Interpreting Blood Gases, Practical and easy approach

ABGs or VBGs interpretation made simple straight forward easy to remember and easy to apply. The presentation is designed to help the residents and junior ER physicians. The second part will discuss the oxygenation and the third part will review the "Stewart Approach" while fourth and last part is meant for the Experts.

1. Interpreting
ABGs
Practical Approach
Muhammad AsimRana
BSc, MBBS, MRCP, SF-CCM, FCCP, EDIC
Department of Adult Critical Care Medicine
KSMC, Riyadh

2. Venous Arterial

3. Arterial Blood Gases
•Written in following manner:
pH/PaCO
2/PaO
2/HCO
3
–pH = arterial blood pH
–PaCO
2 = arterial pressure of CO
2
–PaO
2 = arterial pressure of O
2
–HCO
3= serum bicarbonate concentration

4. Part 1
Acid-Base Disorders

5. Acid-Base
•Acidosis or alkalosis:
–any disorder that causes an alteration in pH
•Acidemiaor alkalemia:
–alteration in blood pH; may be result of one or
more disorders.

6. Some important concepts
•The determinants of extracellular fluid pH
indicate that tight control of the pH requires a
fairly constant PCO2/HCO3ratio.
•Thus, a change in one of the determinants
(PCO2or HCO3) must be accompanied by a
proportional change in the other determinant
to keep the PCO2/HCO3ratio (and the pH)
constant.

7. Some important concepts
•Thus, an increase in PCO2(respiratory
acidosis) must be accompanied by an increase
in HCO3 (metabolic alkalosis) to keep the pH
constant.
•This is how the control system for acid-base
balance operates.
•A respiratory disorder (change in PCO2) always
initiates a complementary metabolic response
(that alters the HCO3), and vice-versa

8. PrimaryDisorderPrimary ChangeCompensatory Change*
Respiratory acidosis Increased PCO2 Increased HCO3
Respiratory alkalosis Decreased PCO2 Decreased HCO3
Metabolic acidosis Decreased HCO3 Decreased PCO2
Metabolic alkalosis Increased HCO3 Increased PCO2
Primary Acid-Base Disorders and Associated Compensatory Changes
[H+] = 24 ×PCO2/HCO3
* Compensatory changes keep the PCO2/HCO3ratio constant.

9. Check if data is consistent
{H} = 24 [ PaCO2/HCO3]
{H} = (7.8 –pH) x 100
Each 0.01 unit change in pH {H} will change by 1mEq/L
{H} = 40+(delta pH) (1mEq/L)/0.01
pH--------------{H}
7.3---------------50
7.2---------------63
7.1---------------80
7.0--------------100
6.9--------------125
6.8--------------160

10. Check if data is consistent
{H} = 24 [PaCO2/HCO3]
{H} = (7.8 –pH) x 100
•The {H} in extracellular fluid normally varies less than
10 nEq/L
•The values of {H} should be within 10 for both
calculations !
•If it is beyond or more than 10 the blood gas analysis
is not interpretable.
•The reasons may include improper caliberationor
others

11. Here are some examples
•Written in following manner:
pH/PaCO
2/PaO
2/HCO
3
•7.8/36.6/76.4/55.4
•7.7/35.5/80.3/50.6
•7.54/53.1/63.7/44.6
24 x 36.6/55.4 = 15.85
7.8-7.8 x 100 = 0
The data is inconsistent
24 x 35.5/50.6 = 16.8
7.8-7.7 x 100 = 10
The data is inconsistent
24x53.1/44.6 = 28.57
7.8-7.54 x 100 = 26
The data is consistent

12. Case Study
•A 13 years old female presented in ER with
pain abdomen and drowsiness.
•Blood gas revealed
•6.87/20.6/88/3.7
•Na 140.4, K 4.41, Cl102

13. Step-wise Approach
1.Acedemiaor Alkalemia
2.Metabolic or Respiratory (Primary Pathology)
3.For metabolic is it anion gap or non anion gap.
4.For AG acidosis, are there other disturbances.
5.Respcompensation for the metabolic disturbances.
6.For respiratory disturbances is it acute or chronic.

