Here are the key points about assessing ventilation from an ABG: - PaCO2 is used to assess ventilation. The normal range is 35-45 mmHg. - A PaCO2 higher than 45 mmHg indicates respiratory acidosis from hypoventilation or obstruction. - A PaCO2 lower than 35 mmHg indicates respiratory alkalosis from hyperventilation. - The pH and HCO3- will be affected by changes in PaCO2 based on the type of respiratory problem (acidosis vs alkalosis). They move in opposite directions for respiratory issues. - Compensation by the kidneys will cause the HCO3- to rise or fall in the same direction as PaCO

1. Dr Manisha Sahay
 Currently i/c Professor and HOD, Nephrology Department,
OMC/OGH
 MBBS Rajasthan
 MD Ped Niloufer
 DNB Neph ,Osmania General Hospital (Gold medal) 2003
 10 gold medals till date
 9 best paper awards at zonal and national conferences
 Best young researcher award –Indian Society of Nephrology
 Executive committee member –Young Nephrologist’s committee
International Society of nephrology
 Executive committee member Indian society of Nephrology
 Executive member of South Zone –Indian society of Nephrology
 Young Key Opinion leader in Transplantation –India
 Publications in national and international journals

2. ABC ‘s of ABG
Dr. Manisha Sahay
i/c Professor & HOD
Dept of Nephrology
Osmania General Hospital
Hyderabad

3. Collection of sample
 use local anaesthesia over the
radial artery before puncture.
(brachial/femoral)
 Use a 20 or 21 gauge needle
 Pre heparinised syringe (glass
better)
 Express the heparin from the
syringe before taking the
sample
 At least 3 ml of blood is
required to avoid a dilution
effect from the heparin.
 Any sample with more than
fine air bubbles should be
discarded. Manisha Sahay

4.  Press for 5 minutes
 Allen's test. The
radial and ulnar
arteries are occluded
 Avoid contact with air by firm pressure while
the fist is clenched.
 Put in ice if delay The hand is opened
and the arteries
released one at a time
to check their ability
to return blood flow to
the hand
Manisha Sahay

5. NORMAL ABG
Arterial pH 7.35 - 7.45 ; Venous 7.32 - 7.42
HCO3 art 22 to 26 mEq/L, venous 19 to 25 mEq/l
PaCO2 35-45 mm Hg, PvCO2 38-52 mm Hg
B.E. –2 to +2 mEq/liter
 value outside normal - metabolic acidosis
Standard Bicarbonate:
 Calculated value. No added info.
PaO2
SO2
FiO2
CaO2
A-a
Definitely Confusing!

6. No click
Steps for
Successful
Blood Gas
Analysis

7. ABC’s of ABG
 1Acidosis or alkalosis
 2 Metabolic or
respiratory
 3 Compensation
 4 Anion gap
 5 ∆ AG and ∆ HCO3
 6 Urine AG Manisha Sahay

8. Step 1
Look at the pH (Normal 7.35-7.45)
Is the patient acidemic pH < 7.35
or alkalemic pH > 7.45

9. Step 2 (pH & CO2)
 Look at the PCO2
(Normal =35-45 mmHg)
 pH and pCO2 change in opposite direction in
respiratory problem
 pH and pCO2 change in same direction in metabolic
problem
Manisha Sahay

10. Step 2
Metabolic OR Respiratory ?
Disorder pH PCO2
Metabolic  
acidosis Metabolic
Metabolic   Same direction
alkalosis
Respiratory   Respiratory
Opposite direction
acidosis
Respiratory  
alkalosis
Manisha Sahay

11. Step 3
Compensation(CO2 & HCO3)
 Body tries to
compensate to
normalise pH
 HCO3 and CO2
always move in CO2
HCO3
same direction
Manisha Sahay

12. Step 3
Compensation
For compensation HCO3 and CO2 follow each other
Primary event Compensation
Disorder pH PCO2 HCO3
Respiratory   
acidosis
Respiratory   
alkalosis
Metabolic   
acidosis
Metabolic   
alkalosis Manisha Sahay

13. Step 3:
Calculation of compensation
Disorder pH Primary Compensatory Equation
change Response
Metabolic   [HCO3-]  PCO2 ΔPCO2  1.2  ΔHCO3
Acidosis
Metabolic   [HCO3-]  PCO2 ΔPCO2  0.7  ΔHCO3
Alkalosis
Respiratory   PCO2  [HCO3-] Acute:
Acidosis ΔHCO3-  0.1  ΔPCO2
Chronic:
ΔHCO3-  0.3  ΔPCO2
Respiratory   PCO2  [HCO3-] Acute:
Alkalosis ΔHCO3-  0.2  ΔPCO2
Chronic:
ΔHCO3-  0.5  ΔPCO2

