Kathamandu Medical college

1. Basics of Arterial blood gas
analysis
Dr. Uttam Laudari
MS Gerneral surgery
1st year Resident
Department of surgery
KMCTH

2. objectives
• Normal ABG parameters
• Definitions of acid base disturbances
• Systematic approach to ABG
• Mixed acid base disturbances analysis
• Examples of common ABG analysis and
interpretation

3. • The normalECF pH is 7.35 to 7.45
• pH= 6.1+log {HCO3}
pCO2*0.3
• Maintenance of pH is essential for normal cellular function

5. 3 general mechanism exist to keep pH within a
narrow window
1. Chemical buffering-
– Mediated by HCO3 in the ECF and by protein and
phosphate buffer in the ICF
2. Alveolar ventilation by altering the pCO2
3. Renal H+ handling-
allows kidney to adapt to changes in acid base balance
status via HCO3 absorption and excretion of titrable acid
(H2PO2) and NH4+

6. Definition
• Acidemia and alkalemia refers to process that lower and raise the
pH regardless of mechanism
• Metabolic acidosis- decresed in plasma HCO3 due to loss or
accumulation
• Metabolic alkalosis- elevation in the plasma HCO3 due to H+ loss or
HCO3 gain
• Respiratory acidosis- elevated pCO2 due to alveolar hypoventilation
• Respiratory alkalosis- decresed pCO2 due to hyperventillation

7. Systematic approach to acid base disorder
1. Checking ABG
2. Establishing a primary disorder
3. Assessing compensation
4. Calculating Anion GAP
5. Evaluating delta gap

8. Step 1- check ABG
• Acidemia if pH< 7.35
• Alkalemia if pH >7.45

9. Step 2 – establishing primary disorder
• Determine whether the change in HCO3 or pCO2 can account
for the observed deflection in pH
• Alkalemia
– metabolic alkalosis -Elevated HCO3
– respiratory alkalosis -Decreased pCO2
• Acidemia
– Metabolic acidosis- decreased HCO3
– Respiratory acidosis- elevated pCO2

10. • Combined disorder
– pH normal but pCO2 and HCO3 are both abnormal
– Changes in both pCO2 and HCO3 can cause change in pH

11. Step-3 Assessing compensation
• Compensation refers to respiratory and renal mechanism that
brings the arterial pH close to but not into normal pH range
• Importance- identifies more than one primary acid base disorder (
mixed acid base disorder)
• Expected compensation if remained in normal
rangeuncompensated disorder is present
• If compensation moves outside the normal range but does not
bring the pH into normal range partially compensated disorder
• Compensation brings pH to normal full compensation ( rarely
occurs)

12. • calculated expected compensation closely
approximated the measured compensation single
primary disorder
• Obvious disparity between calculated expected
compensation and measured compensation more
than one primary disorder

13. Respiratory Acidosis
• Pathogenesis- alveolar hypoventilation with CO2
retention
• pH= HCO3/pCO2
• Compensation with metabolic alkalosis
1. Serum HCO3 <30 in acute respiratory acidosis
2. > 30 in chronic Respiratory acidosis ( indicating renal
compensation)

14. respiratory acidosis
• Expected HCO3 compensation=0.1* pCO2 (acute)
• Expected HCO3 compensation=0.4* pCO2( chronic)
• pH=7.2 pCO2=74 HCO3=27
• Expected HCO3 compensation= 0.1 * (74-40)= 3.4mEq/l increase
above normal
• 24+3.4=27.4 mEq/L
• Measured and expected HCO3 almost similar single disorder

15. respiratory alkalosis
• Alveolar hyperventilation CO2 wash out
• pH= HCO3/PCO2
• Compensation with metabolic acidosis
1. Serum HCO3 >18mEq/L in acute respiratory alkalosis
2. <18 mEq/l but >12 mEq/l in chronic Respiratory
alkalosis ( indicating renal compensation)
• Expected HCO3 compensation:
• Acute- 0.2* PaCO2
• Chronic- 0.5 * PaCO2

16. Metabolic acidosis
• Serum HCO3 < 22mEq/l
• pH-HCO3/ pCO2
• addition of an acid ( increased Anion gap)
• Loss of HCO3 OR inability to synthesize HCO3(
normal anion gap)
• Compensation with Respiratory alkalosis

17. • Calculation of expected compensation in metabolic acidosis
• Expected PaCo2=1.2 * HCO3 +/- 2
• Measured HCO3 is subtracted from the mean HCO3 of
24mEq/l

18. • pH- 7.27, PaCo2 27 HCO3 12
1. Expected compensation
2. Expected PaCo2=1.2 * HCO3 +/- 2
=1.2 * (24-12)= 14.4 ,mm Hg less than
normal
= 40-14.4= 25.6 mm Hg +/- 2 ( 23.6 to
27.6)
measured Paco2 is with in the expected range
Single disorder is only present

