- The document defines terms related to acid-base disorders including acidemia, alkalemia, acidosis, and alkalosis. - It provides normal ranges for arterial blood gas values and describes primary and secondary changes in respiratory and metabolic acid-base disorders. - Formulas are given to calculate expected bicarbonate and pH values based on changes in PCO2 for acute and chronic respiratory acidosis and alkalosis as well as metabolic acidosis and alkalosis.

2.  Acidemia : Blood pH < 7.35
 Alkalemia : Blood pH > 7.45
 Acidosis : a physiologic process that tends to cause
acidemia
 Alkalosis : a physiologic process that tends to

3. pH 7.35 - 7.45
PaCO2 35 - 45 mm Hg
PaO2 70 - 100 mm Hg **
SaO2 93 - 98%
HCO3
¯ 22 - 26 mEq/L
Base excess -2.0 to 2.0 mEq/L
* At sea level, breathing ambient air
** Age-dependent
Normal Arterial Blood Gas Values*

4. Primary & Secondary Acid – Base Disorder
Acid Base Disorder Primary Change Secondary Change
Respiratory Acidosis  PaCO2  HCO3
-
Alkalosis  PCO2  HCO3
-
Metabolic Acidosis  HCO3
-  PCO2
Alkalosis  HCO3
-  PCO2

5. Changes In Actual HCO3
- Levels
Respiratory acidosis
Acute: ↑0.1 mmol of HCO3- per 1 mm Hg ↑ Pco2
or
HCO3- = 24 + 0.1 X Δ PCO2
Chronic: ↑ 0.35 mmol of HCO3- per 1 mm Hg ↑ Pco2
or
HCO3- = 24 + 0.35 X Δ PCO2

6. Changes In Actual HCO3
- Levels
Respiratory alkalosis
Acute: ↓ 0.2 mmol of HCO3- per 1 mm Hg ↓ Pco2
or
HCO3- = 24 – 0.2X Δ PCO2
Chronic: ↓ 0.4 mmol of HCO3- per 1 mm Hg ↓ Pco2
or
HCO3- = 24 – 0.4X Δ PCO2

7. Metabolic acidosis
ΔPCO2 = 1.3 X Δ HCO3-
Or
PCO2 = (1.5 X HCO3-) + 8 ± 2

8. Metabolic alkalosis
ΔPCO2 = 0.6 X Δ HCO3-
Or
PCO2 = (0.7 X HCO3-) + 21 ± 2

9. • Acid – base interpretation is based on
three variables - pH, PCO2 & HCO3
-
• If outside normal range, it is abnormal
Normal range
pH 7.35 to 7.45
PaCO2 35 to 45 mm of Hg
HCO3
- 22 to 26 mEq/L

10. If pH ‹ 7.35 - acidosis
If pH › 7.45 - alkalosis

11. Look at Pco2 and HCO3-
Primary Change Acid Base Disorder Secondary Change
 PaCO2 (› 45mmhg ) Respiratory Acidosis  HCO3
-
 PCO2 (‹35mmhg) Alkalosis  HCO3
-
 HCO3
-(‹22mmol/L) Metabolic Acidosis  PCO2
 HCO3
-(›26mmol/L) Alkalosis  PCO2

12. If primary respiratory disorder
 Acute: Expected pH= 0.008 × ∆
Pco2
 Chronic: Expected pH= 0.003 × ∆
Pco2

13. Respiratory acidosis
Ac : ↑0.1 mmol of HCO3- per 1 mm Hg ↑ Pco2
or
HCO3- = 24 + 0.1 X Δ PCO2
Chr : ↑ 0.35 mmol of HCO3- per 1 mm Hg ↑
Pco2
or
HCO3- = 24 + 0.35 X Δ PCO2
Expected
change in
HCO3-

14. Respiratory alkalosis
Acute: ↓ 0.2 mmol of HCO3- per 1 mm Hg ↓ Pco2
or
HCO3- = 24 – 0.2X Δ PCO2
Chronic: ↓ 0.4 mmol of HCO3-per 1 mm Hg ↓ Pco2
or
HCO3- = 24 – 0.4X Δ PCO2
Expected
change in
HCO3-

15. Metabolic acidosis
ΔPCO2 = 1.3 X Δ HCO3-
or
PCO2 =(1.5 X HCO3-) + 8 ± 2
Metabolic alkalosis
ΔPCO2 = 0.6 X Δ HCO3-
or
PCO2 = (0.7 X HCO3-) + 21 ± 2
Expected
Change
in
pCO2

