arterial blood gas analysis arnel gerald q. jiao, md, fpps, fpapp pediatric pulmonologist philippine...

ARTERIAL BLOOD GASANALYSIS

Arnel Gerald Q. Jiao, MD, FPPS, FPAPPPediatric Pulmonologist

Philippine Children’s Medical Center

Guidelines for Interpreting ABG’S

• The body always tries to maintain a normal ph

• The lungs compensate rapidly; the kidneys compensate slowly

• There is no overcompensation.

• Consider the underlying disease

• Maintain an adequate level of hemoglobin

Bicarbonate-Carbonic AcidBuffer System:

CO2 + H2O H2CO3 H+ + HCO3-

Normal Arterial Blood Gas Values

pH: 7.35 – 7.45

paCO2: 35 – 45 mm Hg

paO2: 80 – 100 mm Hg

HCO3: 22 – 26 mEq/LBE/BD: - 2 to + 2

O2 Sat: > 95 %

Clinically Acceptable LevelspH: 7.30 – 7.50paCO2: 30 – 50 mm HgpaO2

Neonates: 60 – 80 mm Hg2 months above: 80 – 100 mmHgDecreases with age: Subtract

1 mm Hg from 80 mm Hg for every year past the age of 60

Nomenclature for Clinical interpretation

• Acidosis: patho- physiologic state where a significant base deficit is present

(HCO3 < 22mEq/L)

• Alkalosis: patho-physiologic state where a significant base excess is present

(HCO3 > 26mEq/L)

• Mathematical interrelationship among pH, pCO2 and HCO3

• Basis for all Acid-Base interpretation:

pH= HCO3/pCO2

Clinical Approach to Interpretation:

Steps

1. Assessment of the pCO2 and pH:ventilatory status and acid-base balance

2. Assessment of Arterial Oxygenation

Step 1

• Classify carbon dioxide tension

• Consider pH and determine classification

• Consider BE/BD or HCO3 levels and determine classification

Step 1Classification of PaCO2

< 35 mmHg: alveolar hyperventilation

(respiratory alkalosis)

35 – 45 mmHg: Normal alveolar ventilation

> 45 mmHg: ventilatory failure (respiratory acidosis)

Step 1Three questions to ask:

• Is the PaCO2 abnormal?• Is the pH explained by the level of PaCO2?

Yes: respiratoryNo: metabolic

• Is the pH:Abnormal: acute/uncompensatedNormal: chronic/ compensated

Step 1PaCO2 < 35 mmHg

pH < 7.35 7.35 – 7.39 7.41 - 7.45 pH > 7.45

Partially Compensated

Metabolic

Acidosis

Compensated

Metabolic

Acidosis

Chronic

Respiratory

Alkalosis

Acute

Respiratory

Alkalosis

PCO2 < 35 mm Hg

pH < 7.35

HCO3 decreased

partly compensated metabolic acidosis

PCO2 < 35 mm Hg

pH 7.35 – 7.45

HCO3 decreased

chronic respiratory alkalosis

PCO2 < 35 mm Hg

pH > 7.45

HCO3 normal

acute respiratory alkalosis

PCO2 < 35 mm Hg

pH > 7.45

HCO3 decreased

partly compensated respiratory alkalosis

PCO2 < 35 mm Hg

pH > 7.45

HCO3 increased

combined respiratory and metabolic alkalosis

Step 1PaCO2 35 – 45 mmHg

pH < 7.35 7.35 – 7.45 pH > 7.45

Acute

Metabolic

Acidosis

Normal

Acid- Base

Balance

Acute

Metabolic

Alkalosis

PCO2 35 – 45 mm Hg

pH < 7.35

HCO3 decreased

acute metabolic acidosis

PCO2 35 – 45 mm Hg

pH 7.35 – 7.45

HCO3 normal

normal acid-base balance

PCO2 35 – 45 mm Hg

pH > 7.45

HCO3 increased

acute metabolic alkalosis

Step 1PaCO2 > 45 mmHg

pH < 7.35 7.35-7.39 7.41-7.45 pH > 7.45

Acute Resp.

Acidosis

Chronic Resp.

