Arterial Blood Gas Analysis …..1

Dr Satish DeopujariPediatricianHon. Prof. ( Pediatrics) JNMCChairman NationalIntensive care chapterIndian academy of pediatricsdeopujari@rediffmail.com

Visit us at…. http://rdsoxy.org

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The Goal :

To provide Bedside approach to ABG analysis

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H ION CONC.N.MOLS / L. pH

20 7.70

30 7.52

40 7.40

50 7.30

60 7.22H 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”

Bicarbonate:

Henderson - Hasselbach equation:

pH = pK + Log HCO3

Dissolved CO2

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Standard Bicarbonate:Plasma HCO3 after equilibrationto a PaCO2 of 40 mm Hg

: Reflects non-respiratory acid base change: No quantification of the extent of the buffer base abnormality

Base Excess: base to normalise HCO3 (to 24) with PaCO2 at 40 mm Hg(Sigaard-Andersen)

: Reflects metabolic part of acid base : No info. over that derived from pH, pCO2 and HCO3

: Misinterpreted in chronic or mixed disorders

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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 )

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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

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Oxygen delivered to tissues

with normally placed curve

Delivered oxygen with Rt. Shift curve

Alveolar-arterial DifferenceInspired 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

Alveolar- arterial Difference

O2

CO2

Oxygenation Failure WIDE GAPpiO2 = 150pCO2 = 40

palvO2= 150 – 40/.8=150-50 =100

PaO2 = 45

= 100 - 45 = 55

Ventilation Failure NORMAL GAPpiO2 = 150

pCO2 = 80

palvO2= 150-80/.8 =150-100= 50PaO2 = 45 = 50 - 45 = 5

PAO2 (partial pres. of O2. in the alveolus.) = 150 - ( PaCO2 / .8 )760 – 45 = 715 : 21 % of 715 = 150

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20 × 5 = 100

Expected PaO2 =

FiO2 × 5 = PaO2

Normal situation

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The Blood Gas Report: normals…

pH 7.40 + 0.05PaCO2 40 + 5 mm HgPaO2 80 - 100 mm Hg

HCO3 24 + 4 mmol/L

O2 Sat >95Always mention and see FIO2

The essentials

HCO3

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5The

Steps forSuccessfulBlood Gas

Analysis

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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 = metabolicWith PCO2 >45 mm hg = respiratory

Alkalemia:With HCO3 >28 mmol/L = metabolicWith PCO2 <35 mm Hg = respiratory

Step 1Look at the pH

Is the patient acidemic pH < 7.35or alkalemic pH > 7.45

Step 3If there is a primary respiratory disturbance, is it acute ?

.08 change in pH ( Acute )

.03 change in pH ( Chronic )

10 mm Change PaCO2

=

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Step 4If the disturbance is metabolic is the respiratorycompensation 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 = 6 mm for 10 mEq. rise in Bicarb.

Suspect if ............. actual PaCO2 is more than expected :

additional …respiratory acidosis actual PaCO2 is less than expected :

additional …respiratory alkalosis

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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 ethanolU remiaD iabetic KetoacidosisP araldehydeI nfection (lactic acid)E thylene GlycolS alicylate

Common pediatric causes1) Lactic acidosis2) Metabolic disorders3) Renal failure

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th step

Clinical correlation5

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HCO3 META.pH

PaCO2 pH RESP.

Same direction

Opposite direction

Same direction

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Remember the format

pHPaCO2

PaO2

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Primary lesionPrimary lesion

Compensation

pH

Bicarbonate

PaCO2

METABOLIC ACIDOSIS

HYPER VENTILATION

BICARB CHANGES pH in same direction

Low Alkali

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Primary lesion

Compensation

pH

Bicarbonate

PaCO2

METABOLIC ALKALOSIS

HYPO VENTILATION

BICARB CHANGES pH in same direction

High Alkali

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Primary lesion

compensation

pH

PaCO 2

BICARB

Respiratory acidosis

CO 2 CHANGES pH in opposite direction

HighCO2

Three clicksWait for red circle

Primary lesionPrimary lesion

compensation

pH

PaCO 2

BICARB

Respiratory alkalosis

PaCO 2 CHANGES pH in opposite direction

LowPaCO2

Three clicksWait for red circle

PaCO2 of 10 pH

Acute change .08

Chronic change .03

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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

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compensatio

n

considered complete when the pH returns to normal range

Clinical blood gases by Malley

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CO

MP

EN

SIO

N

LIMIT

S

METABLIC ACIDOSISPaCO2 = Up to 10 ?

METABOLIC ALKALOSISPaCO2 = Maximum 6O

RESPIRATORY ACIDOSISBICARB = Maximum 40

RESPIRATORY ALKALOSISBICARB = Up to 10

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Blood Gas Report

Measured 37.0o

CpH 7.523PaCO2 30.1 mm HgPaO2 105.3 mm Hg

Calculated DataHCO3 act 22 mmol / L

O2 Sat 98.3 %PO2 (A - a) 8 mm Hg PO2 (a / A) 0.93

Entered DataFiO2 21.0 %

Case 1

16 year old female withsudden onset of dyspnea.

No Cough or Chest Pain

Vitals normal but RR 56,anxious.

