CLINICAL INTERPRETATION OF ABG : CLINICAL INTERPRETATION OF ABG Dr Vishram Buche
Director, NICU
Central InDIA’S CHILD hOSPITAL
& Research INSTITUTE
NAGPUR
INDIA
Slide 2: Central India’s
C H I L D
HOSPITAL
& Research Institute
Slide 3: pH PCO2 HCO3 PO2
Slide 4: …A respiratory component
…A respiratory acid
…Moves opposite to the direction of pH.
…A metabolic component
…It is a base (Metabolic)
…Moves in the same direction of pH.
…Moves in same direction... Primary disorder
…Moves in opposite direction …Mixed Disorder CO2 HCO3 CO2
HCO3 Facts about Acid-Base balance…… Remember format………..
pH………….. 7.4 (7.35 - 7.45)
PCO2 …….…40 (35 -45)
HCO3 ……… 24 (22 -26)
Slide 5: Getting a Feel Of Blood Gases
Slide 6: Primary lesion compensation pH HCO3 CO2 METABOLIC ACIDOSIS HYPER VENTILATION HCO3 changes
pH in same direction Low
Alkali LOW HCO3
LOW pH
LOW pCO2 (compensated)
Slide 7: Primary lesion compensation pH HCO3 CO2 METABOLIC ALKALOSIS HYPO VENTILATION BICARB CHANGES
pH in same direction HIGH HCO3
HIGH pH
HIGH pCO2 (compensated) High
Alkali
Slide 8: CO 2 CHANGES
pH in opposite direction Primary lesion compensation pH CO 2 BICARB Respiratory acidosis HIGH pCO2
LOW pH
HIGH HCO3 (compensated) High
CO2
Slide 9: Primary lesion compensation pH CO 2 BICARB Respiratory alkalosis Low
CO2 CO 2 CHANGES
pH in opposite direction LOW pCO2
HIGH pH
LOW HCO3 (compensated)
Slide 10: Body’s physiologic response to Primary disorder
in order to bring pH towards NORMAL limit Full compensation
Partial compensation
No compensation…. (uncompensated) BUT never overshoots,
If a overshoot pH is there,
Take it granted it is a MIXED disorder Compensation….
Slide 11: Remember…….
Respiratory compensation
is always FAST …12-24 hrs
Metabolic compensation
is always SLOW...5 -7 days Respiratory disturbances are better compensated than metabolic ones.
Predictable in……… Metabolic acidosis.
Un-predictable in……. metabolic alkalosis.
How to identify the type of compensation…..? : How to identify the type of compensation…..? pH HCO3 CO2
7.20 15 40
7.25 15 30
7.37 15 20 Un Compensated Partially Compensated Fully Compensated (pH abnormal) (pH in normal range)
Slide 13: COMPENSATION LIMITS Compensation Beyond Limits…………..
Mixed disorder
Slide 14: xygenation
Slide 17: …To calculate A-a gradient….
Is the baby hypoxic?
Type and severity of Hypoxia.
…Relationship of PaO2 and FiO2?
FiO2 X 5 = Expected PaO2
…Whether PaO2 is appropriate for the given FiO2?
…Is the O2 content (CaO2) enough to prevent hypoxia?
Alveolar-arterial O2 Difference : Alveolar-arterial O2 Difference * When FiO2 = 21 % :
PiO2 = (760-45) x .21= 150 mmHg O2
CO2 (calculated)PAO2 = 150 – 1.2 (PCO2)
= 150 – 1.2 40
= 150 – 50 = 100 mm Hg (measured) PaO2 = 90 mmHg ………..PAO2 – PaO2 = ?
PAO2 = PiO2* -(PCO2/0.8) PAO2 – PaO2 = 10 mmHg PaO2 PAO2 Classify Respiratory Failure
Ventilation–perfusion mismatch
……FiO2 dependant derivation
Alveolar-arterial Difference : Alveolar-arterial Difference O2
CO2 Alveolar – arterial G.
100 - 45 = 55
……………….Wide A-a Oxygenation Failure
Wide Gap
PCO2 = 40
PaO2 = 45
PAO2 = 150 – 1.2 (40)
= 150 - 50
= 100 Ventilation Failure
Normal Gap
PCO2 = 80
PaO2 = 45
PAO2 = 150-1.2(80)
= 150-100
= 50 Alveolar arterial G.