14. Step 1: Acidemicor Alkalemic?
•Acidemic: PH < 7.35
•Alkalemic: PH > 7.45
An acid-base abnormality is present if either the PaCO2 or the
pH is outside the normal range.
(A normal pH or PaCO2does not exclude the presence of an
acid-base abnormality)

15. Type of disturbance
pH 6.87
Acidemia

16. Primary Acid-Base Disorders
•A change in either the PCO
2or the HCO
3will cause a
change in the [H
+
] of extracellular fluid.
•When a change in PCO
2is responsible for a change in
[H
+
], the condition is called a respiratoryacid-base
disorder
–an increase in PCO
2is arespiratory acidosis
–a decrease in PCO
2is a respiratory alkalosis.
•When a change in HCO
3is responsible for a change in
[H
+
], the condition is called a metabolicacid-base
disorder
–a decrease in HCO
3is ametabolic acidosis
–an increase in HCO
3is a metabolic alkalosis.

17. Step 2: Primary disturbance metabolic or Respiratory
If the pH and PaCO2 are both abnormal, compare the
directional change. If both change in the same
direction (both increase or decrease), the primary
acid-base disorder is metabolic, and if both change in
opposite directions, the primary acid-base disorder is
respiratory.
If either the pH or PaCO2 is normal, there is a mixed metabolic
and respiratory acid-base disorder (one is an acidosis and the
other is an alkalosis).
•If the pH is normal, the direction of change in PaCO2
identifies the respiratory disorder
•If the PaCO2is normal, the direction of change in the pH
identifies the metabolic disorder.

18. Type of disturbance
pH 6.87
Acidemia
Metabolic
pH: 6.8
PaCO2: 14.5

19. Step 3 : What is the Anion Gap
•Anion gap measures the difference between
Anions(-) and Cations(+) present in blood
•AG = Na –(Cl+ HCO
3)
•Normal Anion gap is 12 mEq/L

20. Anion Gap
Unmeasured Anions Unmeasured Cations
Proteins 15 mEq Calcium 5 mEq
Organic acid 5 mEq Potassium 4.5 mEq
Phosphate 2 mEq Magnesium 1.5 mEq
Sulfates 1 mEq
Total 23 mEq Total 11 mEq
Difference : 23 –11 = 12

21. Extra for the experts
•Albumin carries negative charge .
•Hypo-albuminemiacauses falsely low AG.
•To correct for that
•AG
adjusted = AG
Observed+ 0.25 ×(4.5 –pt’salb)
Other causes of low AG
Paraproteinemia,
Bromism,
lithium toxicity,
Profound hypocalcemia,
hypomagnesemia
hyponatremia

22. Extra for the experts
•In metabolic alkalosis AG can be high but it
could be due to unmeasured anions,
specifically the albumin.

23. Type of disturbance
pH 7.10
Acidemia
Metabolic
High AG
Anion Gap?
Na = 140.4
Cl= 104
HCO3= 3.7
AG = 140.4 -104 -3.7 = 32.3

24. Causes Of Anion Gap Acidosis
•Methanol
•Uremia
•DKA
•Paraldehyde
•INH
•Iron
•Lactic Acidosis
•Ethanol
•Ethylene Glycol
•Salicylic Acid
MUDPILES

25. Step 4: Is there other metabolic disturbances
coexisting with AG Acidosis
•In the presence of high AG metabolic acidosis, it is
possible that patient may have another metabolic
acid base disorder.
•A normal AG metabolic acidosis or a metabolic
alkalosis
•This can be discovered by comparing the AG excess
to the HCO3deficit.

26. Step 4: Is there other metabolic disturbances
coexisting with AG Acidosis
•Delta Anion Gap or ΔAG:
•Difference between measured and normal AG
–ΔAG = AG -12
•Delta HCO3 or ΔHCO3:
–Difference between measured and normal HCO3
–ΔHCO3 = 24 –Measured HCO3
Delta Anion Gap or ΔAG is sometimes simply called Δgap

