Abc of abg richa j

1. ABC OF ABG
STUDENT - DR RICHA JOSHI
GUIDE – DR NILOFER MUJAWAR

2. 0verview of discussion
 Basics of acid-base balance.
 Role of Renal/Respiratory system in acid-base
homeostasis.
 Step-wise approach in diagnosis of acid-base
disorders.
 Some practical examples

3. Acid Base Balance
H+ ion concentration in the body is precisely
regulated
The body understands the importance of H+
and hence devised DEFENCES against any
change in its concentration-
BICARBONATE
BUFFER SYSTEM
Acts in few seconds
RESPIRATORY
REGULATION
Acts in few minutes
RENAL
REGULATION
Acts in hours to days
A
C
I
D
B
A
S
E

4. Regulation of Acid Base
Bicarbonate Buffer System
CO2 + H2O carbonic anhydrase H2CO3 H+ + HCO3
-
In Acidosis - Acid = H+
H+ + HCO3 H2CO3 CO2 + H2O
In Alkalosis - Alkali + Weak Acid = H2CO3
CO2 + H20 H2CO3 HCO3
- + H+
+
ALKALI

5. Respiratory Regulation of Acid Base
Balance-
H+ PaCO2
H+ PaCO2
ALVEOLAR
VENTILATION
ALVEOLAR
VENTILATION

6. Renal Regulation of Acid Base Balance
Kidneys control the acid-base balance by excreting
either an acidic or a basic urine,
This is achieved in the following ways-
Reabsorption Secretion of H+
of HCO3 ions in tubules
in blood and excretion
•Proximal Convulated
Tubules (85%)
•Thick Ascending Limb of
Loop of Henle (10%)
•Distal Convulated Tubule
•Collecting Tubules(5%)
ECF
Volume Angiotensin II
Aldosterone

7. • Another mechanism by which the kidney
controls the acid base balance is by the
Combination of excess H+ ions in urine
with AMMONIA and other buffers- A
mechanism for generating NEW
Bicarbonate ions
GLUTAMINE
2HCO3
- 2NH4
+REABSORBED EXCRETED
+
H+, Cl-

8. pH
• pH equals the negative logarithm (log) to the
hydrogen ion.
• pH = - log [H+]
• pH = log 1 / H+concentration
• H+ concentration in extracellular fluid (ECF)
4 X 10 -8 that is (0.00000004)

9. • Low pH values = high H+concentrations
• H+concentration in denominator of formula
• 1Unit changes in pH represent a tenfold
change in H+concentrations.

10. H ION CONC.
ION CONC.
N.MOLS / L.
pH
20 7.70
30 7.52
40 7.40
50 7.30
60 7.22
H ION
OH ION
0
14
pH stand for "power of hydrogen"
H+ = 80 - last two
digits of pH
Don’t click wait …..till
Last message …….. “H = 80-last two digits of pH”
pH

11. HENDERSONS EQUATION
• pH = - log [H+]
Henderson-Hasselbalch equation
pH = 6.1 + log HCO3
-
0.03 x PCO2
The [HCO3-] mentioned on the ABG is actually calculated
using this equation from measured values of PCO2 nd pH
• [H+] = 24 X (PCO2 / HCO3)
• pH = -log [ H+]
pHexpected = pHmeasured = ABG is authentic

12. Definition and terminology
Component Terminology
 Acidosis/Alkalosis
 Respiratory/Metabolic
 Compensated/Uncompensated

13. Basic terminology
• pH – signifies free hydrogen ion concentration. pH is inversely related to
H+ ion concentration.
• Acid – a substance that can donate H+ ion, i.e. lowers pH.
• Base –a substance that can accept H+ ion, i.e. raises pH.
• Anion – an ion with negative charge.
• Cation – an ion with positive charge.
• Acidemia – blood pH< 7.35 with increased H+ concentration.
• Alkalemia – blood pH>7.45 with decreased H+ concentration.
• Acidosis – Abnormal process or disease which reduces pH due to increase
in acid or decrease in alkali.
• Alkalosis – Abnormal process or disease which increases pH due to
decrease in acid or increase in alkali.

14. • Compensation – The body’s response to
neutralise the effect of the initial insult on ph
homeostasis is called compensation .
ph is maintained by ratio of HCO3/PaCO2(
hendersons equation ) . Thus primary
metabolic disorder leads to compensatory
respiratory response .
compensatory change is in the same direction
as primary change .

