4. PaO2 and SaO2
PaO2 / FiO2 ratio
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
5. 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?
6. The hydrogen ion concentration [H+] in extracellular fluid
is determined by the balance between the PaCO2 and
HCO3- in the fluid.
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
7. 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 pH
Low HCO3- Metabolic acidosis low pH
High HCO3- Metabolic
alkalosis
High pH
High PaCO2 Respiratory
acidosis
Low pH
Low PaCO2 Respiratory
alkalosis
High pH
8. Respiratory acidosis is due to a primary rise in CO2
Hypercapnia almost always results from alveolar
hypoventilation due to one of the following causes:
1. Respiratory center depression
2. Neuromuscular disorder
3. Upper airway obstruction
4. Pulmonary disease
9. A respiratory alkalosis is due to decrease in PaCO2.
It results from hyperventilation leading to decrease in
CO2.
Causes of respiratory alkalosis:
1. Hypoxemia from any causes
2. Respiratory center stimulation
3. Mechanical hyperventilation
4. Sepsis, pain
10. 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
11. 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.
17. 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.
18. 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
19. A1 : calculate AG in case of metabolic acidosis
High denotes raised AG metabolic acidosis, and
normal or narrow denotes non-AG acidosis.
20. 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%]
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. 7.23 / 34 /88 /17 : Metabolic Acidosis
Na : 135 / Cl: 99 / K: 3.5
AG = Na - [ Cl + HCO3-] = 135 – [ 99 + 17] = 19
High AG
23. 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
24. Pneumonic Causes
H Hyper alimentation
A Acetazolamide
R Renal tubular acidosis
D Diarrhea
U Uremia (acute)
P Post ventilation hypocapnia
25. 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
26. 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-].
This is called the Delta gap or gap –gap.
27. 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.
28. 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.
29. 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?
30. 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. 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.
33. 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
34. 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
35. High AG metabolic acidosis with non-anion metabolic
acidosis
36. High AG metabolic acidosis is due to BUN 119
Non anion metabolic acidosis is due to diarrhea
37. 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
43. Patient has a long standing pulmonary disease so
bicarbonate is in the compensation.
44. 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
50. Patient lost bicarbonate in diarrhea leading to non
anion gap metabolic acidosis.
51. 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
56. Patient has lost lot of gastric juice and hydrochloric
acid in vomit leading to metabolic alkalosis.
57. 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?
