Arterial Blood Gases (ABG) interpretation, a simplified approach

ABG interpretation, a simplified approach for ER, ICU, Internal Medicine, Pulmonary Medicine Physicians.

1. ABG INTERPRETATION
A SIMPLIFIED PRACTICAL APPROACH
DR ADEL HAMADA
Assistant Consultant Pulmonary Medicine (KAMC)
Lecturer of Pulmonary Medicine (ZagazigUniversity, EGYPT)
MD Chest Diseases
European Diploma in Intensive Care Medicine

2. Accurate and timely interpretation of an acid–base disorder
can be lifesaving.
Establishment of correct diagnosisof acid base
status (specially in mixed acid base disorders)
may be challenging.
In spite of that

3. By the end of this presentation we will:
Know the basic principles of ABG interpretation
Correlate the ABG data to the clinical context of the
patients.
Consider the concept of mixed acid base disorders and
how to detect.
Apply all these concepts to our clinical practice

4. What are the data the ABG Provide???
Oxygenation
Acid base Status
Ventilation
Hb, Glucose , Electrolytes
lactate

5. pH 7.38 –7.42
Pa Co
2
38 –42 mmHg
HCo
3
22 –26 m mol/L
Normal ABG
PaO
2 >= 80 mmHg
Spo
2 >=95%
P A-a o
2
(on room air)(Age/4) + 4
Lactate < 1.6 mmol/L
N Engl J Med. 2014 Oct
9;371(15):1434-45
Dynamed :( accessed 11/2017)
VBG
pH 7.36 –7.38
Pa Co
2
43 –48 mmHg
HCo
3
25 –26 m mol/L

6. Acidemia -arterial pH below the normal range (< 7.38)
Alkalemia -arterial pH above the normal range (> 7.42)
Acidosis -a processthat lowers extracellular fluid pH.
Alkalosis -a process that raises extracellular fluid pH.
Definitions

7. Respiratory (Co2) or Metabolic (HCo3)
pH change
Process

8. Simple acid –base disorder
involvement of single primary abnormality

9. Mixed acid –base disorder
involvement of ≥ 2 primary abnormalities

10. Henderson Equation and
Consistency of ABG
H + = 24 (PaCo2)/HCO3

11. H + = 24 (PaCo2)/HCO3
H + = 80 –(2 digits after Decimal point in ABG)

12. Clinical importance of Henderson Equation
Assure that the ABG is consistent and accurately recorded
pH 7.38
Pa Co
2
41
HCo
3
23
H+= 24(41)/23= 42
H+= 80-38 = 42
So ABG is consistent and
accurately recorded
pH 7.42
Pa Co
2
39
HCo
3
20
H+= 24(39)/20= 46.8
H+= 80-42 = 38
So ABG is inconsistent and
inaccurately recorded
Can be interpreted Cant be interpreted

13. ABG interpretation Steps
pH 7.38
Pa Co
2
41
HCo
3
23
PaO
2
95
Na 143
Cl 98

14. ABG interpretation
Oxygenation
Ventilation
Status
Acid Base
Status
General approach

15. History and Physical Examination
Check consistency of ABG
pH:
Pa Co2 and HCo3
Compensation
AG and Other GAPs
Primary
Disorder
Degree of Compensation.
Presence of Other Primary
(mixed) disorders

16. History and Physical
Examination
Underlying medical conditions.
Vitals.
Consciousness.
Signs of infection.
Respiratory status.
GIT symptoms ( Vomiting and Diarrhea).
Medications
Signs of Intoxication.

18. Acid Base DisorderpH Pa Co
2 HCo
3
RespiratoryAcidosis
Acute
RespiratoryAcidosis
Chronic
Respiratory Alkalosis
Acute
Respiratory Alkalosis
Chronic
Metabolic Acidosis
Metabolic Alkalosis

19. compensation
Zero point
Change from
PaCo240 mmHg
Hco324 mMol/L

20. Fishman’s Pulmonary
Diseases and Disorders
Fifth Edition (2015)

21. +10+1
-10 -2
+10 +3.5
-10 -5
Acute respiratory acidosis
Acute respiratory alkalosis
Chronic respiratory acidosis
Chronic respiratory alkalosis
Metabolic acidosis
Metabolic alkalosis
Delta HCo3
From 24
DeltaPaCo2
From40
-10 -10
+10+15
Chest Seek
TM
2014
It is a simple alphabetical
calculation

22. Anion Gap
Must be calculated even in apparently normal ABG.
Represent Unmeasured Anions
AG= Na –(Hco3+ Cl)= 8-12

23. In patients with low albumin (chronic illness, starvation, cancer,……)
anion gap should be corrected according to the Figgeequation
AG= Na –(Hco3+ Cl)= 8-12
Anion Gap
EmergMed ClinNorth Am 2014 May;32(2):403.