17.  ABG :
pH
HCO3
PCO2
 Obtain a minimum diagnosis
Electrolyte (Na+, K+,Cl-
and HCO3)
Calculate the Anion
gap
K+
SO2
Hb.
 Complete the diagnosis
 Match with the clinical diagnosis and
treat accordingly
Step-5 :
Calculate the Anion gap
a. Check the anion gap (AG) :
AG =Na+ -(HCO3 + Cl- )
(normal = 12 ± 2)
Elevated AG = Acidosis

18. Step 5 :
b. Compare fall in HCO3 with increase in plasma anion
gap.
 i) In high AG metabolic acidosis,
rise in the plasma AG (AG - 12)
matches with fall in serum HCO3 ( 24 – HCO3)
(Rise in AG = Fall in HCO3)
 ii) If increase in AG exceeds the fall in HCO3 (Rise in
AG > Fall in HCO3), it suggests co-existing
metabolic alkalosis.
 iii) If increase in AG is lesser than the fall of
HCO3 (Rise in AG < Fall in HCO3), it suggests loss
of HCO3 (diarrhoea) causing non-AG metabolic
acidosis.

19.  AG = Na+ – (Cl- + HCO3
-)=Normal value is 12 ± 2
 Compare the rise in AG with the decrease in HCO3-is Useful
to identify additional or hidden metabolic disorders
1. change(rise) in AG = the decrease in HCO3= simple
metabolic acidosis
2.change(rise) in AG < the decrease in HCO3= hidden
metabolic alkalosis,
3.change(rise) in AG > the decrease in HCO3= hidden non
AG acidosis

20.  Actual PCO2 = Expected PCO2  respiratory
alkolosis
 Actual PCO2 > Expected PCO2  hidden non AG
acidosis
 Actual PCO2 < Expected PCO2  hidden metabolic
alkalosis

21. Q. A 19 years old boy brought in emergency with
history of injecting some drug. O/E respiration
shallow 8/mt, responded to painful stimuli.
Arterial blood gases shows:
pH 7.14
PaO2 86 mm of Hg
PaCO2 72 mm of Hg
ABC 26 mmol/L
BE 1.4 mmol/L

22. pH 7.14  Acidosis
PaCO2 72 mm of Hg  Respiratory
ABC 26 mmol/L
 0.008 X (72-40) = 0.256  7.4 – 0.256 = 7.144
 0.003 X (72-40) = 0.096  7.4 – 0.096 = 7.304
 HCO3- = 24 + 0.1 X Δ PCO2 = 24+ 0.1(72-40) = 27.2
 Uncompensated
 Uncompensated Acute Respiratory Acidosis
Acute
 Not Chr.

23. Q. A 67 years old male with H/O cough and
respiratory distress and chronic smoking came in
emergency :
Arterial blood gases shows:
pH 7.34
PaO2 76 mm of Hg
PaCO2 60 mm of Hg
ABC 31 mmol/L
BE +4 mmol/L

24. pH 7.34  Acidosis
PaCO2 60 mm of Hg  Respiratory
ABC 31 mmol/L
 0.008 X (60-40) = 0.16  7.4 – 0.16 = 7.24
 0.003 X (60-40) = 0.06  7.4 – 0.06 = 7.34
 HCO3- = 24 + 0.35 X Δ PCO2 = 24+ 0.35(60-40) =
31
 Compensated
 Compensated Chronic Respiratory Acidosis
Not Acute
 Chronic

25. Q. A 18 years old girl was admitted in the hospital
after an argument with her boy friend. She
denied taking any medication. On examination
chest clear, respiratory rate 34 / mt. Blood gas
estimation shows :
pH 7.51
PaO2 82 mm of Hg
PaCO2 26 mm of Hg
ABC 24 mmol/L
BE -1 mmol/L

26. pH 7.51  Alkalosis
PaCO2 26 mm of Hg  Respiratory
ABC 24 mmol/L
 0.008 X (40-26) = 0.112  7.4 + 0.112 = 7.512
 0.003 X (40-26) = 0.042  7.4 + 0.042 = 7.442
 HCO3- = 24 – 0.2X Δ PCO2 = 24- 0.2(40-26) = 21.2
 Uncompensated
 Uncompensated Acute Respiratory Alkalosis
Acute
 Not Chr.

