arterial blood gas analysis calculation of anion gap respiratory acidosis respiratory alkalosis metabolic acidosis metabolic alkalosis

1. Dr. Anand Bhabhor
Senior Intensivist
Jaslok Hospital & Research Centre

2.  ABG analysis is an essential component of diagnosing
and managing critically ill patients in the ICU.
 A step wise approach to the evaluation of these
disorders helps delineate underlying causes,
compensatory mechanism, and the correct approach
to management.

4. Range
For calculation
pH
7.35 – 7.45
7.4
PaCO2
35 – 45 mmHg
40 mmHg
PaO2
> 80 mmHg
> 95 mmHg
HCO3-
22 – 26 mEq/L
24 mEq/L

5.  PaO2 and SaO2
PaO2 / FiO2 ratio
e.g. PaO2 :84 mmHg with 0.21 FiO2 [room air]
PaO2 / FiO2 = 84 / 0.21 = 400
 Normal = 300 – 500 mmHg
 < 300 = acute lung injury [previous definition]
 < 200 = ARDS [previous definition]
Berlin definition:
 200 – 300 [with PEEP/CPAP > 5] = mild ARDS
 < 200 [with PEEP > 5]
= moderate ARDS
 <100 [with PEEP > 5]
= severe ARDS

6.  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?

7.  The pH is the –log [H]. So by altering either the PCO2
or the HCO3-, [H] will change, and so will pH.
 An acidemia(low pH) can result from either a low
HCO3- or a high CO2
 An alkalemia (high pH) can result from either a high
HCO3 or a low CO2

8.  If pH and PaCO2 changes in same direction, the
primary disorder is metabolic and if they change in
opposite direction ,the primary disorder is respiratory.
Chemical
change
Primary
disorder
Compensation
pH
Low HCO3-
Metabolic
acidosis
Respiratory
alkalosis
low pH
High HCO3-
Metabolic
alkalosis
Respiratory
acidosis
High pH
High PaCO2
Respiratory
acidosis
Metabolic
alkalosis
Low pH
Low PaCO2
Respiratory
alkalosis
Metabolic
acidosis
High pH

9.  Respiratory acidosis is due to a primary rise in CO2
 Hypercapnia almost always results from alveolar
hypoventilation due to one of the following causes:
Respiratory center depression
2. Neuromuscular disorder
3. Upper airway obstruction
4. Pulmonary disease
1.

10.  A respiratory alkalosis is due to decrease in PaCO2.
 It results from hyperventilation leading to decrease in
CO2.
 Causes of respiratory alkalosis:
Hypoxemia from any causes
2. Respiratory center stimulation
3. Mechanical hyperventilation
4. Sepsis, pain
1.

11.  Normal pH is 7.4
 Calculate the change in pH (from 7.4)
A.
in acute respiratory disorder (acidosis / alkalosis)
change in pH = 0.008 X [PaCO2 -40]
expected pH = 7.4 +/-change in pH
B. in chronic respiratory disorder (acidosis/alkalosis)
change in pH = 0.003 X [PaCO2 -40 ]
expected pH = 7.4 +/- change in pH
Compare the pH on ABG
if pH on ABG is close to A, it is acute disorder
if pH on ABG is close to B, it is chronic disorder

12.  M/60 yrs, k/c/o C.O.P.D. admitted with U.T.I.
 ABG: 7.26 / 84 / 74 / 37 / 94%
 [A]. For Acute change in pH;
change in pH = 0.008 X [84 – 40 ] =0.008 X [44] =0.35
Expected pH = 7.4 – 0.35 = 7.05
 [B]. For chronic change in pH’
Change in pH = 0.003 X [84 – 40 ] = 0.003 X [44] = 0.13
Expected pH =7.4 -0.13 = 7.27
So B is near to the patient’s ABG which is 7.26; so primary disorder
is chronic respiratory acidosis.

13. Primary
disorder
Initial Compens Compensa
chemic a-tory
al
response tory
change
mechanis
s
m
Respiratory
acidosis
Expected level of compensation
↑PCO2 ↑HCO3-
Acute
Bufferingrule of 1
↑[HCO3-] = 1 mEq/L for every
10 mmHg delta PCO2
Chronic
Generation
of new
HCO3 –
rule of 3
[HCO-] = 3 mEq/L for every
10mmHg delta PCO2

14. Primary
disorder
Initial
chemic
al
change
s
Compen
sa-tory
respons
e
Compensatory
mechanism
Expected level of
compensation
Respiratory
alkalosis
↓PCO2 ↓HCO3-
Acute
buffering- rule
of 2
↓[HCO-] = 2 mEq/L for
every 10 mmHg delta PCO2
chronic
Decreased
reabsorption
of HCO3- rule
or 4
↓[HCO3-] = 4 mEq/L for every
10 mmHg delta PCO2

