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1. Arterial Blood Gas Analysis
Dr. Bikram K Gupta
MD,PDCC,EDIC,FACEE,FICCM
Professor & Head
Division of Critical Care Medicine
Dept. of Anaesthesiology
Heritage Institute of Medical Sciences
Email: – bikramgupta03@gmail.com

2. Application of ABG
• To document Respiratory failure and assess its severity
• To monitor patients on ventilators and assist in weaning
• To assess acid base imbalance in critical illness
• To assess response to therapeutic interventions and mechanical
ventilation
• To assess pre-op patients for high risk surgeries

5. What all in blood ‘gases’ do we measure ?

6. NORMAL VALUE

7. Oxygenation

9. Alveolar Arterial Oxygen Gradient (AaDO2)
= 0.21 (760 - 47) – 40/0.8 = 100 mm Hg
AaDO2 = 0.21 (760 - 47) – 40/0.8 – 90 = 10 mm Hg
The normal A-aPO2 gradient increases 5 to 7 mm Hg for every 10% increase in FIO2.

11. AaDO2 Implications…
Pramod Sood et al. Interpretation of arterial blood gas. IJCCM 2010;14:57-64.
Hypoxemic Respiratory Failure

12. • Unaffected by FIO2
• The normal a/A PO2 ratio is 0.74 to 0.77 when breathing room air, and 0.80 to 0.82
when breathing 100% oxygen.

13. • The oxygenation status of the patient is judged by the paO2;however,
never comment on the oxygenation status without knowing the
corresponding FiO2. Calculate the expected paO2 (generally five times
the FiO2).

15. Acid Base Analysis

16. Boston approach – Rule of Thumb
HCO3/PCO2 Equation
Primary Disorder Initial Chemical
change
Compensatory
Response
Comp mechanism Expected level of compensation
Metabolic Acidosis Decreased HCO3 Decreased pCO2 Hyperventilation pCO2 = 1.5 X HCO3 + 8 ± 2
Metabolic Alkalosis Increased HCO3 Increased pCO2 Hypoventilation pCO2 = 0.7 X HCO3 +21 ± 2
Primary Disorder Initial Chemical
change
Compensatory
Response
Comp mechanism Expected level of compensation
Respiratory Acidosis
• Acute
• Chronic
Increased pCO2 Increased HCO3 Buffering
• Rule of 1
• Rule of 4
Change in HCO3 = 1 meq/l
for every 10 mm Hg delta PCO2
& Similarly 4 mEq/l
Respiratory Alkalosis
• Acute
• Chronic
Decreased pCO2 Decreased HCO3 Buffering
• Rule of 2
• Rule of 5
Change in HCO3 = 2 meq/l
for every 10 mm Hg delta PCO2
& Similarly 5 mEq/l

17. Copenhagen approach

18. Copenhagen approach

19. Boston approach Vs. Copenhagen approach
• In reality, there is little difference between two; both equations and normograms were derived from in
vivo patient data.
• For most patients, either approach is accurate but misleading.
Boston approach Copenhagen approach

20. (Na + K + Ca + Mg) – (Cl + Lactate + SO4 + Keto ions) – (Albumin + Pi) - (HCO3) = 0
SID ATOT
Metabolic Acid – Base Abnormality
pCO2
So alterations in the SID or ATOT or Both affect dissociation of water Hence affect H+ ion
Conc.
Decreased SID (More anions) or Increased ATOT – Acidosis
Increased SID (More Cations) or Decreased ATOT – Alkalosis
Stewart Approach

21. TEN STEPS

22. • Detailed history and scenario of the patient
• Know the normal values

23. Obtain a relevant clinical history..
• A patient with a history of hypotension, renal failure, uncontrolled
diabetic status, of treatment with drugs such as metformin is likely to
have metabolic acidosis.
• A patient, with a history of diuretic use, bicarbonate administration,
high-nasogastric aspirate, and vomiting, is likely to have metabolic
alkalosis.
• Respiratory acidosis would occur in COPD, muscular weakness,
postoperative cases, and opioid overdose.
• Respiratory alkalosis is likely to occur in sepsis, hepatic coma, and
pregnancy.