27. Q. A 52 years old man brought in emergency with
history of cough and pleuritic chest pain for few
days increasingly becoming breathless:
Arterial blood gases shows:
pH 7.47
PaO2 67 mm of Hg
PaCO2 14 mm of Hg
ABC 15 mmol/L
BE -8.2 mmol/L

28. pH 7.47  Alkalosis
PaCO2 14 mm of Hg  Respiratory
ABC 15 mmol/L
 0.008 X (40-14) = 0.208  7.4 + 0.208 = 7.608
 0.003 X (40-14) = 0.078  7.4 + 0.078 = 7.478
 HCO3- = 24 – 0.4X Δ PCO2 = 24- 0.4(40-14) = 13.6
 Uncompensated
 Uncompensated Chronic Respiratory Alkalosis
Not Acute
 Chronic

29. Q.60 year old admitted with gluteal abscess,
known diabetic
pH 7.18
PCO2 18
HCO3- 9
Na 138
K 4.1
Cl 110
AG 23.1

30.  pH <7.4  acidosis
 Metabolic  low HCO3-
 Expected PCO2 = 1.5 X HCO3- + 8 = 1.5 X 9 + 8 =
21.5
 Actual PCO2 (18) < Expected PCO2 (21.5) 
respiratory alkalosis
 Diag : Metabolic acidosis with respiratory alkalosis
 Increased AG (23) metabolic acidosis
 Clinical Diag : Diabetic ketoacidosis with sepsis

31. Q.50 – year old with CRF, smoker
pH 7.1
PCO2 50
HCO3- 15
Na 140
K 5
Cl 105
AG 23

32.  pH <7.4  acidosis
 Metabolic  low HCO3-
 Expected PCO2 = 1.5 X 15 + 8 = 30.5
 Actual PCO2 (50) > Expected PCO2 (30.5) 
respiratory acidosis
 Diag : Metabolic acidosis with respiratory acidosis
 Increased AG (23) metabolic acidosis
 Clinical Diag : CRF with COPD and increased AG
metabolic acidosis

33. Q.35 – year old, collapsed on physical exertion was
brought to hospital
pH 6.99
PCO2 34
HCO3- 8
Na 141
K 6
Cl 105
AG 28

34.  pH <7.4  acidosis
 Metabolic  low HCO3-
 Expected PCO2 = 1.5 X 8 + 8 = 20
 Actual PCO2 (34) > Expected PCO2 (20) 
respiratory acidosis
 Diag : Metabolic acidosis and respiratory acidosis
with hyperkalemia
 Increased AG (28) metabolic acidosis
 Clinical Diag : Cardiorespiratory arrest

35. Q.70 – kg healthy male had acute airway
obstruction during induction of anaesthesia
pH 7.1
PCO2 70
HCO3- 21

36.  pH <7.4  acidosis
 Respiratory  high PaCO2 (>40)
 Acute hypercapnia – HCO3- increases 0.1 mmol for
every increase in 1 mm Hg of PCO2 above 40 mm Hg
 HCO3- should be 3 mmol above 24, if PCO2 only is
changed
 Predicted HCO3- is 27, but actual is 21  deficit of 6
mmol/ L
 Combined respiratory and metabolic acidosis

37. Q.56 – year old male patient, with COPD
having resting PCO2 of 70 sustains
perioperative MI, BP is 80/60 mm Hg, is
sweating, cool and clammy
pH 7.1
PCO2 70
HCO3- 21

38.  pH <7.4  acidosis
 Respiratory  high PaCO2(>40)
 Chronic hypercapnia – HCO3- increases 0.4 mmol for
every increase in 1 mm Hg of PCO2 above 40 mm Hg
 Predicted HCO3- is 36, but actual is 21  deficit of 15
mmol/ L
 Combined respiratory and metabolic acidosis
 Metabolic component much greater than previous case

39. Step 1: Acidemic, alkalemic, or normal?
Step 2: Is the primary disturbance respiratory or metabolic?
Step 3: For a primary respiratory disturbance, is it acute or
chronic?
Step 4: For a metabolic disturbance, is the respiratory system
compensating OK?
Step 5: For a metabolic acidosis, is there an increased anion
gap?
Step 6: For an increased anion gap metabolic acidosis, are
there other derangements?

