ARTERIAL BLOOD GAS
ANALYSIS
Module A
Objectives
• List the normal values for parameters found in a
blood-gas analysis.
• List the normal values for parameters found in a
CO-Oximetry analysis.
• Differentiate between measured and calculated
(derived) blood gas data.
• List the three physiologic processes assessed
with blood gas data.
• State the PaCO2 equation.
• Describe how alveolar minute ventilation is
derived.
• Describe the relationship between PaCO2, CO2
production and Alveolar Minute Ventilation.
Objectives
• Describe the effects of altitude on partial
pressure, barometric pressure and fractional
concentrations.
• Given appropriate data, use Dalton’s Law to
determine the resultant partial pressures of a gas
in a mixture.
• Given appropriate data, calculate the Alveolar Air
Equation.
• Explain how changes in the PIO2 or PaCO2 levels
affect the PAO2.
• State the formula for Oxygen Content and
Oxygen Delivery.
Arterial Blood-Gas Analysis
• Two Components
• Acid Base Balance/Ventilation
• pH, PaCO2, HCO3-, BE
• Electrolytes (primarily K+)
• Oxygenation
• PaO2, Hb, CaO2, SaO2, MetHb%, COHb% &
any other abnormal Hemoglobin species.
• Oxygenation Indices: PaO2/FIO2, A-aDO2,
s/ t.
Acid-Base Balance
• Non-Respiratory Acid Base Component
(Metabolic Indices)
• HCO3• BE
• Respiratory Indice (Respiratory Index)
• PaCO2
Definition of Blood-Gas
• Any element or compound that is a gas
under ordinary conditions and dissolves
in the blood.
• A blood-gas would exert a partial
pressure
•
•
•
•
O2
CO2
N2
CO
Technology
• Blood can be analyzed on either or both of
two different machines (or one machine
with two distinct components)
• Blood-Gas Analyzer
• CO-oximeter
Measured vs. Derived
• Most values are directly measured with
various electrodes:
• Clark: PO2
• Severinghaus: PCO2
• Sanz: pH
• Some are calculated or derived Values are:
• HCO3• Base Excess (BE)
• CaO2
Normal Values
•
•
•
•
•
pH: 7.35 – 7.45
PaCO2: 35 – 45 torr
PaO2: 80 – 100 torr
SaO2: 97%
HCO3-: 22-26 mEq/L
• %MetHb: < 2%
• %COHb: < 2%
• Smokers: 5 – 10%
• BE: +/- 2 mEq/L
• CaO2: 18 – 20 vol%
* Vol% = mL/100 mL of
blood
Hemoglobin Saturation
• %SaO2 + %COHb + %MetHb  100%
• Example of error:
• SaO2 97%, %COHb 50%, MetHb% 0%
Interpretation of an ABG
• Three Areas of information are necessary
• Information about the patient’s immediate
environment.
• Additional Lab Data.
• Clinical Information obtained through patient
assessment.
Interpretation of an ABG
• Immediate Environment
•
•
•
•
•
FIO2
Barometric Pressure
Toxic gases/smoke
Level of consciousness
Environmental information
• Empty Pill Bottle
• Accident
Interpretation of an ABG
• Lab Data
•
•
•
•
•
•
•
Previous analyses
Hemoglobin or hematocrit (from lab)
Electrolytes (K+, Na+, Cl-)
Blood Glucose
Blood Urea Nitrogen (BUN)
Chest x-ray
PFT test
Interpretation of an ABG
• Clinical Information
•
•
•
•
•
History and physical exam.
Vital Signs.
Respiratory effort & ventilatory pattern.
Mental Status.
State of tissue perfusion.
Assessing Oxygenation
• FIO2
• Barometric Pressure
• Age
Composition of the
Environment
• These values stay constant even with
changes in barometric pressure.
Dalton’s Law of Partial
Pressures
• All pressures in a gas mixture must add up
to the total pressure (PBARO).
• Dry Gas
• Pgas = PBARO x FIO2
• Inspired Gas (ex. PIO2)
• Pgas = (PBARO - 47 torr) x FIO2
Calculating Partial Pressures
for dry gases
• PO2 = 760 x .21
160 mm Hg or torr
• PN2 = 760 x .78
593 mm Hg or torr
• PCO2 = 760 x .0003
0.23 mm Hg or torr
• PAr = 760 x .0093
7 mm Hg or torr
NOTE: 160 + 593 + .23 + 7 = 760
Altitude’s Effect on Partial Pressure
High Altitude Response
• Increase Altitude
• PBARO
 PIO2
 PAO2
 PaO2
• To adapt to high altitudes
• Change the environment
• Airplanes are pressurized to 7000-8000 feet.
