arterial blood gas : analysis 1 by dr. deopujari

Arterial Blood Gas Analysis …..1

Dr DeopujariPediatrician Nagpur

The Goal :

To provide simple and bedside approach to ABG report

Not to:To teach physiology .

To teach theories on acid-base regulation

To look for alternative approaches to interpretation

In details

A Systematic and pointed ………approach

Use of pH for Hydrogen Ion Activity ……..

The credit (or Blame) for introducing the term pH, the negative log of hydrogen ion (H+) concentration, goes to S. P. L. Sørensen (1868-1939), who apparently was tired of writing seven zeros in a paper on enzyme activity and wanted a simpler designation…..?.

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

The Anatomy of a Blood Gas Report

----- XXXX Diagnostics ------

Blood Gas Report248 05:36 Jul 22 2000Pt ID 2570 / 00

Measured 37.0o

CpH 7.463pCO2 44.4 mm HgpO2 113.2 mm Hg

Corrected 38.6o


Calculated DataHCO3 act 31.1 mmol / LHCO3 std 30.5 mmol / LBE 6.6 mmol / LO2 CT 14.7 mL / dlO2 Sat 98.3 %ct CO2 32.4 mmol / LpO2 (A - a) 32.2 mm HgpO2 (a / A) 0.79

Entered DataTemp 38.6 oCct Hb 10.5 g/dlFiO2 30.0 %

Measured Values the most important

Temperature Correction:Is there any value to it?

Calculated Data:Which are the useful ones?

Entered Data:As important

Bicarbonate:----- XXXX Diagnostics ------

Blood Gas Report

Measured 37.0o


Corrected 38.6o

C

Calculated DataHCO3 act 31.1 mmol / LHCO3 std 30.5 mmol / LBE 6.6 mmol / LO2 CT 14.7 mL / dlO2 Sat 98.3 %t CO2 32.4 mmol / LpO2 (A - a) 32.2 mm HgpO2 (a / A) 0.79


Henderson - Hasselbach equation:

pH = pK + Log HCO3

Dissolved CO2


Blood Gas Report

Measured 37.0o


Corrected 38.6o

C



Standard Bicarbonate:Plasma HCO3 after equilibrationto a PCO2 of 40 mm Hg

: reflects non-respiratory acid base change: does not quantify the extent of the buffer base abnormality : does not consider actual buffering capacity of blood

Base Excess: base to normalise HCO3 (to 24) with PCO2 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


Blood Gas Report

Measured 37.0o


Corrected 38.6o

C



Oxygenation Parameters:O2 Content of blood:Hb x O2 Sat + Dissolved O2

Oxygen Saturation:( remember this is calculated )

Alveolar / arterial gradient:

Arterial / alveolar ratio:

Rt. Shift

Lt.Shift

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

partO2 = 90 mmHg

palvO2 – partO2 = 10 mmHg

Alveolar- arterial Difference

O2

CO2

Oxygenation FailurepiO2 = 150

pCO2 = 40

palvO2= 150 – 40/.8=150-50 =100

pO2 = 45

= 100 - 45 = 55

Ventilation FailurepiO2 = 150

pCO2 = 80

palvO2= 150-80/.8 =150-100

= 50

pO2 = 45

= 50 - 45 = 5 PAO2 (partial pres. of O2. in the alveolus.)

= 150 - ( PaCO2 / .8 )760 – 45 = 715 : 21 % of 715 = 150


Blood Gas Report

Measured 37.0o


Corrected 38.6o

C

Calculated DataHCO3 act 31.1 mmol / L

O2 CT 14.7 mL / dlO2 Sat 98.3 %t CO2 32.4 mmol / LpO2 (A - a) 32.2 mm HgpO2 (a / A) 0.79


Oxygenation: Limitations of parameters:

O2 Content of blood:Useful in oxygen transport calculationsDerived from calculated saturation

Oxygen Saturation:Ideally measured by co-oximetryCalculated values may be error-prone

Alveolar / arterial gradient:Reflects O2 exchange with fixed FiO2 ImpracticalDifferentiates hypoventilation as cause

Arterial / alveolar ratio:Proposed to be less variableSame limitations as A-a gradient