24. Check for Validity of Gas
A) First calculate H+ by putting PCO2 and HCO3 in the equation
H+ = 24 X (PaCO2/HCO3)
B )Then
• For every 0.1 decrease in PH, multiply H+ sequentially by 1.25
• For every 0.1 increase in PH, multiply H+ sequentially by 0.08
C) Match H+ by both A & B, if matches then ABG is valid
PH = 7.4 (H+ =40)
PH = 7.2
For PH 7.3, H+ = 40 X 1.25 = 50
Next
For PH 7.2, , H+ = 50 X 1.25 = 62.5

25. pH Concentration of
H+ ions
7.0 98
7.1 79
7.2 63
7.3 50
7.4 40
7.5 32
7.6 26
7.7 21
H+ = 80 – last two point of decimal
Check for Validity of Gas

26. Assess for Oxygenation
• Room air
• PAO2-PaO2 =5-15mm Hg
• PaO2 for age= 109-0.45xAge
• Ventilator/oxygen supplementation
• PaO2/FiO2 =400-500
PaO2
Normal 60-80
Mild 50-59
Moderate 40-49
Severe <40
P/F
Mild Hypoxemia 200-300
Moderate 100-200
Severe <100

27. Look at pH
• <7.35 (academia) / >7.45 (alkemia)
• pH is inversely proportional to H+ ions.

28. Identify the primary disorder
• In primary respiratory disorders, the pH and PaCO2 change
in opposite directions.
• In metabolic disorders the pH and PaCO2 change in the same direction.
In a normal ABG
• pH and paCO2 move in opposite directions.
• HCO3
-and paCO2 move in same direction.

29. Identify the primary disorder
• When the pH and paCO2 change in the same direction (which normally should not),
the primary problem is metabolic; when pH and paCO2 move in opposite directions
and paCO2 is abnormal, then the primary problem is respiratory.
• Mixed Disorder – if HCO3
- and paCO2 change in opposite direction (which they
normally should not), then it is a mixed disorder: pH may be normal with abnormal
paCO2 or abnormal pH and normal paCO2.

30. Mixed Acid Base Disorders
• Presence of more than one acid base disorder simultaneously
• Clues to a mixed disorder :-
• Normal PH with abnormal HCO3 or PCO2
• PCO2 and HCO3 move in opposite directions
• PH changes in an opposite direction for a known primary disorder

31. Identify the primary disorder……..
• If the trend of change in paCO2 and HCO3
- is the same, check the percent difference. The one, with
greater % difference, between the two is the one that is the dominant disorder.
e.g.: 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
Therefore, respiratory acidosis as the dominant disorder.

32. Respiratory : is it Acute or Chronic ?
• Respiratory disturbance: Acute /Chronic
• Acute: pH changes by 0.008 with each changes in CO2
(For every 10 change in PaCO2 ; pH will change by 0.08)
• Chronic: pH changes by 0.003 with each changes in PaCO2
(For every 10 change in PaCO2 ;pH will change by 0.03)
• Acute on Chronic : For every 10 change in PaCO2 ;pH will change in between
0.03 – 0.08.

34. Is there appropriate compensation for the primary
disturbance? Usually, compensation does not return the pH
to normal (7.35 – 7.45).
Disorder CO2 change HCO3 change
Acute respiratory acidosis For every 10 mmHg rise in
CO2
HCO3 will rise by 1 mmol/l
Chronic respiratory acidosis For every 10 mmHg rise in
CO2
HCO3 will rise by 4 mmol/l
Disorder CO2 change HCO3 change
Acute respiratory alkalosis For every 10 mmHg fall in CO2 HCO3 will fall by 2 mmol/l
Chronic respiratory alkalosis For every 10 mmHg fall in CO2 HCO3 will fall by 5 mmol/l

35. • If paCO2↓ and HCO3
- is also ↓→ primary metabolic acidosis
• Calculate Anion Gap

36. Anion Gap

38. • If metabolic acidosis, then Anion Gap (AG) should be examined.
• AG= Na+ - (Cl-+HCO3-) , normal value : – 12 ± 2
• In patients with hypoalbuminemia, the normal anion gap is lower than 12 meq/L.
• Correction factor = 2.5 X {4 – albumin (gm/dl)}
• Corrected AG = Correction factor + AG
• If AG is unchanged → then it is hyperchloremic metabolic acidosis/ NAGMA.
• If AG is ↑ → then it is High AG acidosis (HAGMA).

