake is kept constant by increasing extraction when delivery drops.

24. Critical Oxygen Delivery
Maximal extraction ~
Once this is reached a decrease in delivery = decrease in uptake
Known as ‘critical oxygen delivery’
O2 uptake and aerobic energy production is now supply dependent = dysoxia

25. Tissue Oxygenation
In order for tissues to engage in aerobic metabolism they need oxygen.
Allows conversion of glucose to ATP
Get 36 moles ATP per mole glucose

26. Tissue Oxygenation If not enough oxygen, have anaerobic metabolism
Get 2 moles ATP per mole glucose and production of lactate
Can follow VO2 or lactate levels

27. Oxygen Transport O2 is transported by the blood either,
Only about 3 ml of O2 are dissolved in each litre of plasma.
Assuming we have a total plasma volume of 3 to 5 litres, only about 9 – 15 ml of O2 can be carried in the dissolved state.
O2 is transported by the blood either,
Combined with haemoglobin (Hb) in the red blood cells (>98%) or,
Dissolved in the blood plasma (<2%).

28. 3. Properties of the Lung Area of the respiratory membrane
Distance of the diffusion
VA/Qc

31. Function of Haemoglobin

34. If hypoxemia PaO2<80mmHg

35. Getting an ABG sample

36. Sample source and collection:-
Arterial blood sample is common utilized clinically but venous blood may be useful in determining acid base status.
Blood sample should be in heparin coated syringe.
The sample should be analyzed as soon as possible.
Air bubble should be eliminated.
The syringe should be capped and placed in ice.

37. Problem associated with obtaining ABG:
Arterial puncture may result in acute hyperventilation. To minimize that: we should use local anesthetic with small needle.
When would you withdraw ABG sample after beginning or stopping O2 supplementation?
In absence of significant lung disease we should wait from 5-7 minutes before withdraw ABG sample while patient with obstructive lung disease we should wait 25 min.

38. Interpretation of ABG 7.30-7.40 35-40 30-50 40-50 60%-85% 60%-80%
Normal blood gas values:
Interpretation of ABG
Measurement
Arterial blood
Mixed venous
Venous PH
PO2
80-100
35-40
30-50
PCO2
36-44
40-50
O2 saturation
>95%
60%-85%
60%-80%
HCO3
22-26
22-28
Base difference (deficit excess)
-2 to 2

39. or more simply: The Henderson equation:
What is PH?
Relationship between pH & [H+] pH
[H+] (nanomoles/l)
6.8
158
6.9
125
7.0
100
7.1 79
7.2 63
7.3 50
7.4 40
7.5 31
7.6 25
7.7 20
7.8 15
PH is –ve log of H+ concentration.
Henderson-Hasselbalch Equation pH = pK’a + log ([HCO3] / 0.03 x pCO2)
or more simply: The Henderson equation:
[H+] = 24 x ( pCO2 / [HCO3] )

40. The Seven Step Approach to Solving Acid-Base Disorders

41. STEPS for interpretation of ABG
Determine if numbers fit:
H+ =
H+ = (7.8-PH)×100.
The Rt side of the equation should be within 10% of the Lt Side. If not so:
Another ABG
Chemistry panel for HCO3 should be done.

42. STEP 2:
Determine if:
Acidemia (PH<7.37) OR Alkalemia(PH >7.44) is present.
STEP 3:
Identify primary disturbance: PH
Increase
Decrease
Alkalosis
Acidosis
Look at PCO2
Increased
Decrease
Increased
Decreased
Metabolic Alkalosis
Respiratory Alkalosis
Respiratory acidosis
Metabolic acidosis

43. Look at the direction of the change of HCO3/PCO2:
STEP 4:
Look at the direction of the change of HCO3/PCO2:
If it is in the same direction it is either simple or mixed change.
But if it is in the opposite direction so it is mixed change.
STEP 5:
Calculate rate of change of Hco3 and co2
(Expected compensation)
Disturbance
Response
Expected change
Metabolic acidosis
↓Paco2
1.2×(24-HCO3 measured)
Metabolic alkalosis
↑Paco2
0.7×(HCO3-24)
Acute respiratory acidosis
↑Hco3
0.1×(PaCO2-40)
Chronic respiratory acidosis
↑ Hco3
0.4×(PaCO2-40)
Acute respiratory alkalosis
↓ Hco3
0.2×(40-PaCO2)
Chronic respiratory alkalosis
0.4×(40-PaCO2)