14. Compensation Formula Simplified
1.2
Acidosis
Metabolic
0.7
Alkalosis
0.1
Acidosis 0.3
Respiratory
0.2
Alkalosis 0.5
AcuteChronic

15. Step 4 Check Anion Gap
Manisha Sahay

16. What is anion gap?
 [Na+] – ([HC03-] + [Cl-])
 140 - (24 + 105) = 11
 Normal = 12 + 2
Manisha Sahay

17. 1. Anion gap helps in etiology
of Metabolic acidosis
High AG (high acid) NAG (loss of HCO3) Cl 
(MUDPILES) (DURHAM)
M ethanol  Diarrhea
U remia Ureterosigmoid fistula
D KA
P araldehyde Renal tubular acidosis
I nfection Hyperalimentation
L actic acidosis Acetazolamide
E thylene Glycol Miscellaneous conditions:
S alicylate - pancreatic fistula,
cholestyramine, calcium chloride
Manisha Sahay

18. 2. Anion gap may identify
hidden acidosis
pH may be normal but if Anion
gap is high it indicates metabolic
acidosis hence always calculate
anion gap
AG is called the footprint of
metabolic acidosis
No use of ABG without
electrolytes
Manisha Sahay

19. Step 5 check difference bet
AG and change in HCO3
Increase in AG should be equal to fall in
HCO3
Δ AG = Δ HCO3
Δ AG > Δ HCO3 - metabolic alkalosis
Δ AG < Δ HCO3 - metabolic acidosis
Manisha Sahay

20. Step 6-Urine anion gap
 Urine Anion-Gap = Na + K – Cl
 Normal Positive (+30- + 50) mmol/l
 In metabolic acidosis,
 if Urine anion gap is negative-acidosis is
extrarenal (Kidneys excreting NH4)
 Positive - DRTA

21. Step 7
Oxygenation and ventilation
Manisha Sahay

22. Normal arterio/venous difference No click
0 10 20 30 40 50 60 70 80 90 100 PaO2
100
80 Rt. Shift Oxygen delivered
to tissues
60 with normally placed curve
Delivered oxygen
40 with Rt. Shift curve
Normal
20
Shift of the curve ……changes saturation for a given PaO2

23. A Oxygenation
TCO2 19-20 ml/gm Hb
 Hb X SO2 X 1.34 + 0.003X PaO2
PaO2 80-95 mm Hg. Pv 28 - 48 mm
 1.34 ml O2 bound to each gm Hb
Reflects only dissolved oxygen not
 Best measure of hypoxemia
bound to Hb
Anemia, CO poisoning, type of
normal in anemia ,hypoxemia may
Hb all affect TCO2
exist with normal O2,not
affected by Hb
FIO2 - fractional inspired O2.
SaO2-95 - 100%; SvO2 50 – 70% N= 21% in room air
 only reflects % saturation of Hb FiO2 × 5 = PaO2
 normal in anemia ,hypoxemia may 21 × 5 = 100
exist with normal SO2 ,not
affected by type of Hb
Manisha Sahay

24. State which of the following situations would be
expected to lower PaO2.
 a) anemia.
 b) carbon monoxide toxicity.
 c) an abnormal hemoglobin that holds oxygen with half
the affinity of normal hemoglobin.
 d) an abnormal hemoglobin that holds oxygen with
twice the affinity of normal hemoglobin.
 e) lung disease with intra-pulmonary shunting.
Manisha Sahay

25. More on oxygenation…..
CONDITION PaO2 SaO2 CaO2
Severe Anemia n n 
CO Poisoning n  
Severe V-Q   
High Altitude   
Manisha Sahay

26. Body needs O2 molecules,
so oxygen content (CaO2)
takes precedence over
partial pressure
in determining degrees
of hypoxemia
Which patient is
more hypoxemic?
Patient A: PaO2 Patient A
CaO2 = .95 x 7 x 1.34 = 8.9 ml O2/dl
85 mm Hg, SaO2 Patient B
95%, Hb 7 gm% CaO2 = .85 x 15 x 1.34 = 17.1 ml O2/dl
Patient A, with the higher
PaO2, is more hypoxemic.
ALWAYS CHECK CaO2 CONTENT
Patient B: PaO2
55 mm Hg, SaO2
85%, Hb 15 gm% Manisha Sahay