50 – 45 = 5
…………….Normal A-a
Slide 20: 20 × 5 = 100 Expected PaO2 = FiO2 × 5 = PaO2 Normal
Slide 21: It is essential to have
ELECTROLYTES
for
crucial interpretation of ABG.
esp. Na, Cl, K We always correlate PaO2 with FiO2
BUT………………………….
never forget to correlate with PaCO2
Slide 22: It Is Incomplete without…… FiO2
Hb ct The essentials of Blood gas… pH
Slide 23: Now that I have this data, what does it mean? ----- XXXX Diagnostics ------
Blood Gas Report
248 05:36 Jul 22 2000
Pt ID 2570 / 00
Measured 37.0o C
pH 7.463
pCO2 44.4 mm Hg
pO2 113.2 mm Hg
Corrected 38.6o C
pH 7.439
pCO2 47.6 mm Hg
pO2 123.5 mm Hg
Calculated Data
TPCO2 49
HCO3 act 31.1 mmol / L
HCO3 std 30.5 mmol / L
BE 6.6 mmol / L
O2 CT 14.7 mL / dl
O2 Sat 98.3 %
ct CO2 32.4 mmol / L
pO2 (A - a) 32.2 mm Hg
pO2 (a / A) 0.79
Entered Data
Temp 38.6 oC
ct Hb 10.5 g/dl
FiO2 30.0 % output
Slide 24: Experience is the ability to
make the same mistake
repeatedly with increasing
confidence
Slide 25: The Anatomy
of a Blood Gas Report -----XXXX Diagnostics-----
Blood Gas Report
328 03:44 Feb 5 2006
Pt ID 3245 / 00
Measured 37.0 0C
pH 7.452
pCO2 45.1 mm Hg
pO2 112.3 mm Hg
Corrected 38.6 0C
pH 7.436
pCO2 47.6 mm Hg
pO2 122.4 mm Hg
Calculated Data
HCO3 act 31.2 mmol / L
HCO3 std 30.5 mmol / L
B E 6.6 mmol / L
O2 ct 15.8 mL / dl
O2 Sat 98.4 %
ct CO2 32.5 mmol / L
pO2 (A -a) 30.2 mm Hg
pO2 (a/A) 0.78
Entered Data
Temp 38.6 0C
FiO2 30.0 %
ct Hb 10.5 gm/dl Measured values…
most important Temperature Correction :
Is there any value to it ? Calculated Data :
Which are useful one? Entered Data :
Important
Slide 26: Uncorrected pH & pCO2 are reliable reflections of in-vivo acid base status
Temperature correction of pH & pCO2 do
not affect calculated bicarbonate
“ There is no scientific basis ... for applying temperature corrections to blood gas measurements…” Shapiro BA, OTCC, 1999.
pCO2 reference points at 37o C are well established as a reliable reflectors of alveolar ventilation
Reliable data on DO2 and oxygen demand are unavailable at temperatures other than 37o C Measured values should be considered
And
Corrected values should be discarded
Slide 27: Bicarbonate is calculated on the basis of the
Henderson equation:
[H+] = 24 pCO2 / [HCO3-]
or
for the
Mathematically inclined… Act Bicarbonate: -----XXXX Diagnostics-----
Blood Gas Report
328 03:44 Feb 5 2006
Pt ID 3245 / 00
Measured 37.0 0C
pH 7.452
pCO2 45.1 mm Hg
pO2 112.3 mm Hg
Corrected 38.6 0C
pH 7.436
pCO2 47.6 mm Hg
pO2 122.4 mm Hg
Calculated Data
HCO3 act 31.2 mmol / L
HCO3 std 30.5 mmol / L
B E 6.6 mmol / L
O2 ct 15.8 mL / dl
O2 Sat 98.4 %
ct CO2 32.5 mmol / L
pO2 (A -a) 30.2 mm Hg
pO2 (a/A) 0.78
Entered Data
Temp 38.6 0C
FiO2 30.0 %
ct Hb 10.5 gm/dl
Slide 28: -----XXXX Diagnostics-----
Blood Gas Report
328 03:44 Feb 5 2006
Pt ID 3245 / 00
Measured 37.0 0C
pH 7.452
pCO2 45.1 mm Hg
pO2 112.3 mm Hg
Corrected 38.6 0C
pH 7.436
pCO2 47.6 mm Hg
pO2 122.4 mm Hg
Calculated Data
HCO3 act 31.2 mmol / L
HCO3 std 30.5 mmol / L
B E 6.6 mmol / L
O2 ct 15.8 mL / dl
O2 Sat 98.4 %
ct CO2 32.5 mmol / L
pO2 (A -a) 30.2 mm Hg
pO2 (a/A) 0.78
Entered Data
Temp 38.6 0C
FiO2 30.0 %
ct Hb 10.5 gm/dl
Slide 29: Oxygenation
Parameters: /limitations
O2 Content of blood:
(Hb x1.34x O2 Sat + 0.003x Dissolved O2 )
Remember Hemoglobin
Oxygen Saturation:
( remember this is calculated …error prone)
Alveolar / arterial gradient:
( classify respiratory failure)
Arterial / alveolar ratio:
Proposed to be less variable
Same limitations as A-a gradient -----XXXX Diagnostics-----
Blood Gas Report
328 03:44 Feb 5 2006
Pt ID 3245 / 00
Measured 37.0 0C
pH 7.452
pCO2 45.1 mm Hg
pO2 112.3 mm Hg
Corrected 38.6 0C
pH 7.436
pCO2 47.6 mm Hg
pO2 122.4 mm Hg
Calculated Data
HCO3 act 31.2 mmol / L
HCO3 std 30.5 mmol / L
B E 6.6 mmol / L
O2 ct 15.8 mL / dl
O2 Sat 98.4 %
ct CO2 32.5 mmol / L
pO2 (A -a) 30.2 mm Hg
pO2 (a/A) 0.78
Entered Data
Temp 38.6 0C
FiO2 30.0 %
ct Hb 10.5 gm/dl
Slide 30: A Systematic and Pointed
………. approach
Slide 31: Steps for
Successful
Blood Gas
Analysis 7
Slide 32: 1.
