1. ABG Interpretation
ยิ่งศักดิ์ ศุภนิตยา
นนท์

2. Case 1
FiO2 0.21
pH 7.21
PaCO2 64
BE 2
PaO2 48
SpO2 80
• Acute respiratory acidosis with
moderate hypoxemia

3. Normal composition of clean, dry air
near sea level
Nitrogen 78.084
Oxygen 20.9476
Argon 0.934
Carbon dioxide 0.0314
Neon 0.001818
Helium 0.000524
Krypton 0.000114
Xenon 0.000087
Hydrogen 0.00005
Methane 0.0002
Nitrous oxide 0.00005

4. Dalton’s Law of Partial Pressure
GAS PARTIAL PRESSURE = % GAS
CONC x 7.13 mmHg

5. ABG DATA
pH 7.40 ( 7.35 - 7.45 )
H+
40 nM/L
PaO2 97 ( 90- 100 ) torr
PaCO2 40 ( 35 - 45 ) torr
Actual HCO3 24 ( 22 - 26 ) mEq/L
Base excess +/- 2 mEq/L
Hb 15 gm%
O2 sat 95-100%
O2 content 15-23%

6. IMPLICATIONS OF ABG
• LUNG FUNCTION
• OXYGENATION, VENTILATION, ACID BASE
STATUS
• EARLY DIAGNOSIS OF ARF
• DIAGNOSIS OF SECONDARY POLYCYTHEMIA
DUE
TO PULMONARY DISEASE
• HEART AND CIRCULATORY FUNCTION
• KIDNEY FUNCTION
• METABOLISM
• THE USE OF SOME MEDICATIONS
• QUALIFY PATIENTS FOR HOME OXYGEN USE
• DETECT EXPOSURE TO CARBON MONOXIDE
AND OTHER
CHEMICALS

7. TECHNIQUES
SITES: radial, brachial, femoral arteries.
Allen’s test
VOLUME: 1 mL

9. ERRORS
• PLASTIC SYRINGE
• AIR MIXTURE
• OVER HEPARINIZATION (acid)
• VENOUS SAMPLE
• PAIN
• DELAY OR UNCOOL SPECIMEN
• ERROR OF GAS ANALYSER

10. PaCO2
• 80% plasma NaHCO3
• 10% carboxy Hb + 2% carbamino
comp
• 8% dissolved in plasma

12. EXCRETORY RATE OF CO2 = VA x Pa
CO2
Pa CO2 = 1 / VA
VA = ( VT - VD ) x f
INTERPRETATION OF PaCO2
PaCO2 > 45 mmHg = alveolar
hypoventilation = resp acidosis
PaCO2 < 35 mmHg = alveolar
hyperventilation = resp alkalosis

17. PaO2
Interpretation
> 100 mmHg hyperoxemia
90-100 normal
60-80 mild hypoxemia
40-60 moderate
hypoxemia
< 40 severe
hypoxemia

18. Oxygenation and external respiration
Causes of hypoxemia
Low FIO2
Hypoventilation
Diffusion defect
Ventilation perfusion mismatch
Dead space and shunting
Venous admixture

19. Oxygen transport and internal
respiration
Arterial oxygen content
Volume of dissolved oxygen
+
Volume of combined oxygen with
hemoglobin
-------------------------------------
Total oxygen content
--------------------------------------

20. volume of dissolved O2 = PaO2 x CsO2 = 0.3
vol%O2
volume of combined O2 = Hb x SaO2 x 1.34
= 19.7vol%
CaO2 = dissolved O2 + combined O2 = 20
vol%
CaO2 - CvO2 = 20 - 15.2 = 4.8 vol%
O2 consumption = Q x C(a - v) O2 = 250 mL
O2/min
total O2 transport = cardiac output x
CaO2 = 1000mLO2/min

21. Oxygenation ratio (PaO2/%FiO2)
Pulmonary status
O2 ratio
normal 4.0
- 5.0
moderate pulmonary
dysfunction 2.0 - 3.9
substantial pulmonary
dysfunction < 2.0

22. pH
Henderson’s equation
Kc = [H] [HCO3] / [H2CO3]
Hasselbalch’s equation
pH = pKc + log [HCO3] /
[H2CO3]
pH = pKc + log [HCO3 /
dissolve CO2]
pH = 6.1 + log 24 / 1.2
pH = 7.4

23. Determination of primary problem
pH
> 7.4 Alkalosis is primary;
acidosis is
compensatory
< 7.4 Acidosis is primary;
alkalosis is
compensatory

24. Severity of generalized acid-base
disturbances
pH Degree of impairment
< 7.20 severe acidemia
7.20-7.29 moderate acidemia
7.30-7.34 mild acidemia
7.35-7.45 normal pH
7.46-7.50 mild alkalemia
7.51-7.55 moderate alkalemia
> 7.55 severe alkalemia

25. Calculated bicarbonate
• Actual bicarbonate
• It is a calculated value based on the
Henderson-Hasselbalch equation.
• Henderson’s equation
Kc = [H] [HCO3] / [H2CO3]
Hasselbalch’s equation
pH = pKc + log [HCO3] / [H2CO3]
pH = pKc + log [HCO3 /
dissolve CO2]
pH = 6.1 + log
24 / 1.2

26. Base excess of blood
BE = Observed BB - normal BB

31. Classification of laboratory metabolic
acid-base compensation
Classification BE HCO3
normal metabolic component 0 +/-2
24+/-2
metabolic acidosis < - 2 <
22
metabolic alkalosis > + 2
> 26