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Acute respiratory alkalosis And why acute ?

Case 2 6 year old male with progressive respiratory distress

Muscular dystrophy .

Blood Gas Report

Measured 37.0o

CpH 7.301PaCO2 76.2 mm HgPaO2 45.5 mm Hg

Calculated DataHCO3 act 35.1 mmol / L

O2 Sat 78 %PO2 (A - a) 9.5 mm Hg PO2 (a / A) 0.83

Entered DataFiO2 21 %

pH <7.35 :acidemia

Res. Acidemia : High PaCO2 and low pH

HypoxemiaNormal A-a gradient

CO2 =76-40=36Expected pH for ( Acute ) = .08 for 10Expected ( Acute ) pH = 7.40 - 0.29=7.11Chronic resp. acidosis

Hypoventilation

Chronic respiratory acidosisWith hypoxia due to hypoventilation

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7.60 20 7.50 30 7.4040

7.3050

7.2060

7.1070

pHPaCO2

Acute respiratory change

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Last two digits

pH80 – PaCO2

Case 38-year-old male asthmatic;3 days of cough, dyspneaand orthopnea notresponding to usualbronchodilators.

O/E: Respiratory distress;suprasternal and intercostal retraction;tired looking; on 4 L NC.

Blood Gas Report

Measured 37.0o

CpH 7. 24PaCO2 49.1 mm HgPaO2 66.3 mm Hg

Calculated DataHCO3 act 18.0 mmol / L

O2 Sat 92 %PO2 (A - a) mm Hg PO2 (a / A)

Entered DataFiO2 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

CO2 = 49 - 40 = 9Expected pH ( Acute ) = 9/10 x 0.08 = 0.072Expected pH ( Acute ) = 7.40 - 0.072 = 7.328Acute resp. acidosis

8-year-old male asthmatic with resp. distress Six clicks

Case 4 8 year old diabetic with respi. distress fatigue and loss of appetite.

Blood Gas Report

Measured 37.0o

CpH 7.23PaCO2 23 mm HgPaO2 110.5 mm Hg

Calculated DataHCO3 act 14 mmol / L

O2 Sat %PO2 (A - a) mm Hg PO2 (a / A)

Entered DataFiO2 21.0 %

pH <7.35 ; acidemia

HCO3 <22; metabolic acidemia

Last two digits of pHCorrespond with co2

If Na = 130, Cl = 90Anion Gap = 130 - (90 + 14)

= 130 – 104 = 26

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Blood Gas Report

Measured 37.0o

CpH 7.46PaCO2 28.1 mm HgPaO2 55.3 mm Hg

Calculated DataHCO3 act 19.2 mmol / L

O2 Sat %PO2 (A - a) mm Hg PO2 (a / A)

Entered DataFiO2 24.0 %

Case 5 : 10 year old child with encephalitis

pH almost within normal rangeMild alkalosis

PaCO2 is low , respiratorylow by around 10 ( Acute ) by .08 (Chronic ) by .03

BICARBINATURIA

Bicarb looks low ?Is it expected ?

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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 COMPENSATIONExpected PaCO2 = (1.5 x [HCO3]) + 8 ) + 2(Winter’s equation)PCO 2 ……SHOULD BE 20

Case 6………….

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Adolescent boy with appendicitis , posted for surgery , he is a known case of SLE.His pre-op ABG shows: Room air pH 7.39 pCO2 l5mmHg paO2 90 mmHgHCO3 8mmol/L

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.

What is the probable cause for the above findings ? Are they OK as far as oxygenation is concerned ?

Case 7……….

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Patient was hypo volumic , received Normal Saline bolus... Corrected acidosisHe was operated ….but post-op became drowsy His ABG……..FiO2….30%

pH 7.38PaCO2 38PaO2 60

1) Why hypoxemia ?2) Were the lungs bad to begin with ? ( Pre OP PaO2 90 mmHg )

3) Micro atelectesis during surgery ? Anesthetist goofed up the case 4) Pure and simple hypoventilation …..Sedation ?

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Why hypoxemia ?Lungs were bad to begin with ?Micro atelectesis during surgeryPure and simple hypoventilation ? sedation

PRE OP ….ABG on room airpH 7.39 PaCO2 l5mmHg PaO2 90 mmHgHCO3 8mmol/L

Pre OP .....A/a gradient palvO2 = PiO2 – PaCO2 / RQ

= 150 – 15 / 0.8= 150 – 18 = 132 mm Hg

132 – 90= 42 WIDE A / a gradient

Oxygenation status good …..?

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Apparently the lungs looked good with PaO2 of 90…….But have a good look at the ABG again With wash out of CO 2 ……….The expected PaO2 should have been more than 90 .

This coupled with correction of acidosis( normalizing PaCO2 )Lowered the PaO2 …post operatively.Conclusion …….. Lungs were not normal to begin with ( SLE )……..

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Correlate PaO2 with FiO2

But please also correlate with PaCO2

Learning point

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Respiratory Alkalosis

Is it acute ?

What is the Diagnosis

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Case 8,,,,,,,,,,,,,,,,,,

pH 7.583PCO2 19.8HCO3 18.7

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