Acidosis

Compens Metabolic

Alkalosis

Partially CompensMetabolic

Alkalosis

PCO2 > 45 mm Hg

pH < 7.35

HCO3 normal

acute respiratory acidosis

PCO2 > 45 mm Hg

pH < 7.35

HCO3 decreased

combined respiratory and metabolic acidosis

PCO2 > 45 mm Hg

pH < 7.35

HCO3 increased

partly compensated respiratory acidosis

PCO2 > 45 mm Hg

pH 7.35 – 7.45

HCO3 increased

chronic respiratory acidosis

PCO2 > 45 mm Hg

pH > 7.45

HCO3 increased

partly compensated metabolic alkalosis

Approximate PaCO2-pH Relationship

PaCO2 ( mm Hg) pH

80 7.2

60 7.3

40 7.4

30 7.5

20 7.6

Determining Base Excess/ Deficit

1. Determine pCO2 variance: difference between measured pCO2 & 40, move decimal

point two places to the left2. Determine the predicted pH:

pCO2 > 40, subtract half pCO2 variance from 7.40

pCO2 < 40, add pCO2 variance to 7.403. Estimate BE/BD:

Difference between measured and predicted pH

Move decimal point two places to right.Multiply by 2/3

Base Excess: measured pH > predicted pHBase Deficit: measured pH < predicted pH

pH 7.04 pCO2 76 predicted pH 7.227.22 – 7.04 = 0.1818 x 2/3 = 12 mEq/L (BD)

pH 7.21 pCO2 90 predicted pH 7.157.21 – 7.15 = 0.066 x 2/3 = 4 mEq/L (BE)

Causes of AcidosisMetabolic

Diabetes (ketoacidosis)

Renal failure (impaired H+ secretion)

Diarrhea (loss of base)

Tissue hypoxia (lactic acidosis)

Respiratory

Respiratory insufficiency

Causes of AlkalosisMetabolic

Excessive loss of HCl (e.g. pyloric stenosis)

Excessive citrate/bicarbonate load

Respiratory

Hyperventilation (fever, psychogenic)

TreatmentMetabolic Acidosis

HCO3 administration

Empiric: 1-2 meq/kg

Calculated:

(Desired – actual) x k x KBW =

meqs required

k = 0.5 - 0.6 (represents fraction of body wt. where material is apparently distributed)

TreatmentMetabolic Alkalosis

Volume expansion; Cl and K replacement

Respiratory Acidosis

Inc. RR, PIP, or both

Respiratory Alkalosis

Dec. RR

Step 2: Assessment of Arterial Oxygenation

Evaluation of Hypoxemia

Room Air (Patient < 60 y/o):Mild: PaO2 < 80 mmHgModerate: PaO2 < 60 mmHgSevere: PaO2 < 40 mm Hg

Step 2On Oxygen Therapy:

• Uncorrected hypoxemia:

PaO2 < 80 mm Hg• Corrected hypoxemia:

PaO2 = 80 – 100 mm Hg• Overcorrected hypoxemia:

PaO2 > 100 mm Hg

FiO2 (Fractional InspiredOxygen Concentration)

the measurable amount of oxygen received by the patient

21% - room air > 21% - supplemental oxygen

Inspired Oxygen to PaO2 Relationship

FiO2 Predicted Minimal PaO2

30 % 150

40 % 200

50 % 250

80 % 400

If PaO2 < minimal predicted (FiO2 x 5), the patient can be assumed to be hypoxemic at room air.

Treatment of Hypoxemia

For ventilated patients

Increase:

FiO2

RR

PIP

PEEP

Inspiratory time

Flow rate

Exercises

pH 7.44

PCO2 40

PO2 99

HCO3 22

BE +2

SaO2 95

FiO2 21%

normal

acid-base

balance

with adequate

oxygenation

Exercises

pH 7.44

PCO2 40

PO2 99

HCO3 22

BE +2

SaO2 95

FiO2 21%

pH 7.36

PCO2 25

PO2 78

HCO3 15

BE -10

SaO2 95

FiO2 35%

pH 7.36

PCO2 25

PO2 78

HCO3 15

BE -10

SaO2 95

FiO2 35%

chronic

metabolic

acidosis

with

uncorrected

hypoxemia

pH 7.24

PCO2 60

PO2 80

HCO3 26

BE -2

SaO2 95

FiO2 60%

pH 7.24

PCO2 60

PO2 80

HCO3 26

BE -2

SaO2 95

FiO2 60%

acute

respiratory

acidosis

with corrected

hypoxemia

pH 7.55

PCO2 52

PO2 70

HCO3 44

BE +17

SaO2 97

FiO2 90%

pH 7.55

PCO2 52

PO2 70

HCO3 44

BE +17

SaO2 97

FiO2 90%

partly

compensated

metabolic

alkalosis

with uncorrected

hypoxemia

pH 7.19

PCO2 56

PO2 120

HCO3 17

BE -30

SaO2 94

FiO2 45%

pH 7.19

PCO2 56

PO2 120

HCO3 17

BE -30

SaO2 94

FiO2 45%

combined

metabolic and

respiratory

acidosis with

overcorrected

hypoxemia