27. ABG shows SO2 75%
Pulse oximeter shows SO2 97%
Which would you believe? Explain.
ABG is reliable as it measures oxy Hb
separately while on pulse oximeter any
bound Hb is shown as saturated Hb eg
Hb bound to CO will also increase SO2 in
pulseox. Manisha Sahay

28. B. Ventilation
PaCO2 important for assessing ventilation
• (N= 35-45 mmHg)
• > 45 hypoventilation
• < 35 hyperventilation
Manisha Sahay

29. Hypoventilation
Ventilation
failure
CO2 , ,O2
Lung
dz
Oxygenation failure
CO2 N,O2
Arterial blood pAO2
Manisha Sahay

30. C. Oxygenation vs Ventilation failure
Alveolar-arterial O2 gradient
PAO2-PaO2
PAO2 generally given on ABG
pAO2 = piO2 – pCO2 / RQ (respiratory quotient)
piO2= (Barometric Pr-Pr H2O) X FiO2
piO2 = (760-45) x . 21 = 150 mmHg

O2 
CO2
PAO2= 150 – 40 / 0.8
= 150 – 50 = 100 mm Hg
PaO2 = 90 mmHg
pAO2 – paO2 = 10 mmHg One click and wait

31. No click
Alveolar- arterial Difference
Oxygenation Failure Ventilation Failure
WIDE GAP NORMAL GAP
piO2 = 150 piO2 = 150
pCO2 = 40
pCO2 = 80
PaO2 = 45
O2 PaO2 = 45
CO2
palvO2= 150 – 40/.8 palvO2= 150-80/.8
=150-50 =150-100
=100 = 50
A-a = 100 - 45 = 55
A-a = 50 - 45 = 5

32. ABC’s of ABG
 1 Acidosis or alkalosis
 2 Metabolic or respiratory
 3 Compensation
Metabolic acidosis: 1.2(∆HCO3 = ∆ PCO2
Metabolic alkalosis: 0.7(∆ HCO3 = ∆ PCO2
Respiratory acidosis AcutC 0.1(ΔPCO2) = ΔHCO3
Respiratory acidosis: Chronic 0.3(ΔPCO2) = ΔHCO3
Respiratory alkalosis Acute 0.2(ΔPCO2) = ΔHCO3
Respiratory alkalosis chronic0.5(ΔPCO2) = ΔHCO3
 4 Anion gap
 5 Δ anion gap = Δ HCO3- metabolic alkalosis
 6 urine AG
 7 Oxygenation and ventilation
Remember the “golden rule” of acid-base
interpretation: always look at a patient’
Manisha Sahay

33. HANDS ON ABG
Manisha Sahay

34. Case 1
A 16-year-old male with diabetes mellitus presents after
having eaten no food and taken no insulin for the last 3
days . He is hypotensive, tachycardic, and markedly
tachypneic (respiratory rate 36). He smells strongly of
acetone and is dehydrated, and clinical findings are
consistent with left lower lobe pneumonia. Results of
ABG testing are:
• PaO2 = 68 mm Hg
• PaCO2 = 17 mm Hg
• HCO3- = 6 mEq/L
• pH = 7.30.
What is the acid-base abnormality?
Manisha Sahay

35. Case 2
A 16-year-old female presents with 24
hours of unremitting emesis. She is
dehydrated and hypotensive. Tests of
her ABG
• PaO2 = 104 mm Hg,
• PaCO2 = 46 mm Hg,
• HCO3- = 35 mEq/L
• pH = 7.49.
What is the acid-base
disturbance here?
Manisha Sahay

36. Case 3
Blood Gas Report
o
Measured 37.0 C
pH 7.523
PaCO2 30.1 mm Hg
PaO2 105.3 mm Hg
Calculated Data
HCO3 act 22 mmol / L
O2 Sat 98.3 %
PO2 (A - a) 8 mm Hg
DPO2 (a / A) 0.93
FiO2 21.0 %

37. Case 4 8 year old diabetic with respi. Distress
fatigue and loss of appetite.
Blood Gas Report
o
Measured 37.0 C
pH 7.23
PaCO2 23 mm Hg
PaO2 110.5 mm Hg
Calculated Data
HCO3 act 14 mmol / L
Entered Data
FiO2 21.0 %
Na = 130, Cl = 90

38. Case 5
A 16-year-old male presents with anorexia
and unremitting emesis for 4 days
ABG testing
 HCO3 - = 18 mEq/L
 PaCO2 = 33 mm Hg
 pH = 7.36
 Na = 130 meq/L
 Cl = 89 meq/L
Manisha Sahay

39. Case 6
 A 15 yr old type 1 diabetic presents with following
abnormalities after missing insulin
 ABG:
pH 7.31
PaCO2 26 mmHg
HCO3 12 mEq/L Serum Electrolytes:
PaO2 92 mm Hg Na 140 mEq/L
K 5.0 mEq/L
 Evaluate the acid-base Cl 100 mEq/L
disturbance(s)?

