1. The patient has a mixed disorder of metabolic acidosis with respiratory compensation and high anion gap metabolic acidosis. 2. The primary disorder is metabolic acidosis as pH and HCO3 are decreased while PCO2 is normal. 3. The anion gap is elevated indicating a high anion gap metabolic acidosis likely due to an unknown toxin given the clinical context.

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