2. Look at pH?
3. Who is the culprit ?...Metabolic / Respiratory
4. If respiratory…… acute and /or chronic
5. If metabolic acidosis,
Anion gap ↑ed and/or normal or both?
Is more than one disorder present?
Correlate clinically Consider the clinical settings! Anticipate the disorder 7 steps to analyze ABG
Slide 33: Look at the pH
Is the patient acidemic pH < 7.35
or alkalemic pH > 7.45
If pH = 7.4 …… Normal
Mixed or Fully compensated Step 2
Step 3 ……. CULPRIT? : Step 3 ……. CULPRIT? HCO3…… METABOLIC
> 26 ….. Met. Alkalosis
< 22 ……Met. Acidosis
PCO2 ……RESPIRATORY
> 45 …… Resp. Acidosis
< 35 …… Resp. Alkalosis HCO3 = Base
Normal…22-26 CO2 = ACID
Normal…35-45
Slide 36: If there is a primary Respiratory disturbance,
is it acute ? .08 change in pH ( Acute )
.03 change in pH (Chronic) 10 mm
Change
PaCO2 = Remember………… relation of CO2 and pH Step 4 …
Slide 37: PCO2 of 10 Acute change .08
Chronic change .03 pH Step 4 continued…
Slide 38: 7.60 20 7.50 30 7.40 40 7.30 50 7.20 60 7.10 70 pH PaCO2 Acute respiratory change pH
Last two digits 80 – PaCO2 Step 4 continued…
Slide 39: Step 4 continued… RESPIRATORY disorders…
Expected HCO3 for a Change in CO2 ......... 1 2 3 4 Acidosis…. (expected) HCO3 = 0.1 x ∆ CO2
Alkalosis…. (expected) HCO3 = 0.2 x ∆ CO2
Acidosis…. (expected) HCO3 = 0.35 x ∆ CO2
Alkaosis…. (expected) HCO3 = 0.4 x ∆ CO2 Acute respiratory Chronic respiratory
Slide 40: If it is a primary Metabolic disturbance,
whether respiratory compensation appropriate?
For metabolic acidosis:Expected PCO2 = (1.5 x [HCO3]) + 8 + 2
(Winter’s equation) For metabolic alkalosis:
Expected PCO2 = 6 mm… for 10 mEq. rise in Bicarb.
………UNCERTAIN COMPENSATION CO2 is equal to
Last two digits
of pH Remember If :
Suspect .............
actual PaCO2 is more than expected
additional...respiratory acidosis
actual PaCO2 is less than expected
additional...respiratory alkalosis Step 5
Slide 41: If metabolic acidosis is there
How is anion gap ? Is it wide ...
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
Lactic acidosis
Metabolic disorders
Renal failure Step 5 cont.
Step 6… : Step 6… Is more than one DISORDER present?
