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
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