15. Aerobic respiration
During aerobic respiration your
heart and lungs work to supply the
tissues with oxygen.
The equation for respiration is:
glucose + oxygen =
CO2 + H2O + energy
Respiration 8

16. Anaerobic respiration
Respiration 11
glucose + NO oxygen
lactic acid + energy
During anaerobic respiration the
tissues are NOT supplied with
oxygen.

17. Why Order an ABG?
 Aids in establishing a diagnosis
 Helps guide treatment plan
 Aids in ventilator management
 Improvement in acid/base management allows
for optimal function of medications
 Acid/base status may alter electrolyte levels
critical to patient status/care.
 Pre operative fitness.

18. The Goal :
To provide Bedsideapproach to
ABG analysis
No click

19. Oxygenation
Indices:
O2 Content of blood:
Hb. x O2 Sat + Dissolved O2
(Don’t forget hemoglobin)
Oxygen Saturation: reported as ABG report
( Derived from oxygen dis. curve
not a measured value )
Alveolar / arterial gradient:
( Useful … to classify respiratory failure )
No click

20. Determination of PaO2
PaO2 is dependant upon Age, FiO2, Patm
As Age the expected PaO2
• PaO2 = 109 - 0.4 (Age)
As FiO2 the expected PaO2
• Alveolar Gas Equation:
• PAO2= (PB-P h2o) x FiO2- pCO2/R
O
X
Y
G
E
N
A
T
I
O
N
PAO2 = partial pressure of oxygen in alveolar gas, PB = barometric pressure
(760mmHg), Ph2o = water vapor pressure (47 mm Hg), FiO2 = fraction of
inspired oxygen, PCO2 = partial pressure of CO2 in the ABG, R = respiratory
quotient (0.8)

21. Determination of the PaO2 / FiO2 ratio
Inspired Air FiO2 = 21%
PiO2 = 150 mmHg
PalvO2 = 100 mmHg
PaO2 = 90 mmHg
O2CO2

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

23. Alveolar-arterial Difference
Inspired O2 = 21 %
piO2 = (760-45) x . 21 = 150 mmHg
O2
CO2
palvO2 = piO2 – pCO2 / RQ
= 150 – 40 / 0.8
= 150 – 50 = 100 mm Hg
PaO2 = 90 mmHg
palvO2 – partO2 = 10 mmHg One click and wait

24. Alveolar- arterial Difference
O2
CO2
Oxygenation Failure
WIDE GAP
piO2 = 150
pCO2 = 40
palvO2= 150 – 40/.8
=150-50
=100
PaO2 = 45
D = 100 - 45 = 55
Ventilation Failure
NORMAL GAP
piO2 = 150
pCO2 = 80
palvO2= 150-80/.8
=150-100
= 50
PaO2 = 45
D = 50 - 45 = 5
PAO2 (partial pres. of O2. in the alveolus.)
= 150 - ( PaCO2 / .8 )
760 – 45 = 715 : 21 % of 715 = 150
No click

25. 20 × 5 = 100
Expected PaO2 =
FiO2 × 5 = PaO2
Normal situation
No click

26. The Blood Gas
Report: normals…
pH 7.40 + 0.05
PaCO2 40 + 5 mm Hg
PaO2 80 - 100 mm Hg
HCO3 24 + 4 meql/L
O2 Sat >95
Always mention and see FIO2
The essentials
HCO3
No click

27. Normal values in newborns

28. Oxygen cascade

29. Arterial and venous sample
ARTERIAL VENOUS
pH 7.38-7.42 7.36-7.39
PaO2 80-100 mm hg 38-42 mm hg
PaCO2 36-44 mm hg 44-48 mm hg
HCO3 22-26 meq/l 20-24 meq/l
SaO2 95-100 % 75 %

30. The
Steps for
Successful
Blood Gas
Analysis
No click

31. Step 2
Who is responsible for this change in pH ( culprit )?
 CO2 will change pH in opposite direction
 Bicarb. will change pH in same direction
Acidemia: With HCO3 < 20 mmol/L = metabolic
With PCO2 >45 mm hg = respiratory
Alkalemia: With HCO3 >28 mmol/L = metabolic
With PCO2 <35 mm Hg = respiratory
Step 1
Look at the pH
Is the patient acidemic pH < 7.35
or alkalemic pH > 7.45