15.  Metabolic acidosis results from a primary decrease in
plasma [HCO3-]
 Check the respiratory compensation by winter’s
formula:

16. Primary
disorder
Initial
chemica
l
changes
Compens Compensaa-tory
tory
response mechanism
Expected level of
compensation
Metabolic
acidosis
↓HCO3-
↓PCO2
Hyperventilation
Metabolic
alkalosis
↑HCO3-
↑PCO2
Hypoventilation PCO2 = 0.7 X [HCO3] +
21+/- 2
PCO2 = 1.5 X [HCO3-]+
8 +/-2

17.  7.23 / 34 /88 /17 [ metabolic acidosis]
 Winter ‘s formula:
 Expected PaCO2 = 1.5 X [HCO3-] + 8 +/- 2
= 1.5 X [17] + 8 +/ -2
= 33.5 + / -2 = 31.5 – 35.5 mmHg
 7.45 / 43 / 95 / 30 [ metabolic alkalosis]
 Winter ‘s formula:
 Expected PaCO2= 0.7 X [HCO3-] + 21 + / - 2
= 0.7 X [30] + 21 + / - 2
=42 + / -2 = 40 – 44 mmHg

18.  Metabolic alkalosis reflects an increase in plasma
[HCO3-]
 It can be classified into saline responsive or
nonresponsive.
 More than 20 mEq/L urinary chloride is saline
unresponsive and less than 20 mEq/L is saline
responsive.

19. Causes of Urine CL > 20 mEq/L
Causes of Urine CL < 20 mEq/L
Mineralocorticoid excess
Vomiting, nasogastric suctioning
K + and Mg ++ deficiency
chloride-wasting diarrhea
Liddle’s syndrome
Villous adenoma of colon
Barter’s syndrome
Posthypercapnia
Hypercalcemia with secondary
hyperparathyroidism
Poorly reabsorbed anions like
carbenicillin
Milk – alkali syndrome
Diuretic therapy

20.  Calculate AG in case of metabolic acidosis
 High denotes raised AG metabolic acidosis, and
normal or narrow denotes non-AG acidosis.

21.  It is used to determine if a metabolic acidosis is due to
an accumulation of non- volatile acids [e.g. lactic
acidosis] or a net loss of bicarbonate [e.g. diarrhea]
 Na + UC = [ Cl + HCO3 ] + UA
 UA – UC [ Anion gap] = Na –[ Cl + HCO3- ]
 AG= Na –[Cl + HCO3]; normal AG is 12+/-2 mEq/L

22. Unmeasured Anions
Unmeasured Cation
Albumin: 15 mEq/L
Calcium: 5 mEq/L
Organic Acids: 5 mEq/L
Potassium: 4.5 mEq/L
Phosphate: 2 mEq/L
Magnesium: 1.5 mEq/L
Sulfate: 1 mEq/L
Total UA: 23 mEq/L
Total UC: 11 mEq/L
Anion AG = UA – UC = 12 mEq/L
Adjusted AG = calculated AG + 2.5 X [4 – S.albumin gm%]

23.  7.23 / 34 /88 /17 : Metabolic Acidosis
 Na : 135 / Cl: 99 / K: 3.5
 AG = Na - [ Cl + HCO3-] = 135 – [ 99 + 17] = 19
 High AG

24. Pneumonic
Causes
M
Methanol
U
Uremia
D
Diabetic ketoacidosis
P
Paraldehyde
I
Isoniazid / iron
L
Lactate
E
Ethanol, ethylene glycol
R
Rhabdomyolysis / renal failure
S
Salicylate / sepsis

25. Pneumonic
Causes
H
Hyper alimentation
A
Acetazolamide
R
Renal tubular acidosis
D
Diarrhea
U
Uremia (acute)
P
Post ventilation hypocapnia

26.  Check urinary AG in non-AG metabolic acidosis
 U Na + U K – U Cl
 Normal :
negative
 Non-renal loss of bicarbonate [diarrhea] : negative
 Renal loss of bicarbonate[ RTA /
H+ excretion]
: positive

27.  In less obvious cases, the coexistence of two metabolic
acid-base disorders may be apparent by calculating the
difference between the change in AG [delta AG] and
the change in serum HCO3- [delta HCO3-].
 e.g. Diabetic ketoacidosis
 This is called the Delta gap or gap –gap.

28.  Delta gap = delta AG – delta HCO3 Where delta AG = patient’s AG – 12 mEq/L
 Delta HCO3- = 24 mEq/L – patient’s HCO3 Normally the delta gap is zero :
 AG acidosis
 A positive delta gap of more than 6 mEq/L :
 metabolic alkalosis and/or HCO3- retention.
 The delta gap of less than 6 mEq/L :
 Hypercholremic acidosis and/or HCO3- excretion.

