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