40. Useful Formulas for Acid Base
Interpretations
Metabolic Acidosis Expected PaCO2 = (1.5 × HCO3) + (8±2)
PRIMARY DISORDER EXPECTED RESULTS
Metabolic Alkalosis Expected PaCO2 = (0.7 × HCO3) + (21±2)
Acute Respiratory
Acidosis
Acute Respiratory
Alkalosis
Chronic Respiratory
Acidosis
Chronic Respiratory
Alkalosis
DpH = 0.003 × DPaCO2
Expected pH = 7.40 + [0.003 × (40 – PaCO2)]
DpH = 0.003 × DPaCO2
Expected pH = 7.40 – [0.003 × (PaCO2 – 40)]
DpH = 0.008 × DPaCO2
Expected pH = 7.40 + [0.008 × (40 – PaCO2)]
DpH = 0.008 × DPaCO2
Expected pH = 7.40 - [0.008 × (PaCO2 – 40)]

41. ABG analysis is the stethoscope of
ICU and is an integral part of
management of critical care patients,
however, one should always
remember that
‘Treat the patient, not the

42. THANK YOU

43. Q. A 66 years old woman had cardiac arrest in
casualty. Basic and advanced life support was
started in casualty. While patient was being
ventilated blood sample was taken:
Arterial blood gases shows:
pH 6.86
PaO2 6o mm of Hg
PaCO2 38.9 mm of Hg
ABC 6.6 mmol/L
BE - 23.9 mmol/L
Ans :Metabolic Acidaemia

44. Q. A 33 years old man with insulin dependent
diabetes brought to emergency department not
well for 3 days c/o frequency of urination, fever,
nausea, sweating had not been eating hence not
taking insulin:
Arterial blood gases shows:
pH 7.2
PaCO2 22 mm of Hg
ABC 5.4 mmol/L
SBC 7.5 mmol/L
BE -26 mmol/L
Ans :Metabolic Acidaemia with poor compensation

45. Q. A 68 years old man c/o severe chest pain and
shortness of breath. O/E cold and clammy R/R 8
per mt. brought in emergency
Arterial blood gases shows:
pH 6.99
PaCO2 109.5 mm of Hg
ABC 15.5 mmol/L
SBC 15.3 mmol/L
BE -12.4 mmol/L
Ans :Combined respiratory and metabolic Acidaemia

46. Q. A 40 years old woman who had taken overdose
of Dothiepin (a tricyclic antidepressant) 2 hrs
previously. She was unconscious with a shallow
breathing RR 8 / mt.
Arterial blood gases shows:
pH 6.76
PaCO2 46.5 mm of Hg
ABC 6.5 mmol/L
SBC 6.2 mmol/L
BE -29 mmol/L
Ans :Mixed Respiratory and Metabolic Acidaemia

47. Q. A patient admitted in surgical ward was on
gastric aspiration through Ryle’s tube because of
persistent vomitings:
Arterial blood gases shows:
pH 7.5
PaCO2 50 mm of Hg
ABC 35 mmol/L
SBC 32 mmol/L
BE +8 mmol/L
Ans : Metabolic Alkalaemia

48. Q. A 26 years old man having vomiting and
diarrhoea from 4 days came in emergency:
Arterial blood gases shows:
pH 7.48
PaCO2 43 mm of Hg
ABC 31 mmol/L
SBC 30 mmol/L
BE +4.5 mmol/L
Ans :Metabolic Alkalaemia

49. Q. A 51 years old woman presented in emergency
department c/o of shortness of breath. The
symptoms were present from years. She had no
other medical problems:
Arterial blood gases shows:
pH 7.48
PaCO2 17.9 mm of Hg
ABC 12.5 mmol/L
SBC 14.5 mmol/L
BE -6.6 mmol/L
Ans :Chronic Respiratory Alkalaemia

50. Q. A 30 years old lady attempted to commit suicide by
throwing herself into a canal having taken overdose of
alcohol . Passing policeman saved her & brought her to
casualty. O/E she was pale, cold and confused. Her
tympanic temperature was 28.40C, R/R 18/mt, PR
108/mt, BP 136/90 chest – bilateral crepts and ronchi.
Arterial blood gases on breathing room air shows:
pH 7.23
PaCO2 41.5 mm of Hg
ABC 16.7 mmol/L
SBC 16.4 mmol/L
BE -10.2 mmol/L
PaO2 86.0 mm of Hg
Ans :Combined metabolic and respiratory acidaemia

51. Q. A patient having fever since 3 days, having
some respiratory difficulty was admitted with
PR=130/ minute and BP = 100/ 60 mm Hg
pH 6.56
PaO2 60 mm of Hg
PaCO2 80 mm of Hg
ABC 25 mmol / L
Ans: IS IT COMPATIBLE?
TREAT THE
PATIENT
NOT THE ABG!