32. Stepwise approach to diagnosing acid-base
disorders
• Step1: Acidemic or alkalemic?
• Step2: Is the primary disturbance
respiratory or metabolic?
• Step3: For a respiratory disturbance,
determine whether it is acute or
chronic.
• Step4: For a metabolic acidosis,
determine whether an anion gap is
present.
• Step5: Determine whether other
metabolic disturbances coexist with
an anion gap acidosis.
• Step6: Assess the normal compensation
by the respiratory system for a

33. Step1: Acidemic or Alkalemic?
Normal arterial blood pH = 7.40
+/- 0.05
Acidemic: pH < 7.35
Alkalemic: pH > 7.45

34. Step2: Is the primary disturbance
respiratory or metabolic?
A respiratory disturbance alters the arterial
PaCO2 (normal value 40, range 38-42). Go
to step 3.
A metabolic disturbance alters the serum
HCO3 (normal value 24, range 22-26)
• If HCO3 < 22, metabolic acidosis is
present. Go to step 4.
• If HCO3 > 26, metabolic alkalosis is
present, is respiratory compensation
adequate? Go to step 6.

35. Step3: For a respiratory disturbance, determine
whether it is acute or chronic.
• Ac resp acid: pH decrease = 0.08*(PaCO2-
40)/10
• Ch resp acid: pH decrease = 0.03*(PaCO2-
40)/10
• Ac resp alka: pH increase = 0.08*(40 -
PaCO2)/10
• Ch resp alka: pH increase = 0.017*(40 -
PaCO2)/10

36. Step4: For a metabolic acidosis, determine
whether an anion gap is present.
• Anion gap = Na - (Cl + HCO3)
• Anion gap metabolic acidosis, anion gap
> 12
• Normal or non anion gap acidosis, anion
gap </= 12

37. Anion gap reflects the unmeasured
anion and cation.
Unmeasured
Anions
Proteins, mostly
albumin 15 mEq/L
Organic acids 5 mEq/L
Phosphates 2 mEq/L
Sulfates 1 mEq/L
Total: 23 mEq/L
Measured Anions
Chloride 104 mEq/L
Bicarbonate 24 mEq/L
Total: 128 mEq/L
Unmeasured
Cations
Calcium 5 mEq/L
Potassium 4.5 mEq/L
Magnesium 1.5 mEq/L
Total: 11 mEq/L
Measured Cations
Sodium 140 mEq/L
Total: 140 mEq/L

41. Step5: Determine whether other metabolic
disturbances coexist with an anion gap acidosis.
Corrected HCO3 = measured HCO3 +
(anion gap - 12)
If the corrected HCO3 varies significantly
above or below 24, then a mixed or more
complex metabolic disturbance exists.
To be more specific, if the corrected HCO3
is greater than 24, a metabolic alkalosis
coexists. If the corrected HCO3 is less
than 24 then a non anion gap acidosis
coexists.

46. Step6: Assess the normal compensation by the
respiratory system for a metabolic disturbance.
Winter’s Formula
Expected PaCO2 = (1.5*HCO3) +(8+/-2)
Winter’s Formula does not predict the resp
response to a metabolic alkalosis.
Two general rules
• a pt will increase PaCO2 above 40 but not greater
than 50-55 to compensate for a metabolic
alkalosis.
• a pt will be alkalemic if the PaCO2 is elevated to
compensate for a met alk ( If the patient is
acidemic,PH < 7.38, then an additional resp acid
is present).

53. Steps in evaluation and
classification of acid-base
compensation
• Evaluate for the presence of
compensation.
• Determine the probable primary
problem.
• Classify the degree of
compensation.

54. Alerts to mixed acid-base
disturbances
• If respiratory and metabolic parameters change
proportionately, pH remains unchanged.
• Both parameters are altered in fashion that
changes the pH in the same direction.
• Fails to compensate in the expected manner for
a primary disorder after sufficient time has
elapse.
• A metabolic alkalosis is accompanied by an
increase in the anion gap.
• Absent of compensation.
• Long standing pulmonary or renal disease.
• Excessive compensation.
• Respiratory assistance.
• Settings conducive to mixed disturbances.
• Triple disorders may also be encountered.

55. Case 2
FiO2 0.21
pH 7.22
PaCO2 25
HCO3 10
PaO2 96
SaO2 95
creatinine 11 mg/dL
•Simple metabolic acidosis with
normoxemia

56. Case 4
FiO2 0.21
pH 7.35
PaCO2 22
HCO3 12
PaO2 41
SaO2 75
•Mixed respiratory alkalosis and
metabolic acidosis.

57. Case 5
FiO2 0.21
pH 7.10
PaCO2 95
BE - 5
HCO3 29
PaO2 60
SpO2 78%
• Partially compensated respiratory
acidosis.

58. Case 6
FiO2 0.21
pH 7.53
PaCO2 49
HCO3 39
PaO2 92
SaO2 98
• Partially compensated metabolic
alkalosis with normoxemia.

59. Case 7
FiO2 0.21
pH 7.58
PaCO2 31
HCO3 28
PaO2 65
SaO2 96
• Combined respiratory alkalosis and
metabolic alkalosis with mild
hypoxemia.

60. Case 9
FiO2 0.21
pH 7.04
PaCO2 15
BE -22
PaO2 125
SaO2 95
•Partially compensated metabolic
acidosis with hyperoxemia.

61. Case 10
FiO2 0.21
pH 7.25
PaCO2 80
HCO3 34
PaO2 39
SaO2 52
•Partially compensated
respiratory acidosis with severe
hypoxemia.