19. Incresed anion gap metabolic acidosis
• Anion gap= Na- (Cl + HCO3)
• 12 +/- 2 mEq/l
• 12mEq/l represents the anions that are not accounted for in
the formula ( phosphate, albumin, sulphate)
• If AG> 12 +/- 2  addition of anions that should not be there
( lactate, salicylate, acetoacetate)
• Increased AG metabolic acidosis excess H+ ion of acid (lactic
acid) are buffered by HCO3 which decreases the serum HCO3

20. • HCO3 loss due to buffering is counter balanced by anions of
acid ( e.g-lactate ions)
• Cause of increase AG metabolic acidosis
• Lactic acidosis ( shock, CN poisoing, CO poisoning,
severe hypoxemia, severe anemia, alcholosim, Liver
disease
• Ketacidosis
• Renal failure
• Salicylate, ethylene glycol and methy alcohol poisoning

21. Normal AG metabolic acidosis
• Due to loss of HCO3 or an inability to synthesize or reclaim HCO3 in kidney
• Chloride ion increases to counterbalance the loss of HCO3
• {AG= Na- (Cl + HCO3)}
• Produces hyperchloremic normal AG metabolic acidosis
• Cause-
– Diarrhoea
– Drainage of bile or pancreatic secretions
– Renal tubular acidosis
– Cholestyramine
– Ureteroenterostomy

22. Metabolic alkalosis
• Loss of H + or gain in HCO3-
• Serum HCO3 >28 mEq/l
• Compensation by respiratory acidosis
• Expected compensation
– PaCo2=0.7* HCO3
• Causes
– Vomiting
– Excess mineralocorticoids
– Thiazide and loop diuretics

23. Mixed acid base disorders
• Two or more primary acid base disorders occurring at same time
• Clues suggesting mixed disorder
– Normal pH
– Extreme change in pH
• Normal pH due to combination of primary
acidosis and primary alkalosis ( salicylate
poisoning)
• Extreme change in pH due to primary metabolic
acidosis plus primary respiratory acidosis (
cardiorespiratory arrest)

24. Identifying mixed disorder
• pH 7.26 PaCo2 38mmHg HCO3 17
• presumptive diagnosis
metabolic acidosis without compensation
( HCO3 < 22 but Paco2 is within normal range)
• Formula for calculating expected
compensation in metabolic acidosis
Expected PaCo2= 1.2 * HCO3 +/- 2

25. • pH 7.26 PaCo2 38mmHg HCO3 17
• Expected PaCo2= 1.2 * HCO3 +/-2
= 29.6 to33.6
• Measure Pco2 is 38 mm Hg
• Expected is 29.6 to 33.6
• Indicating respiratory acidosis also present as primary
disorder

26. Example of mixed acid base disorder
• pH 7.38
• PaCO2 70mmHg
• HCO3- 41 mEq/l
• Presumptive diagnosis
• Mixed disorder( pH normal)
• with Chronic Respiratory
acidosis ( PaCo2 >
45mmHG, HCO3> 30 mEq/l)
• and primary metabolic
alkalosis

27. • pH 7.38 PaCO2 70mmHg HCO3- 41
Using metabolic alkalosis formula
• Expec. PaCo2=0.7* HCO3 +/-2
= 49.9- 53.9
Final diagnosis has additional primary
respiratory acidosis also

28. examples
pH (7.35 – 7.45) PaCo2(33-45) HCO3 (22-28) interpretation
7.00 52 13 MIXED DISORDER
EXTREME
ACIDEMIA:
PRIMARY
METABOLIC
ACIDOSIS WITH
PRIMARY
REPIRATORY
ACIDOSIS
Carido respi arrest
7.20 74 28 ACUTE
RESPIRATORY
ACIDOSIS,
UNCOMPENSATED (
paco2>45mm hg,
HCO3 <30 )

29. pH (7.35 – 7.45) PaCo2(33-45) HCO3 (22-28) interpretation
7.33 60 31 Chronic respiratory
acidosis with
partially
compensated
metabolic alkalosis
(Paco2>45,Hco3>
30)
Chronic bronchitis,
cyystic fibrosis
7.28 28 12 Metabolic acidosis
with partially
compensated
respiratory alkalosis
7.42 22 14 Primary metabolic
acidosis with
primacy respiratory
alkalosis
Salicylate poisoning
and septic shock

30. References
The ICU book 3 rd Edition
The Washington Manual of Surgery
Rapid review of Pathology Edward F. Golzan
Clinical surgery in general – RCS Course manual

31. • Thank you
Life is a struggle not against sin, not
against money, power…. But against
Hydrogen ion. ( H.L Mencken)