32. Step 3
If there is a primary respiratory disturbance, is it
acute ?
.
No click
For resp alkalosis -With every 10mmHg decrease in PaCO2
below 40, decrease in Bicarb
by 2 : Acute respiratory alkalosis
by 4 to 5 : Chronic respiratory
alkalosis
For respiratory acidosis -With every 10mmHg increase in
PaCO2 above 40, increase in Bicarb
by 1 : Acute respiratory acidosis
by 4 to 5 : Chronic respiratory
acidosis

33. Step 4
If the disturbance is metabolic is the respiratory
compensation appropriate?
For metabolic acidosis:
Expected PaCO2 = (1.5 x [HCO3]) + 8 ) + 2
or simply…
expected PaCO2 = last two digits of pH
For metabolic alkalosis:
Expected PaCO2 =0.7 x( HCO3* − 24 )+ 40 ± 2
Suspect if .............
actual PaCO2 is more than expected : additional
…respiratory acidosis
actual PaCO2 is less than expected : additional
…respiratory alkalosis
No click

34. Step 4 cont.
If there is metabolic acidosis, is there a wide anion gap ?
Na - (Cl-
+ HCO3
-
) = Anion Gap usually <12
If >12, Anion Gap Acidosis :
M ethanol
U remia
D iabetic Ketoacidosis
P araldehyde
I nfection (lactic acid)
E thylene Glycol
S alicylate
Common pediatric causes
1) Lactic acidosis
2) Metabolic disorders
3) Renal failure
No click

35. th step
Clinical correlation
No click

36. •Metabolic
DisorderHCO3
•Respiratory
DisorderPaCO2

37. Remember the format
pH
PaCO2
PaO2
No click

38. Primary lesion
Primary lesion
Compensation
pH
Bicarbonate
PaCO2
METABOLIC ACIDOSIS
HYPER VENTILATION
BICARB CHANGES
pH in same direction
Low
Alkali
Three clicks

39. Primary lesion
Compensation
pH
Bicarbonate
PaCO2
METABOLIC ALKALOSIS
HYPO VENTILATION
BICARB CHANGES
pH in same direction
High
Alkali
Three clicks

40. Primary lesion
compensation
pH
PaCO 2
BICARB
Respiratory acidosis
CO 2 CHANGES
pH in opposite direction
High
CO2
Three clicks
Wait for red circle
RENAL COMPENSATION

41. Primary lesion
Primary lesion
compensation
pH
PaCO 2
BICARB
Respiratory alkalosis
PaCO 2 CHANGES
pH in opposite direction
Low
PaCO2
Three clicks
Wait for red circle
RENAL COMPENSATION

42. INTERPRETATION OF A.B.G.
FOUR STEP METHOD OF DEOSAT
1) LOOK FOR pH
2) WHO IS THE CULPRIT ?
3) IF RESPIRATORY ACUTE / CHRONIC ?
4) IF METABOLIC / COMP. / ANION GAP
CLINICAL CORRELATION
No click

43. compensation considered
complete
when the pH
returns to
normal
range
Clinical blood gases by Malley
No click

44. COMPENSIONLIMITS
METABLIC ACIDOSIS
PaCO2 = Up to 10 ?
METABOLIC ALKALOSIS
PaCO2 = Maximum 6O
RESPIRATORY ACIDOSIS
BICARB = Maximum 40
RESPIRATORY ALKALOSIS
BICARB = Up to 10
No click

47. Blood Gas Report
Measured 37.0
o
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 D
PO2 (a / A) 0.93
Entered Data
FiO2 21.0 %
Case 1
16 year old female with
sudden onset of dyspnea.
No Cough or Chest Pain
Vitals normal but RR 56,
anxious.
One click for answer
Acute respiratory alkalosis

48. Case 2 6 year old male with progressive respiratory distress
Muscular dystrophy .
Blood Gas Report
Measured 37.0
o
C
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 D
PO2 (a / A) 0.83
Entered Data
FiO2 21 %
pH <7.35 :acidemia
Res. Acidemia : High PaCO2 and low
pH
Hypoxemia
Normal A-a gradient
In chronic for every 10 mmhg increase
in pco2 the bicarb increases by 4-5
Chronic resp. acidosis
Hypoventilation
Chronic respiratory acidosis
With hypoxia due to hypoventilation
Five clicks

