This document provides a comprehensive guide to interpreting arterial blood gases (ABG), focusing on acid-base balance, oxygenation, and ventilation status. It outlines assessment steps, compensation mechanisms for different acid-base disorders, and examples of both high and normal anion gap acidosis. The content includes critical formulas and equations for diagnosing respiratory and metabolic disorders, emphasizing the importance of systematic evaluation.

1. ABG INTERPRETATION - METABOLIC
DISORDER AND MIXED ACID BASE
DISORDER
DR JAYABHARATHI PALANIVEL
JUNIOR RESIDENT III
DEPARTMENT OF PULMONARY
MEDICINE
08/08/2022

2. WHAT TO LOOK IN ABG
• Oxygenation status ( po2- 80 to 100 ,
fio2, arteriolar alveolar gradient, RR )
• Ventilation status
• Acid base status
60- 80 : mild hypoxemia
40-60 : moderate hypoxemia
< 40 : severe
Sood P, Paul G, Puri S. Interpretation of arterial blood gas. Indian J Crit Care Med. 2010;14(2):57-64. doi:10.4103/0972-5229.68215

3. CHECK FOR CONSISTENCY OF ABG
• Modified Henderson equation:[H+−][HCO3]/PaCO2 = 24
• Subtract the last two digits of the pH from 80; this value is
approximately equal to the H+ concentration .
• Example :pH: 7.42, pCO2: 30.8, HCO3- : 19.3, H+ : 38.1.
• 80 - last 2 digits of pH = 80-42 = 38 = approximately equal to measured
H + in the report.
• https://www.thoracic.org/professionals/clinical-resources/critical-care/clinical-education/abgs.php

4. Sood P, Paul G, Puri S. Interpretation of arterial blood gas. Indian J Crit Care Med. 2010;14(2):57-64. doi:10.4103/0972-5229.68215
pH value and corresponding H+

5. ACIDEMIA VS ACIDOSIS
ALKALEMIA VS ALKALOSIS
• Acidemia and alkalemia - Physiological state dependent solely upon
arterial pH .
Acidemia pH <7.35
Alkalemia pH >7.45
Sood P, Paul G, Puri S. Interpretation of arterial blood gas. Indian J Crit Care Med. 2010;14(2):57-64. doi:10.4103/0972-5229.68215

6. ACIDOSIS VS ALKALOSIS
• Process which tend to drive pH towards Acidemia and Alkalemia
Sood P, Paul G, Puri S. Interpretation of arterial blood gas. Indian J Crit Care Med. 2010;14(2):57-64. doi:10.4103/0972-5229.68215

7. RESPIRATORY DISORDERS METABOLIC DISORDERS
ACIDOSIS - hypoventilation ACIDOSIS - HCO3 low
ALKALOSIS - Hyperventilation ALKALOSIS - HCO3- high
Disrupts acid base balance due to their effect on the
lungs
Disrupts acid base balance due to their effect on the
Kidneys, GI tract ,etc..,
https://www.thoracic.org/professionals/clinical-resources/critical-care/clinical-education/abgs.php

8. • Step 0 : Is this ABG Authentic?
• Step 1 : Acidosis or Alkalosis?
• Step 2 : Respiratory or Metabolic?
• Step 3 : If Respiratory, Acute or Chronic?
• Step 4 : Is Compensation adequate?
• Step 5 : If Metabolic - Anion Gap?
• Step 6 : Check “Gap Gap" Ratio
• Step 7 :if AG normal –urine anion gap
https://www.thoracic.org/professionals/clinical-resources/critical-care/clinical-education/abgs.php

9. COMPENSATION
Acid base disorder Mechanism of compensation
Metabolic acidosis Increase minute ventilation (dec pco2)
Metabolic alkalosis Decrease minute ventilation
Respiratory acidosis
Re absorption of HCO3 and excretion of H+ by
kidneys
Respiratory alkalosis
Re absorption of HCO3 and excretion of H+ by
kidneys
Marino PL. Arterial Blood Gas Interpretation. 4th edi. Lippincott: Williams and Wilkins Publishers; 2014. pp. 559-601

10. WHICH IS DOMINANT DISORDER ?
• If the trend of change in paCO2 and HCO3- is the same, check the percent difference.
• pH = 7.25 HCO3- =16 paCO2 =60
• Here, the pH is acidotic and both paCO2 and HCO3- explain its acidosis: so look at the %
difference
• HCO3- % difference = (24 - 16)/24 = 0.33
• paCO2 % difference = (60 - 40)/40 = 0.5
RESPIRATO
RY
ACIDOSIS
AS THE
DOMINANT
DISORDER.
Marino PL. Arterial Blood Gas Interpretation. 4th edi. Lippincott: Williams and Wilkins Publishers; 2014. pp. 559-601