24. Delta Ratio
Delta Ratio =
Delta AG( increase above 12)
Delta Hco3 ( decrease below 24)
EmergMed ClinNorth Am 2014 May;32(2):403.

25. Approach to metabolic acidosis
Common
causes AGMA ( KUSMALE)
Ethylene G
Ketones
Uremia
Salicylates
Alcohols
Lactate
Diabetic, Alcoholic, Starvation
Cause also respiratory alkalosis
Early: No AG and + OsmGap.
Late: + AG and no OsmGap
L lactate( Type A and Type B)
D lactate
With osmolarGap
Due to acc. Of phosphate and Sulfates

26. Approach to metabolic acidosis
Common
causes Non-AGMA ( USED CARS)
Amino acids ( ArginineHCL , Lysine)
Uretrosigmoidostomy
Saline
Early renal failure
Diarrhea and pancreatic fistula
Carbonic anhydrase inhibitors
RTA
Supplements( TPN with excess ClvsAcetate)

28. Approach to metabolic acidosis
Clinical context
Internal consistency
Anion Gap
(corrected to Alb.)
Pa Co2
ABG with Met Acidosis
AGMA
NAGMA
Urine AG
Positive : Renal cause
Negative: Extrarenal
Delta
ratio
Osmolar Gap
Metabolic
alkalosis
OrRespiratory
acidosis
Normal or
High
Prim resp
acidosis
Low
+
=
-
Prim resp
alkalosis
No pr. Respdis.
comp

29. 24 Y man with DM, CKD is admitted with altered mental status and hyperglycemia
, blood ethanol negative, ketone + in urine and + opioidin urine
Case 1
pH 7.15
Pa Co
2
35
HCo
3
12
PaO
2
95
Na 140
Cl 112
Alb 2.5
consistency
Met acidosis
A G16 A G c21 d. gap 9
d. ratio 9/12= 0.7
Anion Gap Metabolic Acidosis
Non AGMA.
Respiratory acidosis
Expected
Co2
28

30. Approach to metabolic alkalosis
Common
causes
Volume depletion:
•Vomiting
•NGT loss
•Villous adenoma.
Diuretic use.
Post Hypercapnic
Hypertensive:
•Hyperaldosteronism.
•Cushing
•Exogenous mineralocorticoids
Normotensive:
•Severe hypokalemia.
•Milk alkali syndrome

32. Approach to metabolic alkalosis
Clinical context
Internal consistency
ABG with Met Acidosis
Anion Gap
(corrected to Alb.)
Pa Co2
Check for
urinary
chloride
If low or normal
Prim resp.
alkalosis
High
Compensation
check
-
=
+
No
Resp.
Disorder
Prim
resp.
acidosis
< 20 mmol/l
>20 mmol/l
Saline responsive
Saline resistant

33. Causes of respiratory
acidosis
Drive
Pump
Effectororgan

34. Case 2
pH7.38
Pa
Co
2
41
HCo
323
PaO
295
Na143
Cl98
A 23-year-old man presented with generalized malaise and vomiting.
His ABG showed:
consistency
Disorder
Anion Gap 22 Delta Gap 10
Delta Ratio 10/1 10
This patient has a blood sugar of 510 mg/ dl and ketonesin
the urine. He had diabetic keto-acidosis responsible for his
AGMA and vomiting caused his metabolic alkalosis.
AGMA
Metabolic alkalosis

35. Case 3
pH7.45
Pa
Co
2
44
HCo
324
PaO
290
Na144
Cl112
consistency
pH = 7.45
H+= 35
H+= 24(44)/24= 44≠
Inconsistent ABG

36. Case 4
pH 7.30
Pa Co
260
HCo
329
Na 144
Cl 112
consistency
Disorder
compensation
AG
yes
Respiratory
acidosis
If
Acute
If
Chronic
HCo3 = 26
HCo3 = 31
3
Acute on top of chronic respiratory acidosis.
Chronic respiratory acidosis with NAGMA

37. References
Uptodate (accessed 11/2017)
DynamedPlus ( accessed 11/2017).
N Engl J Med. 2014 Oct 9;371(15):1434-45.
Fishman’s Pulmonary Diseases and Disorders, Fifth Edition (2015).
EmergMed ClinNorth Am 2014 May;32(2):403.
Pocket ICU , second edition (2017).
Arterial Blood Gases Made Easy, second edition (2015).
Chest Seek
TM
2014 (Education Product of ACCP).

38. N Engl J Med. 2014 Oct 9;371(15):1434-45.
Recommended Review for Medical Residents