44. Determine the Anion Gap
[(Na+) + (K+)] – [(Cl-) + (HCO3-)]
The normal A.G is 12meq ± 4.
Normal A.G (Hyperchloremic Acidosis)
↑ GIT loss of HCO3 as in: diarrhea, high output fistula
↑ renal HCO3 loss as in: RTA(I, II)
TPN.
↑ CL containing acids. 
Wide Anion Gap Acidosis (↑ endogenou non-volatile acid)
Keto Acidosis
Uremia
Lactic Acidosis
Salicylism
Toxins : Methanol,Paraldehyde,Ethylene glycol

45. Na+ + UC = (Cl-) + (HCO3-) + UA
All anions and cations
ANIONS
CATIONS
Proteins 15
Calcium 5
Organic acids 5
Magnesium 1.5
Phosphates 2
Potassium 4.5
Bicarbonate 24
Sodium 140
Sulfates 1
Chloride 104
TOTAL 151
Na UC = (Cl-) + (HCO3-) + UA
Na+ – (Cl-) + (HCO3-) = UA – UC

46. Very low or even –Ve A.G

47. Calculation of AG in urine: Urine AG = ( Na+ + K+) – CL-
In a patient with a hyperchloraemic metabolic acidosis:
A negative UAG suggests GIT loss of bicarbonate (eg diarrhoea)
A positive UAG suggests impaired renal distal acidification (ie renal tubular acidosis).

48. If there is metabolic acidosis calculate the A.G
STEP 6:
If there is metabolic acidosis calculate the A.G
Anion Gap = Na+ - (Cl- + HCO3-) = 12meq ± 4.
Corrected A.G = observed A.G (normal albumin - measured albumin).
If the anion gap ↑ proceed to step 7.

49. STEP 7:
If the anion gap metabolic acidosis is present we should evaluate for additional metabolic disorder because the elevation of anion gap above normal ∆ AG = (AG-12) should be buffered by HCO3.
Adding ∆AG to current HCO3 will yield the corrected Hco3 which should be normal value 24 meq/l unless there is another disorder present.
Corrected HCO3 = current HCO3 (measured) +∆A.G
(Normal value 24 meq/l)
If corrected HCO3 >24 → metabolic alkalosis is also present
If corrected HCO3 <24 → a non gap metabolic acidosis is also present
If corrected HCO3 = 24 → it is pure gap metabolic acidosis.

50. Delta ratio = (Increase in anion gap / Decrease in bicarbonate)
Assessment Guideline
< 0.4
Hyperchloraemic normal anion gap acidosis
Consider combined high AG & normal AG acidosis BUT note that the ratio is often <1 in acidosis associated with renal failure
1 to 2
Usual for uncomplicated high-AG acidosis Lactic acidosis: average value 1.6 DKA more likely to have a ratio closer to 1 due to urine ketone loss (esp if patient not dehydrated)
> 2
Suggests a pre-existing elevated HCO3 level: consider a concurrent metabolic alkalosis or a pre-existing compensated respiratory acidosis.

51. Final step:
Be sure that the interpretation of blood gas is consistent and correlated with the clinical picture of the patient.

53. Practice is not a magic
Thursday, April 20, 2017
D.R Mohamad Hamdy Resident in Anaesthesiology & ICU department AIN SHAMS University

54. Case 1
A 75-year-old man presents to the ED after a witnessed out of hospital VF cardiac arrest.
Arrived after 10 minutes, CPR had not been attempted.
The paramedics had successfully restored spontaneous circulation after 6 shocks.
On arrival the man is comatose with a GCS of 3 and his lungs are being ventilated with 50% oxygen via ET tube.
He has a ST with rate of 120 min-1 and a blood pressure of 150/95 mmHg.