11. COMPENSATION
Compensatory
mechanism is poor in
which acid base
disorder ...?
METABOLIC
ALKALOSIS
ALVEOLAR GAS
EQUATION
Marino PL. Arterial Blood Gas Interpretation. 2nd edi. Lippincott: Williams and Wilkins Publishers; 1998. pp. 582–605

12. COMPENSATION FORMULA
Sood P, Paul G, Puri S. Interpretation of arterial blood gas. Indian J Crit Care Med. 2010;14(2):57-64. doi:10.4103/0972-5229.68215

13. COMPENSATION FORMULA
PRIMARY DISORDER FORMULA
Metabolic acidosis
PaCO2 = 1.5(HCO3 )+8
Metabolic alkalosis
PaCO2 = 40 + (0.6 ( HCO3 - 24))
Sood P, Paul G, Puri S. Interpretation of arterial blood gas. Indian J Crit Care Med. 2010;14(2):57-64. doi:10.4103/0972-5229.68215

14. COMPENSATION FORMULAS
Respiratory acidosis
Respiratory alkalosis
Acute 1 2
Chronic 4 5
Sood P, Paul G, Puri S. Interpretation of arterial blood gas. Indian J Crit Care Med. 2010;14(2):57-64. doi:10.4103/0972-5229.68215

15. PH : 7.50/ PaCO2 : 48 / HCO3 : 34
STEP 1: PH - alkalosis
STEP 2: pco2 - metabolic alkalosis
STEP 3 : evaluate compensationPaCO2 = 40 + (0.6 ( HCO3 - 24))
PRIMARY METABOLIC ALKALOSIS

16. PH : 7.12/ PaCO2 : 32/ HCO3 : 10
STEP 1: PH
STEP 2: pco2
STEP 3 : evaluate compensation PaCO2 = 1.5(HCO3 )+8

17. SIMPLE TO ADDITIONAL ACID BASE
DISORDER
• Explanation -additional respiratory acidosis
Primary acidosis
Hco3 = 10 mEq/l
Respiratory compensation
PCO2= 23 mmhg
Measured
PCO2 = 35 mmHg

18. PH :7.34/ PaCO2 : 65/ HCO3 : 34
STEP 1: PH
STEP 2: pco2
STEP 3 : evaluate compensation
24 year old man found unconscious on
the floor soiled with vomit . He was last
seen 4hrs prior , and appeared well at
that time

19. ANION GAP
Marino PL. Arterial Blood Gas Interpretation. 4th edi. Lippincott: Williams and Wilkins Publishers; 2014. pp. 559-601

20. NORMAL AG METABOLIC ACIDOSIS HIGH ANION GAP METABOLIC ACIDOSIS
Marino PL. Arterial Blood Gas Interpretation. 4th edi. Lippincott: Williams and Wilkins Publishers; 2014. pp. 559-601

21. ETIOLOGIES OF NORMAL ANION GAP
ACIDOSIS
LOSS OF HCO3 DECREASED RENAL H+ EXCRETION
Diarrhoea Renal tubular acidosis
Type 2 RTA Type 1RTA
Acetazolamide
TYPE 4 RTA
( hypoaldosteronism )
Urethral diversion
Sood P, Paul G, Puri S. Interpretation of arterial blood gas. Indian J Crit Care Med. 2010;14(2):57-64. doi:10.4103/0972-5229.68215

22. ETIOLOGIES OF HIGH ANION GAP ACIDOSIS
Sood P, Paul G, Puri S. Interpretation of arterial blood gas. Indian J Crit Care Med. 2010;14(2):57-64. doi:10.4103/0972-5229.68215

23. ALTERATION IN ANION GAP UNRELATED TO
METABOLIC ACIDOSIS
HIGH ANION GAP LOW ANION GAP
1) metabolic alkalosis Hypoalbuminemia
2)hyperphosphatemia Increased K/ ca/ Mg
3) anionic Paraproteinemia Severe lithium intoxication
Cationic paraproteinemia
Sood P, Paul G, Puri S. Interpretation of arterial blood gas. Indian J Crit Care Med. 2010;14(2):57-64. doi:10.4103/0972-5229.68215

24. URINE OSMOLAR GAP
• OG>10 mOsm/kg indicates the presence of abnormal, unmeasured
osmotically active molecules
• Measured osmolality – calculated osmolality
• Calculated osmolality = 2 x Na + RBS/18 + BUN/2.8
• ethanol, methanol, ethylene glycol, diethylene glycol, propylene glycol,
and isopropanol
Sood P, Paul G, Puri S. Interpretation of arterial blood gas. Indian J Crit Care Med. 2010;14(2):57-64. doi:10.4103/0972-5229.68215

25. URINE OSMOLAR GAP
• Urine anion gap=(Na+ + K+) – Cl−
• < 10 mEq/L
• negative urinary AG = GI cause
• positive urinary AG=renal cause

26. HYPOALBUMINEMIA
Sood P, Paul G, Puri S. Interpretation of arterial blood gas. Indian J Crit Care Med. 2010;14(2):57-64. doi:10.4103/0972-5229.68215

27. MAINTAINING NEUTRALITY DURING AN
ELEVATED GAP ACIDOSIS
• 1:1 ratio between increased anion gap and decreased bicarbonate .