40. Case 7
 A 14 yr old boy presents with continuous vomiting of
3 days duration and drowsiness and appears
dehydrated
 ABG
pH 7.50
PaCO2 48
HCO3 32
PaO2 90
Na 139
K 3.9
Cl 85
 Evaluate the acid-base disturbance(s)?

41. Case 8
 A 15 yr girl admitted with renal failure
on furosemide now in respiratory failure
and is on ventilator
 ABG
• PaCO2 30 mm Hg
• PaO2 62 mm Hg
• Na+ 145 mEq/L
• K+ 2.9 mEq/L
• Cl- 98 mEq/L
• HCO3- 21 mEq/L
• pH 7.52

42. Case 9
 A 12-year-old girl has been mechanically ventilated for two
days following a drug overdose. Her arterial blood gas
values and electrolytes show:
 ABG
• pH 7.45
• PaCO2 25 mm Hg
• Na + 142 mEq/L
• K+ 4.0 mEq/L
• Cl- 100 mEq/L
• HCO3- 18 mEq/L

43. Case 10
 A 10 year old boy
with renal
 insufficiency admitted
with following values
 ABG
 pH 7.20
 PaCO2 24 mm Hg
 Na + 140 mEq/L
 K+ 5.6 mEq/L
 Cl- 110 mEq/L
 HCO3- 10 mEq/L

44. Types of metabolic acidosis
Patient A B C
ECF volume Low Low Normal
Glucose 600 120 120
pH 7.20 7.20 7.20
Na 140 140 140
Cl 103 118 118
-
HCO3 10 10 10
AG 27 12 12
Ketones 4+ 0 0

45. Urine electrolytes in Metabolic
Acidosis
Patient A B C
U. Na 10 50
U. K 14 47
U. Cl 74 28
Urine AG –50 +69
Dx:
Urine Anion Gap = (U. Na + U. K – U. Cl)
In Normal anion gap Metabolic Acidosis,
Positive Urine AG suggests distal Renal Tubular Acidosis
Negative Urine AG suggests non-renal cause for Metabolic
Acidosis.

46. 6 year old male with progressive respiratory distress
Muscular dystrophy .
Blood Gas Report
Measured 37.0 C
o
Case 11
pH 7.301
PaCO2 76.2 mm Hg
PaO2 45.5 mm Hg
Calculated Data
HCO3 act 35.1
mmol / L
O2 Sat 78 %
PO2 (A - a) 9.5 mm Hg
PO2 (a / A) 0.83
Entered Data
FiO2 21%

47. 8-year-old male asthmatic with resp. distress
Case 12
Blood Gas Report
o 8-year-old male asthmatic;
Measured 37.0 C
3 days of cough, dyspnea
pH 7. 24
and orthopnea not
PaCO2 49.1 mm Hg
responding to usual
PaO2 66.3 mm Hg
bronchodilators.
Calculated Data
HCO3 act18.0 mmol / L O/E: Respiratory distress;
O2 Sat 92 % suprasternal and
PO2 (A – a) mm Hg intercostal retraction;
tired looking; on 4 L NC.
Entered Data
FiO2 30 %

48. ABC’s of ABG
 1 Acidosis or alkalosis
 2 Metabolic or respiratory
 3 Compensation
Metabolic acidosis: 1.2(∆HCO3 = ∆ PCO2
Metabolic alkalosis: 0.7(∆ HCO3 = ∆ PCO2
Respiratory acidosis AcutC 0.1(ΔPCO2) = ΔHCO3
Respiratory acidosis: Chronic 0.3(ΔPCO2) = ΔHCO3
Respiratory alkalosis Acute 0.2(ΔPCO2) = ΔHCO3
Respiratory alkalosis chronic0.5(ΔPCO2) = ΔHCO3
 4 Anion gap
 5 Δ anion gap = Δ HCO3- metabolic alkalosis
 6 urine AG
 7 oxygenation and ventilation
Remember the “golden rule” of acid-base
interpretation: always look Sahay a patient’
Manisha at

49. Practice makes a man
perfect!!
Manisha Sahay