49. Case 3
8-year-old male asthmatic;
3 days of cough, dyspnea
and orthopnea not
responding to usual
bronchodilators.
O/E: Respiratory distress;
suprasternal and
intercostal retraction;
tired looking; on 4 L NC.
Blood Gas Report
Measured 37.0
o
C
pH 7. 24
PaCO2 49.1 mm Hg
PaO2 66.3 mm Hg
Calculated Data
HCO3 act 18.0 mmol / L
O2 Sat 92 %
PO2 (A - a) mm Hg D
PO2 (a / A)
Entered Data
FiO2 30 %
153-66= 87
pH <7.35 ; acidemia
PaCO2 >45; respiratory acidemia
piO2 = 715x.3=214.5 / palvO2 = 214-49/.8=153 Wide A / a gradient
Hypoxia
WITH INCREASE IN CO2 BICARB MUST RISE ?
Bicarbonate is low………
Metabolic acidosis + respiratory acidosis
30 × 5 = 150
resp. acidosis
8-year-old male asthmatic with resp. distress Six clicks

50. Case 4 8 year old diabetic with respi. distress fatigue and loss of appetite.
Blood Gas Report
Measured 37.0
o
C
pH 7.23
PaCO2 27 mm Hg
PaO2 110.5 mm Hg
Calculated Data
HCO3 act 14 mmol / L
O2 Sat %
PO2 (A - a) mm Hg D
PO2 (a / A)
Entered Data
FiO2 21.0 %
pH <7.35 ; acidemia
HCO3 <22; metabolic acidemia
Last two digits of pH
Correspond with co2
If Na = 130,
Cl = 90
Anion Gap = 130 - (90 + 14)
= 130 – 104 = 26
Three clicks

51. These findings are most consistent with….
a) Metabolic acidosis with compensatory Hypocapnia.
b) Primary metabolic acidosis with
respiratory alkalosis.
c) Acute respiratory alkalosis fully compensated.
d) Chronic respiratory alkalosis fully compensated.
pH 7.39
PCO2 l5mmHg
HCO3 8mmol/L
PaO2 90 mmHg
For metabolic acidosis: FULL COMPENSATION
Expected PaCO2 = (1.5 x [HCO3]) + 8 ) + 2
(Winter’s equation)
PCO 2 ……SHOULD BE 20
Case 6………….
One click

52. Respiratory
Alkalosis
Is it acute ?
What is the
Diagnosis
Click for answer
Case 8,,,,,,,,,,,,,,,,,,
pH 7.583
PCO2 19.8
HCO3 18.7

53. Case 7…….
• Known case of COPD develops severe
vomitting …
• ph =7.4
• Hco3= 36meq/l
• paco2= 60mmhg
• Diagnosis ??

54. • Ph is normal – mixed disorder
• Increased bicarb = metabolic alkalosis due to
vomitting …
• Increased paco2 = respiratory acidosis due to
COPD …
• Metabolic alkalosis is expected to increase the
ph and resp acidosis to decrease the ph …
thus ph normal …

56. U should know !!!
• Standard bicarbonate (stHCO3)- It is the bicarbonate
concentration assuming a temp of 37celsius and pco2 of 40
mmhg . It reflects the true metabolic state of the body .
• Buffer base – It is the sum total of all body store of buffer .
Normal 45-50 . If <45 it is metabolic acidosis and >50 it is
metabolic alkalosis .
• Base excess – it is the amount of acid or alkali that must be
added to sample of whole blood so as to restore the body ph
to 7.4 assuming paco2 of 40 mmhg .metabolic alkalosis has
excess base(+BE) and metabolic acidosis has base deficit (- BE)
normal value (0+- 2.5mmol)

57. Application in ventilator settings
Blood gas
abnormality
fio2 rate PIP PEEP Ti comments
Hypercapnea
(paco2 >40mmhg)
- ↑ ↑ - - Adjust PEEP to avoid auto
PEEP
Hypocapnea
(paco2<35mmhg)
- ↓ ↓ - - -
Hyperoxia
(pao2>100mmhg)
↓ - ↓ ↓ ↓ -
Hypoxia
(pao2<60mmhg)
↑ - ↑ ↑ ↑ If chest excursions are
adequate , u can increase
fio2..
Total ventilatory
failure (paco2 too
high and pao2 too
low)
Depends upon the cause
.. Check chest expansion
and ETT patency … check
for air leak …

58. 0.1ml of O2 consumed /dl of blood in 10 min in
pt with normal TLC..

60.  8) I shall practice gentle
mechanical ventilation and not
to try bring ABG to perfect
normal.
 9) I shall treat the patient, not
the ABG report.
 10) I shall always correlate ABG
report clinically.