27. Step 4: Is there other metabolic disturbances
coexisting with AG Acidosis
•If the disturbance is pure AG Acidosis
•ΔAG/ΔHCO3 = unity or 1
•In our example
•HCO3= 3.7 so
•ΔHCO3= 24 –3.7 = 20.3
•Now ΔAG
•AG = 32.3 so
•ΔAG = 32.3 –12 = 20.3
•ΔAG /ΔHCO3= 20.3/20.3 = 1.0
So this patient has pure high AG metabolic acidosis

28. Remember !
•If ΔAG /ΔHCO3<1.0
•The decrease in the HCO3is greater than the
increase in the AG and the ratio falls below 1
•It means there is accumulation of other acid
which does not affect the AG but causes a fall
in HCO3i.e. NON-AG Metabolic Acidosis

29. Remember !
•If ΔAG /ΔHCO3>1.0
•When alkali is added in the presence of high
AG acidosis, the decrease in serum HCO3is
less than the increase in the AG and the ratio
goes above 1
•Therefore, in the presence of high AG
metabolic acidosis a gap-gap ratio of greater
than 1 indicates co-existence of
–metabolic alkalosis

30. Concept of corrected HCO3
•Add rgap to measured HCO
3
–If new value becomes normal (22-26)
–There is no other metabolic problems
–If it still stays < 22, then there is concomitant
metabolic acidosis, non AG metabolic acidosis
–If it goes > 26, then there is concomitant
metabolic alkalosis

31. Revision
rgap + HCO3= N (Only one disorder i.e.↑AGMet Acid)
rgap + HCO3= >N (↑AG Met Acid + Meta Alk)
rgap + HCO3= <N (↑AG Met Acid + Nor AG Meta Acid)

32. Let us apply on our case
•Corrected HCO3= HCO3+ Δ AG
•Corrected HCO3= 3.7 + 20.3 = 24
•Perfect !!

33. In ↑AG metabolic acidosis
Extend your search further
To pin point the diagnosis

34. In case of high AG acidosis
•Always calculate Osmolargap:
•Osmgap = measured Osm–CalcOsm
•CalcOsm=
(2 x Na
+
) + (glucose/18) + (BUN/2.8)
Normal Osmgap < 10 mOsm/kg
•In areas where alcohol is common
•CalcOsm=
(2 x Na
+
) + (glucose/18) +(BUN/2.8) + (EtOH/4.6)

35. In case of high AG acidosis
↑AG acidosis but N osmolargap
•DKA
•Uremia
•Lactic acidosis
•Salisylates
↑AG acidosis and ↑osmolargap
•Ethanol
•Methanol
•EhyleneGlycol

36. Causes of non-Anion Gap Acidosis
•Hyper Alimentation
•Acetazolamide
•Renal Tubular Acidosis
•Diarrhea
•Ureterosigmoidostomy
•Pancreatic Fistula
•Primary Hyperparathyroidism
HARD-UP

37. Compensatory responses
•Compensatory responses are secondary
responses designed to limit the change in [H
+
]
produced by the primary acid-base disorder,
and this is accomplished by changing the
other component of the PaCO
2/HCO
3ratio in
the same direction.

38. Secondary Responses
•If the primary problem is an increase in
PaCO
2(respiratory acidosis)
–The secondary response will involve an increase in
HCO
3, and this will limit the change in [H
+
]
produced by the increase in PaCO
2.
•Secondary responsesshould not be called
“compensatory responses” because theydo
not completely correct the change in [H
+
]
produced by the primary acid-base disorder

39. Secondary/Compensatory responses
•If there is a primary metabolic acidosis or
alkalosis, use the measured HCO3to identify the
expected PaCO2.
•If the measured and expected PaCO2are
equivalent, the condition is fully compensated.
•If the measured PaCO2is higher than the
expected PaCO2, there is a superimposed
respiratory acidosis.
•If the measured PCO2is less than the expected
PCO2, there is a superimposed respiratory
alkalosis.
Metabolic Acidosis
ExpPaCO2 = 1.5 x HCO3+ 8 ±2
Metabolic Alkalosis
ExpPaCO2 = 0.7 x HCO3+ 21 ±2

40. Let’s see our case
pH 7.10
Acidemia
Metabolic
High AG
Compensated or ????
Winter’s Formula :
Expected PaCO2 = (1.5 x HCO3) + 8 ±2
Applying Winter’s Formula :
Expected PaCO2 = (1.5 x 3.7) + 8 ±2 = 13.5-15.5
So in our case it is :
Metabolic acidemiais compensated

41. Mixed Disorders
•If either the pH or PaCO2is normal, there is a
mixed metabolic and respiratory acid-base
disorder
–(one is an acidosis and the other is an alkalosis).
•If the pH is normal, the direction of change in
PaCO2identifies the respiratory disorder, and
if the PaCO2is normal, the direction of change
in the pH identifies the metabolic disorder.