20 × 5 = 100


Blood Gas Report

Measured 37.0o



O2 Sat 98.3 %pO2 (A - a) 32.2 mm Hg

Entered DataFiO2 30.0 %

The Blood Gas Report:

pH 7.40 + 0.05PCO2 40 + 5 mm HgPO2 80 - 100 mm Hg

HCO3 24 + 4 mmol/L

O2 Sat >95Always mention and see FIO2


Blood Gas Report

Measured 37.0o


Corrected 38.6o




The essentials

A ) low PAO2 ( Low Alveolar Pressure )

1) low barometric pressure,2) low fraction of inspired oxygen (FiO2) 3) Hypercarbia – elevated (PaCO2).

B ) Wide A / a gradient ( Normal Alveolar pressure )

1) Shunt ( cardiac or non cardiac )2) Diffusion abnormality

Low PaO2 can be the result of

Technical Errors Glass vs. plastic syringe: Changes in pO2 are not clinically importantNo effect on pH or pCO2

Heparin (1000 u / ml):Need <0.1 ml / ml of bloodpH of heparin is 7.0; pCO2 trends downAvoided by heparin flushing & drawing 2-4 cc blood

Delay in measurement:Rate of changes in pH, pCO2 and pO2 can be

reduced to 1/10 by cooling in ice slush(4o C)No major drifts up to 1 hour

5The

Steps forSuccessfulBlood Gas

Analysis

Step 1Look at the pH

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

Step 2Who is responsible for this change ( culprit )?

Acidemia: With HCO3 < 20 mmol/L = metabolicWith PCO2 >45 mm hg = respiratory

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

The culprit

BICARB pH

CO2 pH

BICARB pH

CO2 pH

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

(Acute)change in pH = 0.08 for 10 mm change in PCO2(Chronic)change in pH = 0.03 for 10 mm change in PCO2

Step 4If the disturbance is metabolic is the respiratorycompensation appropriate?

For metabolic acidosis:Expected PCO2 = (1.5 x [HCO3]) + 8 ) + 2(Winter’s equation)( Last two digits of pH )

For metabolic alkalosis:Expected PCO2 = 6 mm for 10 mEq. rise in Bicarb.

If : actual PCO2 more than expected : additional respiratory acidosisactual PCO2 less than expected : additional respiratory alkalosis

The last two digits

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 : MethanolUremiaDiabetic KetoacidosisParaldehydeInfection (lactic acid)Ethylene GlycolSalicylate

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

th step

Clinical correlation5

HCO3 META.pH

PCO2 pH RESP.

Same direction

Opposite direction

Same direction

24 CO2

BICARBONATE = H ION CONC.

24 40 = 960

BICARBONATE = H ION CONC.

960 24

= H ION CONC. = 40 N.MOLS / L.

H+ N.MOLS / L. = 80 - last two digits of pH

N.MOLS / L.

N.MOLS / L.

Primary lesionPrimary lesion

compensation

pH

HCO3

CO2

METABOLIC ACIDOSIS

HYPER VENTILATION

BICARB CHANGES pH in same direction

Primary lesion

compensation

pH

HCO3

CO2

METABOLIC ALKALOSIS

HYPO VENTILATION

BICARB CHANGES pH in same direction

Primary lesion

compensation

pH

CO 2

BICARB

Respiratory acidosis

CO 2 CHANGES pH in opposite direction

pHCO2+H20=H2CO3 = H + HCO3+

HCO3HCO3

RESP. ACIDOSIS ALKALOSIS META.

ACUTE RISE : PCO2 10 : pH .08CHRONIC RISE : PCO2 10 : pH .03

PCO2

HIGH H HIGH HCO3

+

Primary lesionPrimary lesion

compensation

pH

CO 2

BICARB

Respiratory alkalosis

CO 2 CHANGES pH in opposite direction

CO2 + H20 = H2CO3 = H + HCO3+pH

HCO3LOW H IONS …LOW HCO3

RESP. ALK. ACID. META.

ACUTE FALL : PCO2 10 : pH .08CHRONIC FALL: PCO2 10 : pH .03

CO2

+

Pco2 of 10 pH

Acute change .08

Chronic change .03

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

compensatio

n

considered complete when the pH returns to normal range

Clinical blood gases by Malley

CO

MP

EN

SIO

N

LIMIT

S

METABLIC ACIDOSISCO2 = Up to 10 ?