40. • If the anion gap is elevated, consider calculating the osmolar gap in compatible
clinical situations.
• Elevation in AG is not explained by an obvious case (DKA, renal failure etc)
• Toxic ingestion is suspected
• Osmolar gap = measured OSM – (2[Na+] - glucose/18 – BUN/2.8)
• The OSM gap should be < 10
• If osmolar gap >10 then suspect ingestion of an alcohol, including ethanol,
methanol, ethylene glycol, diethylene glycol, propylene glycol, and
isopropanol (isopropyl alcohol)

41. Normal Anion Gap Metabolic Acidosis

42. • If paCO2↓ and HCO3
- is also ↓→ primary metabolic acidosis
• Expected PaCO2 (Winters Formulae) = 1.5 x HCO3 + 8 ± 2
• Calculate expected paCO2 as follows:
• paCO2 = [1.5 × HCO3+ 8] ± 2 metabolic acidosis only.
• paCO2 < expected paCO2→ concomitant respiratory alkalosis.
• paCO2 > expected paCO2→ concomitant respiratory acidosis.

44. • If paCO2 ↑ and HCO3
- also ↑ → then it is primary metabolic alkalosis.
• Calculate the expected paCO2
• paCO2 = [0.7 × HCO3-+ 21] ± 2 Or 40 + [0.7 ΔHCO3] → metabolic
alkalosis only
• paCO2 < expected paCO2 → concomitant respiratory alkalosis.
• paCO2 > expected paCO2 → concomitant respiratory acidosis

46. Check urinary chloride
• If urinary chloride < 20 → chloride responsive or ECV depletion
• If urinary chloride > 20→ chloride resistant

47. STEP 1 Validity of gas
STEP 2 Assess for Oxygenation
STEP 3 Look at pH
STEP 4 Identify the primary disorder
STEP 5 Respiratory acute or chronic
STEP 6 Metabolic Compensation
STEP 7 Metabolic Acidosis – Anion gap
STEP 8 Respiratory compensation
STEP 9 Concomitant/Mixed disorder
STEP 10 Metabolic alkalosis – Urinary Chloride

48. Case 1
• A 22-year-old man who had been previously healthy was brought to the emergency
room early on a Monday morning, with agitation, fever, tachycardia, and hypertension.
• He was confused and incapable of providing a meaningful history.
• Examination revealed pulse 124 and regular; respirations 30; blood pressure 180/118;
and temperature 101.6°F.
• Pulse oximetry was consistent with oxygen saturation of 95%.
BE=-5.8 mmol/l; Albumin =4 gm/dL; Hb=15 gm %

49. • Step : 1 – Validity
a) H+= 24 x 40/18 = 53.33,
b) for every 0.1 decrease in PH, multiply H+ sequentially by 1.25, here PH is 7.27
Hence H+ CONC = 40 x 1.25 = 50.
A & B matched hence ABG is valid
• Step : 2 – Oxygenation 84 mm Hg means normal on room air
• Step: 3 – PH is 7.27 i.e. academia
• Step: 4 - Identify the primary disorder (PCO2 – 40, HCO3 -18); Metabolic acidosis
• Step: 7 - Metabolic Acidosis, see Anion gap (AG) = Na+ - (Cl-+HCO3-) = 128 – (88 + 18) = 22 (HAGMA)
• Step: 8 – Respiratory compensation = (1.5x 18)+8±2 = 35±2
• Step: 9 – Mixed disorder see Gap-gap ratio= (22-12)/(24-18)=1.66
Interpretation: High anion gap metabolic acidosis having respiratory compensation with normal oxygenation

50. • Treat the patient not the gas; correlate with clinical condition
• Always follow the essential Ten steps for ABG interpretation
• Compensation tries to bring pH towards normal but never near normal
• If pH is near normal then coexisting disorder is present