42. Mixed Disorders
•If there is a respiratory acidosis or alkalosis,
use the PaCO2to calculate the expected pH
for respiratory acidosis or for respiratory
alkalosis.
•Compare the measured pH to the expected pH
to determine if the condition is acute, partially
compensated, or fully compensated.

43. Mixed Disorders
•For respiratory acidosis
–If the measured pH is lower than the expected pH
for the acute, uncompensated condition, there is a
superimposed metabolic acidosis
–If the measured pH is higher than the expected pH
for the chronic, compensated condition, there is a
superimposed metabolic alkalosis.

44. •For respiratory alkalosis
–If the measured pH is higher than the expected pH
for the acute, uncompensated condition, there is a
superimposed metabolic alkalosis
–If the measured pH is below the expected pH for
the chronic, compensated condition, there is a
superimposed metabolic acidosis.
Mixed Disorders

45. Formulae for secondary responses
Predicting Timing by pH change
Acute Respiratory Acidosis
Fall in pH or Δ pH = 0.008 x ΔPaCO2
Expected pH = 7.40 –[0.008 x ( PaCO2–40)]
Chronic Respiratory Acidosis
Fall in pH or Δ pH = 0.003 x ΔPaCO2
Expected pH = 7.40 –[0.003 x ( PaCO2–40)]
Acute Respiratory Alkalosis
Rise in pH or ΔpH = 0.008 x ΔPaCO2
Expected pH = 7.40 + [0.008 x ( 40 -PaCO2)]
Chronic Respiratory Alkalosis
Rise in pH or Δ pH = 0.003 x ΔPaCO2
Expected pH = 7.40 + [0.003 x ( 40 -PaCO2)]

46. Predicting Timing by response

47. Another way to cram compensatory responses
•Metabolic Acidosis HCO3 ↓----------------PaCO2↓
•PaCO2↓ by 1.3 for each 1 mEq↓in HCO3
•Metabolic Alkalosis ↑in HCO3------------------PaCO2↑
•PaCO2↑ by 0.7 for each 1 mEq↑in HCO3
•Acute Respiratory Acidosis ↑in PaCO2-----HCO3↑
•HCO3↑ by 1 mEqfor each 10 mmHg ↑in PaCO2
•Acute Respiratory Alkalosis ↓in PaCO2-----HCO3↓
•HCO3↓ by 2 mEqfor each 10 mmHg ↓in PaCO2
•Chronic Respiratory Acidosis ↑in PaCO2 ---HCO3 ↑
•HCO3 ↑ by 3.5 mEqfor each 10 mmHg ↑in PaCO2
•Chronic Respiratory Alkalosis ↓in PaCO2---HCO3↓
•HCO3↓ by 5mEq for each 10 mmHg ↓in PaCO2

48. Causes Of metabolic Alkalosis
•Volume contraction
–(Vomiting, diuresis, ascities)
•Hypokalemia
•Alkali ingestion
•Excess gluco-mineralocorticosteroids
•Bartter’s Syndrome

49. Let’s Solve
PH 7.02/PaCO
219/HCO
32.8, Na 141, Cl111
Acedmia
AG=141-111-3=27
Corrected HCO
3=
3 + (15) = 18
Additional
Non AG acidosis
PaCO2 = 1.5 ×3 + 8 ±2
= 12.5 ±2
RespAcidosis
Metabolic
Status
Met/ Resp
Anion Gap
Other disorder?
Compensation??