• Increase FIO2 (above 20,000 feet).
• Adapt Physiologically
•
•
•
•
Hyperventilation.
Collateral Circulation.
Shift the oxygen dissociation curve.
Increase Hemoglobin levels.
Calculating PBaro at High
Altitudes
• PBARO falls 120 mm Hg per mile of altitude
• Example: Leadville is 2 miles above sea
level. Calculate the PBARO & PO2
• 120 x 2 miles = 240 mm Hg decline
• 760 - 240 = 520 mm Hg (PBARO)
• PO2 = 520 x .21
109 mm Hg or torr (PO2)
Physiologic Processes
• ABG results provide information
on the three physiologic processes
• Alveolar Ventilation
• Acid-Base
• Oxygenation
Equations Used to Reflect the
Physiologic Processes
• PaCO2 Equation
Alveolar Ventilation
• Henderson
Hasselbalch
Acid Base
• Alveolar Air Equation
Oxygenation
• Oxygen Content (CaO2)
Oxygenation
• Oxygen Delivery
Oxygenation
PaCO2 and Alveolar Ventilation
• Alveolar Ventilation is the amount of air in L/min
that reaches the alveoli and takes part in gas
exchange.
•



VA  VE  VD
• The body eliminates the CO2 produced, during
metabolism, via ALVEOLAR ventilation.
Metabolism
• Steady State
• The amount of CO2 added to the blood through
metabolism = the amount of CO2 excreted by
the lungs.
• 200 mL/min
PaCO2 Equation
• PaCO2 = CO2 production x 0.863
Alveolar Minute Ventilation
• 0.863 is a constant which equates
dissimilar units.
• 40 mm Hg = 200 mL/min x 0.863
4.3 L/min
PaCO2 Equation
• If CO2 production doubles (e.g. fever),
alveolar minute ventilation must double to
keep a normal PaCO2 level.
• 40 mm Hg = 400 mL/min x 0.863
8.6 L/min
Henderson-Hasselbalch
Equation
• pH is defined as the negative log of the H+
concentration
• pH = pK + Log
HCO3(Base)
(PaCO2 x 0.03) (Acid)
• pH = pK + Log 24.0 mEq/L
1.20 mEq/L
• “Normal” pH implies 20 times more base than
acid
PAO2
• PAO2 = PBARO – 47 torr x FIO2 – PaCO2
0.8
• PAO2 = PIO2 - PaCO2
0.8
• PAO2 on room air = 100 – 104 mm Hg
• PAO2 on 100% = 600’s
Effects of PaCO2 on PAO2 and PaO2
• A rise in the PaCO2 will lower the PAO2 and
therefore the PaO2.
• Hypoventilation is a cause of hypoxemia.
CaO2
• CaO2 = (SaO2 x Hb x 1.34) +
(PaO2 x 0.003)
• With normal values:
• Oxyhemoglobin (attached) represents 19.7
vol%.
• Dissolved oxygen (PaO2) represents 0.3 vol%.
• Total Oxygen present in the blood 20 vol%.
Vol %
• mL of oxygen/100 mL of blood
Or
• mL of oxygen/dL of blood
Oxygen Delivery
•
•
•
•
Oxygen Delivery = CaO2 x CO x 10
Oxygen Delivery = CaO2 x SV x HR x 10
Normal Value = 1,000 mL/min
Represents amount of oxygen delivered to
the tissues each minute.
Factors that Influence Oxygen
Delivery to the Tissues
•
•
•
•
•
SaO2
Hb
PaO2
Stroke Volume
Heart Rate
Summary of Important Points
•ABG interpretation means evaluating the
acid base and oxygenation status of the
patient.
•Acid Base represent the metabolic and
respiratory indices.
•FIO2 stays the same regardless of changes
in PBaro.
•PBARO decreases as altitude increases.
•Dalton’s Law.
•PO2 is affected by FIO2, PBARO and age.
Summary of Important Points
• PAirway = PBARO.
• To interpret an ABG you need 3 areas of
information.
• Oxygen delivery is influenced by five
factors.
• ABG values are either measured or
derived.
• Understand the 5 equations and the
relationship among the parameters used.