Mixed Acid-Base Disorders : Clues : Mixed Acid-Base Disorders : Clues -- Clinical history
-- pH normal, abnormal PCO2 n HCO3
-- PCO2 n HCO3 moving opposite directions
-- Degree of compensation for primary
disorder is inappropriate
-- Find Delta Gap
Metabolic Acidosis……. + additional disorders : Metabolic Acidosis……. + additional disorders Equivalent rise of AG and Fall of HCO3……
….Pure Anion Gap Metabolic Acidosis
Discrepancy…….. in rise & fall
+ Non AG M acidosis, + M Alkalosis
PURE Anion Gap Acidosis + : PURE Anion Gap Acidosis + Delta gap = HCO3 + ∆ AG
Delta Gap = 24….Pure AG acidosis
< 24 = non AG acidosis (+ AG M Acidosis)
> 24 = metabolic alkalosis (+ AG M Acidosis)
Slide 46: N-HCO3 = 24, N-Anion Gap = 12
Delta Gap = HCO3 + ∆AG
e.g. if HCO3= 12, AG = 24, ∆ AG = 12
Delta gap = 12 + 12 = 24
….Pure AG Metabolic Acidosis Delta Gap = 24 ……AG met Acidosis
< 24 ….. + Non AG Mac
> 24 ….. + Meta. Alkalosis N-HCO3 = 24, N-Anion Gap = 12
Delta Gap = HCO3 + ∆ AG
e.g. if HCO3 = 12, AG = 20, ∆ AG = 8
Delta Gap = 12 + 8 = 20,
< 24 …AG + Non AG metabolic Acidosis N-HCO3 = 24, N-Anion Gap = 12
Delta Gap = HCO3 + ∆ AG
e.g. if HCO3 = 12, AG = 30, ∆ AG = 18
Delta Gap = 12 + 18 = 30
> 24 ….AG + metabolic Alkalosis
Double……… triple……………. Quadruple…….??? : Double……… triple……………. Quadruple…….???
Slide 48: th step Clinical correlation 7
Validity of ABG report… a lab error : Validity of ABG report… a lab error e.g. pH = 7.30, PCO2 = 38, HCO3 = 30 By Henderson-Hasselbach
H+ = 24 x pCO2/HCO3
= 24 x (38/30) = 30
80 - last two digit pH = H+
80 - H+ = last two digit pH (after 7)
pH should be 7.50
Slide 50: Ready Chart………
It’s not magic understanding ABG’ s, it just takes a little practice! : It’s not magic understanding ABG’ s, it just takes a little practice!
Slide 52: Experience is a wonderful
thing. It enables you to recognize a mistake when you make it (again).
Slide 53: pH............7.34
PaCO2.....33.9
HCO3.......18.2 Partially compensated Metabolic Acidosis pH = 7.4
PaCO2 = 40
HCO3 = 24 9 months old male with Acute Enteritis….. Partially compensated Metabolic Acidosis
Slide 54: pH.............7.55
PaCO2.....49.0
HCO3.......48.2 Partially compensated Metabolic Alkalosis pH = 7.4
PaCO2 = 40
HCO3 = 24
Slide 55: pH............7.44
PaCO2.....27.0
HCO3.......18.1 Fully compensated Respiratory Alkalosis pH = 7.4
PaCO2 = 40
HCO3 = 24
Slide 56: pH............7.28
PaCO2.....79.5
HCO3.......37.1 Partially compensated Respiratory Acidosis pH = 7.4
PaCO2 = 40
HCO3 = 24
Slide 57: pH............7.51
PaCO2.....39.4
HCO3.......31.3 Uncompensated
Metabolic Alkalosis pH = 7.4
PaCO2 = 40
HCO3 = 24
Slide 58: pH............7.39
PaCO2.....39.0
HCO3.......23.4 Normal A.B.G. pH = 7.4
PaCO2 = 40
HCO3 = 24
Slide 59: pH............7.25
PaCO2.....58.5
HCO3.......25.1 Uncompensated
Respiratory Acidosis pH = 7.4
PaCO2 = 40
HCO3 = 24
Slide 60: pH............7.46
PaCO2.....34.0
HCO3.......26.0 Uncompensated
Respiratory Alkalosis pH = 7.4
PaCO2 = 40
HCO3 = 24
Slide 61: pH..............7.37
PaCO2.......75.1
HCO3.........42.6 Fully compensated Respiratory Acidosis pH = 7.4
PaCO2 = 40
HCO3 = 24
Slide 62: pH..............7.52
PaCO2.......31.0
HCO3.........29.4 Combined Alkalosis pH = 7.4
PaCO2 = 40
HCO3 = 24
Slide 63: pH..............7.08
PaCO2.......54.0
HCO3.........18.0 Combined Acidosis pH = 7.4
PaCO2 = 40
HCO3 = 24
Slide 64: ▲Respiratory
Alkalosis What is the
Diagnosis ? pH ………7.563
PCO2 ….19.8
HCO3 ….18.7 For a 10 mm change of PCO2
pH changes by 0.08 ……Acute
by 0.03 ……Chronic Is it acute / Chronic? Acute Respiratory Alkalosis
THANKS : THANKS : vbuche@gmail.com