50. 7.50/21.9/88.7/20.3/98.2%
Alkalemia
Acute Respiratory Alk
ExpHCO
3=
24+0.2x40-21.9= 27.6
Respiratory
Status
Met/ Resp
Acute/Chronic
Compensation??
Acute Respiratory Alkalosis
Rise in pH or ΔpH
= 0.008 x ΔPaCO2
Expected pH =
7.40 + [0.008 x ( 40 -PaCO2)]
Chronic Respiratory Alkalosis
Rise in pH or Δ pH =
0.003 x 40 –ΔPaCO2
Expected pH =
7.40 + [0.003 x ( 40 -PaCO2)]
ΔHCO3 = 0.2 x ΔPaCO2
ExpHCO3 = 24 + [ 0.2 x (40 –PaCO2)]
Acute Respalkalosis with metabolic acidosis?
Acute Respalkalosis not yet compensated? PaCO2 = 0.7 x HCO3 + 21 ±2

51. 7.23/58/96/24
Acidosis
Respiratory
Δ PH= 0.008 x (58-40)
=0.08 x 1.8 = 0.144
Δ PH=0.003 x 18 = 0.054
PH=7.326PH=7.236
Chronic
Acute
Compensated or?
ExpHCO3 = 24 + [0.1(PaCO2-40)]
ExpHCO3 = 24 + [0.1(58-40)]
Answer = 25.8
Acute respacidosis
not yet fully compensated

52. 7.35/48/69/29
Mixed Disorder
Respiratory
Δ PH= 0.008 x (48-40)
=0.008 x 8 = 0.064
7.40 –0.064 =
Δ PH= 0.003 x (48-40)
=0.003 x 8 = 0.024
7.40 –0.024 =
PH=7.37PH=7.336
ChronicAcute
Compensated or?
ExpHCO3 = 24 + [0.4(PaCO2-40)]
ExpHCO3 = 24 + [0.4(48-40)]
Answer = 27.2
Chronic compensated respacidosis
With metabolic alkalosis

53. More Examples

54. 7.27/87.4/83.5/40.1
•Acidemia
•Respiratory
•Acute or Chronic ?
•ΔpH:
–Acute: 0.008 x ΔPaCO2 = 0.008 x 47 = 0.379
–Expected pH = 7.40 –0.379 = 7.021
–Chronic: 0.003 x ΔPaCO2 = 0.003 x 47 = 0.142
–Expected pH = 7.40 –0.142 = 7.258
•Chronic Respiratory Acidosis
•Compensation:
–3.5 x 47 / 10 = 16.59
–Expected HCO3 = 24 + 16.59 = 40.59
For each 10 mmHg CO2 rise
HCO3 rises by 3.5

55. 7.24/62/58/22
•Acidemia
•Primary …. Respiratory acidosis as PaCO2↑
•Acute or Chronic ?
•ΔpH:
–Acute: 0.008 x ΔPaCO2 = 0.008 x 22= 0.176
–Expected pH = 7.40 -0.176 = 7.224
–Chronic: 0.003 x ΔPaCO2 = 0.003 x 22 = 0.066
–Expected pH = 7.40 + 0.066 = 7.334
•So it is Acute Respiratory Acidosis
•Compensation for acute respacidosis
–Expected ↓ in HCO3= 22/10 = 2.2
–Expected HCO3= 24 –2.2 = 21.8
HCO3will fall by 1 with each
10 mmHg rise in CO2

56. 7.365/22/110/12.3
•Mixed Disorder ….. pH (N) and CO2↓
•Respiratory alkalosis as PaCO2↓
•ΔpH:
–Acute: 0.008 x ΔPaCO2 = 0.008 x 18 = 0.114
–Expected pH = 7.40 + 0.114 = 7.514
–Chronic: 0.003 x ΔPaCO2 = 0.003 x 18 = 0.054
–Expected pH = 7.40 + 0.054 = 7.454
•In both cases the pH should be higher than what we have !!
•So there is concomitant metabolic acidosis !!
•Expected HCO3for respiratory alkalosis
–Expected HCO3for acute 24 -3.6 = 20.4
–Expected HCO3for chronic 24 -9= 15
Acute: HCO3 ↓by 2 for each
10 mmHg ↓ in CO2
Chronic: HCO3 ↓by 5 for
each 10 mmHg ↓ in CO2
Although the last calculation is not required !!

57. Thank you
See you in part 2
Oxygenation Status