55. ABG Analysis reveals: FiO2 0.5 pH 7.10 PaCO2 6.0 kPa (45 mmHg)
PaO kPa(56 mmHg)
HCO mmol l-1
BE mmol l-1

56. A 65-year-old man with severe COPD has just collapsed in the respiratory high-care unit.
On initial assessment he is found to be apnoeic but has an easily palpable carotid pulse at 90 min-1.
A nurse is ventilating his lungs with a BVM and supplementary O2 (with reservoir)
Case 2

57. ABG Analysis reveals: FiO2 0.85 (estimated) pH 7.20 PaCO2 (80 mmHg)
PaO2 (147 mmHg)
HCO mmol l-1
BE mmol l-1

58. Case 3
A 75-year-old lady is admitted to the ED following a VF cardiac arrest, which was witnessed by the paramedics.
A spontaneous circulation was restored after 4 shocks, but the patient remained comatose and apnoeic.
The paramedics intubated her trachea, and on arrival in hospital her lungs are being ventilated with an automatic ventilator using a tidal volume of 900 ml and a rate of 18 breaths min-1.

59. ABG Analysis reveals: FiO2 1.0 pH 7.60 PaCO2 2.65 kPa (20 mmHg)
PaO kPa (192 mmHg)
HCO mmol l-1
BE -2 mmol l-1

60. Case 4
An 18-year-old male insulin dependent diabetic is admitted to the ED.
He has been vomiting for 48 hours and because he was unable to eat, he omitted his insulin.
He has a ST at a rate of 130 min-1 and his blood pressure is 90/65 mmHg.
He is breathing spontaneously with deep breaths at a rate of 35 min-1 and is receiving oxygen 6 l min-1 via O2 mask. His GCS is 12 (E3, M5, V4).

61. ABG Analysis reveals: FiO2 0.4 pH 6.79 PaCO2 (14 mmHg)
PaO2 (129.2 mmHg)
HCO mmol l-1
BE mmol l-1

62. Case 5 Heart rate 120 min-1 – sinus tachycardia – warm peripheries
His vital signs are:
Heart rate min-1 – sinus tachycardia – warm peripheries
Blood pressure 70/40 mmHg
Respiratory rate 35 breaths min-1
SpO2 on oxygen 92%
Urine output 50 ml in the last 6 hours
GCS 13 (E3, M6, V4)
Case 5

63. ABG Analysis reveals: FiO2 0.4 (approx) pH 7.12
PaCO kPa (36 mmHg)
PaO kPa (62 mmHg)
HCO mmol l-1
BE mmol l-1

64. Which patient is more hypoxemic, and why?
Case 6
Which patient is more hypoxemic, and why?
Patient A: pH 7.48, PaCO2 34 mm Hg, PaO2 85 mm Hg, SaO2 95%, Hemoglobin 7 gm%
Patient B: pH 7.32, PaCO2 74 mm Hg, PaO2 55 mm Hg, SaO2 85%, Hemoglobin 15 gm%

65. Patient A: Arterial oxygen content = .95 x 7 x 1.34 = 8.9 ml O2/dl
Patient B: Arterial oxygen content = .85 x 15 x 1.34 = 17.1 ml O2/dl
Patient A, with the higher PaO2 but the lower hemoglobin content, is more hypoxemic.

66. Case 7
The PO2 in a cup of water open to the atmosphere is always higher than the arterial PO2 in a healthy person (breathing room air) who is holding the cup.
True or False

67. Case 8 What is(are) the acid-base disorder(s)?
A patient is admitted to the ICU with reabeted vomiting the following
BLOOD GASES
pH: 7.40 PCO2: 38 HCO3: 24 PO2: 72
ELECTROLYTES, BUN & CREATININE
Na: 149 K: 3.o Cl: 100 CO2: 24 BUN: 110 Creatinine: 8.7
What is(are) the acid-base disorder(s)?

68. The patient was both uremic (causing metabolic acidosis)
and had been vomiting (metabolic alkalosis).

69. Case 9 What is(are) the acid-base disorder(s)?
55 yrs old pt. who drink fifth of wesky per day has 2 wks history of diarrhea , Anion gap is 20, HCO3 = 10, PH = 7.30, PO2 =0.90 mmHg, PCO2 = 30 mmHg.
What is(are) the acid-base disorder(s)?

70. Case 10 What is(are) the acid-base disorder(s)?
25 yrs pt. come to ER with fever, abd. pain , vomiting, with the history of migrane PH = 7.33, PCO2 = 8mmHg, PO2 = 80 mmHg, HCO3 = 4, Sodium = 140 mmol, K = 3 mmol, CL = 108 mmol.
What is(are) the acid-base disorder(s)?

71. “Life is a struggle, not against pharama, not against the physics, not against Anesthesia Exame, but against hydrogen ions.“
M. Hamdy

72. Any Question?

73. Thank You