METABOLIC ALKALOSISCO2 = Maximum 6O

RESPIRATORY ACIDOSISBICARB = Maximum 40

RESPIRATORY ALKALOSISBICARB = Up to 10


Blood Gas Report

Measured 37.0o


Calculated DataHCO3 act 22 mmol / L

O2 Sat 98.3 %pO2 (A - a) 8 mm Hg pO2 (a / A) 0.93



Blood Gas Report

Measured 37.0o



O2 Sat 98.3 %pO2 (A - a) 8 mm Hg pO2 (a / A) 0.93


Case 1

16 year old female withsudden onset of dyspnea.

No Cough or Chest Pain

Vitals normal but RR 56,anxious.

Case 2 6 year old male with progressive respiratory distress

Muscular dystrophy .----- XXXX Diagnostics ------

Blood Gas Report

Measured 37.0o



O2 Sat 78 %pO2 (A - a) 9.5 mm Hg pO2 (a / A) 0.83

Entered DataFiO2 21 %


Blood Gas Report

Measured 37.0o



O2 Sat 78 %pO2 (A - a) 9.5 mm Hg pO2 (a / A) 0.83


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

pH <7.35 :acidemia

respiratory acidemia : co2 and pH

HypoxiaNormal A-a gradientDue to hypoventilation

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. 24pCO2 49.1 mm HgpO2 66.3 mm Hg


O2 Sat 92 %pO2 (A - a) mm Hg pO2 (a / A)



Blood Gas Report

Measured 37.0o

CpH 7. 24pCO2 49.1 mm HgpO2 66.3 mm Hg


O2 Sat 92 %pO2 (A - a) mm Hg pO2 (a / A)


153-66= 87

pH <7.35 ; acidemia

pCO2 >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 ? 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

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


Blood Gas Report

Measured 37.0o

CpH 7.23pCO2 23 mm HgpO2 110.5 mm Hg


O2 Sat %pO2 (A - a) mm Hg pO2 (a / A)



Blood Gas Report

Measured 37.0o

CpH 7.23pCO2 23 mm HgpO2 110.5 mm Hg




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


Blood Gas Report

Measured 37.0o






Blood Gas Report

Measured 37.0o





Case 5 : 10 year old child with encephalitis

pH almost within normal rangeMild alkalosis

Co2 is low , respiratoryCo2 low by around 10 ( Acute ) by .08 (Chronic ) by .03

Bicarb looks low ?Is it expected ?

More cases

ABG OF THE DAY

The arterial blood gas report : Room air pH 7.39 PCO2 l5mniHg HCO3 8mmol/L PaO2 90 mmHg

PCO2 24

BICARBONATEH ION CONCENTRATION =

= 45 nmol/lit

1) 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 l5mniHg HCO3 8mmol/L PaO2 90 mmHg

For metabolic acidosis: FULL COMPENSATIONExpected PCO2 = (1.5 x [HCO3]) + 8 ) + 2(Winter’s equation)PCO 2 ……SHOULD BE 20

2) What is the oxygenation status a) Normal oxygenation status b) Hypoxemia c) None of the above

palvO2 = piO2 – pCO2 / RQ

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

132 – 90 = 42 WIDE A / a gradient

pH 7.39 PCO2 l5mniHg HCO3 8mmol/L PaO2 90 mmHg

pCO2 pH

70 7.10

60 7.20

50 7.30

40 7.40

30 7.50

20 7.60

When pH is normal and: Bicarbonate is high ( Metabolic alkalosis + respiratory acidosis ) Bicarbonate is low ( Metabolic acidosis + resp. alkalosis) Bicarbonate is normal and: anion gap is high ( Metabolic Acidosis + Metabolic alkalosis) When bicarbonate is normal and: pH is in acidic range ( Chronic resp. acidosis + resp alk.) pH is in alkalemic range ( Metab.alk. + resp alk.) Anion gap is elevated and: clinical and laboratory data suggest a diagnosis other than metabolic acidosis PCO2 level and bicarbonates are shifted from normal in opposing directions.

THANKS

arterial blood gas : analysis 1 by dr. deopujari

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