29.  7.23 / 34 /88 /17 : Metabolic Acidosis
 Na : 138 / Cl: 99 / K: 3.5
 AG = Na - [ Cl + HCO3-] = 138 – [ 99 + 17] = 22
Next step is to calculate the Delta Gap.
 Delta AG = patient’s AG -12 = 22 – 12 = 10
 Delta HCO3- = 24 – patient’s HCO3- = 24 – 17 = 7
 Delta gap = Delta AG- Delta HCO3- = 10 – 7 = 3
 Additional metabolic alkalosis is also present with high AG
metabolic acidosis.

30.  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?

31.  65 years old male with CKD presenting with nausea,
diarrhea and acute respiratory distress
 ABG: 7.23/17/235/7 with 50% FiO2 on V.M.
 Electrolytes: Na: 123 mEq/L, Cl: 97 mEq/L, S.K 3.5
 Renal function: S. Creat: 5.1 mg%, BUN: 119

32.  Acidemic ?
 Alkalemic ?
 Normal ?

33.  Respiratory / metabolic ?
 ABG: 7.23/17/235/7 with 50% FiO2 on V.M.
 pH and PaCO2 goes in same direction; so it is
primarily metabolic disorder.

34.  ABG: 7.23/17/235/7 with 50% FiO2 on V.M.
 Winter’s formula:
 Expected PaCO2 = 1.5 X [7] + 8 +/- 2 = 18.5 +/-2

35.  ABG: 7.23/17/235/7 with 50% FiO2 on V.M.
 Electrolytes: Na: 123 mEq/L, Cl: 97 mEq/L, S.K 3.5
 AG = Na – [Cl +HCO3-] = 123 – [97 + 7] = 19
 High AG metabolic acidosis
 Delta gap = Delta AG – Delta HCO3-
= [19 - 12 ] – [24 – 7 ]

=7–7=0
 Non –anion gap metabolic acidosis


36.  High AG metabolic acidosis with non-anion metabolic
acidosis

37.  High AG metabolic acidosis is due to BUN 119
 Non anion metabolic acidosis is due to diarrhea

38.  Very sick 56 year old man being evaluated for a




possible double lung transplant
Dyspnoea on minimal exertion
On home oxygen therapy
(nasal prongs, 2 lpm)
Numerous pulmonary medications
ABG: 7.30/65/88/31.1

39.  Acidemia ?
 Alkalemia ?
 Normal ?

40.  pH and PaCO2 goes in opposite direction; so primary
disorder is respiratory.

41.  For Acute disorder;
 Change in pH = 0.008 X [65 – 40 ]=0.2
 Expected pH = 7.40 – 0.2 = 7.20
 For chronic disorder;
 Change n pH= 0.003 X [65 – 40 ] = 0.07
 Expected pH = 7.40 – 0.07 = 7.33
 So its chronic respiratory acidosis.

42.  delta PaCO2 = 25
 So rise in HCO3- will be by 7.5 mEq/L, which should
be 24 + 7.5 = 31.5 mEq/L
 Which is expected HCO3-

43.  Primary chronic respiratory acidosis.

44.  Patient has a long standing pulmonary disease so
bicarbonate is in the compensation.

45.  A 44 year old moderately dehydrated man was
admitted with a two day history of acute severe
diarrhea.
 ABG: 7.31 / 33 / 88 /16 / 95%
 Elect: Na: 136 mEq/L, Cl: 103 mEq/L, K: 2.9 mEq/L

46.  Acidemia ?
 Alkalemia ?
 Normal ?

47.  pH and PaCO2 goes in the same direction; so primary
disorder is metabolic.

48.  Winter’s formula
 Expected PaCO2= 1.5 X [16] + 8 +/ -2

= 32 -/+ 2 = 30 – 34 mEq/L
 So it is fully compensated metabolic disorder.

49.  AG = Na – [Cl + HCO3-] = 134 – [104 + 16 ] = 14

50.  Non AG metabolic acidosis

51.  Patient lost bicarbonate in diarrhea leading to non
anion gap metabolic acidosis.

52.  A 38-year-old woman is 12 weeks pregnant. For the last 10
days she has had worsening nausea and vomiting. When
seen by her physician, she is dehydrated and has shallow
respirations. Arterial blood gas data is as follow
 ABG: 7.56/54/110/45

53.  Acidemia ?
 Alkalemia ?
 Normal ?

54.  pH and PaCO2 goes in same direction so the primary
disorder is metabolic alkalosis.

55.  Expected PaCO2 = 0.7 X [ HCO3-] + 21 +/- 2
= 0.7 X [ 45 ] + 21 +/ - 2
= 52.5 +/-2
= 50.5 – 54.5

56.  Metabolic alkalosis

57.  Patient has lost lot of gastric juice and hydrochloric
acid in vomit leading to metabolic alkalosis.

58.  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?

60. THANK YOU ! !!