28. SHIFTS OF DELTA RATIO
Marino PL. Arterial Blood Gas Interpretation. 4th edi. Lippincott: Williams and Wilkins Publishers; 2014. pp. 559-601

29. SHIFTS OF DELTA RATIO
Marino PL. Arterial Blood Gas Interpretation. 4th edi. Lippincott: Williams and Wilkins Publishers; 2014. pp. 559-601

30. DELTA RATIO
MEASURED DELTA RATIO
PATHOLOGICAL METABOLIC DISORDER
Lower than expected range HAGMA + NAGMA
Within expected range HAGMA
HIGHER HAGMA + METABOLIC ALKALOSIS
Marino PL. Arterial Blood Gas Interpretation. 4th edi. Lippincott: Williams and Wilkins Publishers; 2014. pp. 559-601

31. DELTA GAP
DELTA AG + HCO3 = 24 +/- 6
Delta gap equals 0+-6mEq/L
Marino PL. Arterial Blood Gas Interpretation. 4th edi. Lippincott: Williams and Wilkins Publishers; 2014. pp. 559-601

32. THE DELTA GAP: ADD DELTA AG TO THE
MEASURE HCO3
MEASURED DELTA RATIO
PATHOLOGICAL METABOLIC DISORDER
Lower than expected range (<18) HAGMA + NAGMA
Within expected range (18-30) HAGMA
HIGHER (>30) HAGMA + METABOLIC ALKALOSIS
Marino PL. Arterial Blood Gas Interpretation. 4th edi. Lippincott: Williams and Wilkins Publishers; 2014. pp. 559-601

33. PH :7.29/ PaCO2 : 30/ HCO3 : 14
Na: 128/ k: 3.2/ Cl: 94/ HCO3 : 14
STEP 1: PH
STEP 2: pco2
STEP 3 : evaluate compensation ( PaCO2 = 1.5(HCO3 )+8)
STEP 4 : calculate AG ( AG= NA-(cl+HCO3 )
STEP 5 : calculate DELTA RATIO ( delta ag / delta HCO3 )
75 yr old female presents with fever and
profuse diarrhea for 2 days , vitals T:
100.4; HR : 130, BP : 78/30
High anion gap
metabolic acidosis with
normal anion gap
metabolic acidosis
Marino PL. Arterial Blood Gas Interpretation. 4th edi. Lippincott: Williams and Wilkins Publishers; 2014. pp. 559-601

34. APPROACH TO METABOLIC ALKALOSIS

36. BARTER SYNDROME

37. HYPOKALAEMIA CAUSING METABOLIC ALKALOSIS

38. APPROACH TO HAGMA

39. APPROACH TO NAGMA

40. APPROACH TO
PATIENT WITH
NORMAL ANION GAP
ACIDOSIS
Sood P, Paul G, Puri S. Interpretation of arterial blood gas. Indian J Crit Care Med. 2010;14(2):57-64. doi:10.4103/0972-5229.68215

41. MIXED DISORDERS

42. MIXED DISORDERS
Sood P, Paul G, Puri S. Interpretation of arterial blood gas. Indian J Crit Care Med. 2010;14(2):57-64. doi:10.4103/0972-5229.68215

43. PH :7.17/ PaCO2 : 65/ HCO3 : 22
Na: 136/ k: 3.4/ Cl: 98/ albumin:1.6
STEP 1: PH - Acidemia
STEP 2: pco2 - RESPIRATORY ACIDOSIS
STEP 3 : evaluate compensation ( 1/10) --> Metabolic acidosis
STEP 4 : calculate AG ( AG= NA-(cl+HCO3 ) - ELEVATED AG METABOLIC ACIDOSIS (21)
STEP 5 : calculate DELTA RATIO ( delta ag / delta HCO3 ) =5 --> additional Metabolic alkalosis
48 year old alcoholic found unconscious
in his apartment ,soiled with vomit .he
was seen leaving a party 6hrs prior
Respiratory acidosis
with High anion gap
metabolic acidosis with
metabolic alkalosis
Marino PL. Arterial Blood Gas Interpretation. 4th edi. Lippincott: Williams and Wilkins Publishers; 2014. pp. 559-601

44. • Step 0 : Is this ABG Authentic?
• Step 1 : Acidosis or Alkalosis?
• Step 2 : Respiratory or Metabolic?
• Step 3 : If Respiratory, Acute or Chronic?
• Step 4 : Is Compensation adequate?
• Step 5 : If Metabolic - Anion Gap?
• Step 6 : Check “Gap Gap" Ratio
• Step 7 :if AG normal –urine anion gap
https://www.thoracic.org/professionals/clinical-resources/critical-care/clinical-education/abgs.php

45. Thank you