1. ARTERIAL BLOOD GAS ANALYSIS
Module A

2. 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.

3. 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.

4. 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.

5. Acid-Base Balance
Non-Respiratory Acid Base Component (Metabolic Indices)
HCO3- BE
Respiratory Indice (Respiratory Index)
PaCO2

6. 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

7. 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

10. 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

11. Normal Values pH: 7.35 – 7.45 PaCO2: 35 – 45 torr PaO2: 80 – 100 torr
SaO2: 97%
HCO3-: mEq/L
%MetHb: < 2%
%COHb: < 2%
Smokers: 5 – 10%
BE: +/- 2 mEq/L
CaO2: 18 – 20 vol%
* Vol% = mL/100 mL of blood

12. Hemoglobin Saturation
%SaO2 + %COHb + %MetHb » 100%
Example of error:
SaO2 97%, %COHb 50%, MetHb% 0%

13. 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.

14. Interpretation of an ABG
Immediate Environment
FIO2
Barometric Pressure
Toxic gases/smoke
Level of consciousness
Environmental information
Empty Pill Bottle
Accident

15. 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

16. Interpretation of an ABG
Clinical Information
History and physical exam.
Vital Signs.
Respiratory effort & ventilatory pattern.
Mental Status.
State of tissue perfusion.

17. Assessing Oxygenation
FIO2
Barometric Pressure
Age

18. Composition of the Environment
These values stay constant even with changes in barometric pressure.

19. 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

20. Calculating Partial Pressures for dry gases
PO2 = x .21
160 mm Hg or torr
PN2 = x .78
593 mm Hg or torr
PCO2 = x
0.23 mm Hg or torr
PAr = x
7 mm Hg or torr
NOTE: = 760

21. Altitude’s Effect on Partial Pressure

22. High Altitude Response
Increase Altitude
¯ PBARO ¯ PIO2 ¯ PAO2 ¯ PaO2
To adapt to high altitudes
Change the environment
Airplanes are pressurized to feet.
Increase FIO2 (above 20,000 feet).
Adapt Physiologically
Hyperventilation.
Collateral Circulation.
Shift the oxygen dissociation curve.
Increase Hemoglobin levels.

23. 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
= 520 mm Hg (PBARO)
PO2 = x .21
109 mm Hg or torr (PO2)

24. Physiologic Processes
ABG results provide information on the three physiologic processes
Alveolar Ventilation
Acid-Base
Oxygenation

25. Equations Used to Reflect the Physiologic Processes
PaCO2 Equation
Henderson Hasselbalch
Alveolar Air Equation
Oxygen Content (CaO2)
Oxygen Delivery
Alveolar Ventilation
Acid Base
Oxygenation

26. PaCO2 and Alveolar Ventilation
Alveolar Ventilation is the amount of air in L/min that reaches the alveoli and takes part in gas exchange.
The body eliminates the CO2 produced, during metabolism, via ALVEOLAR ventilation.

27. Metabolism Steady State
The amount of CO2 added to the blood through metabolism = the amount of CO2 excreted by the lungs.
200 mL/min

28. 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
4.3 L/min

29. 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
8.6 L/min

30. Henderson-Hasselbalch Equation
pH is defined as the negative log of the H+ concentration
pH = pK + Log HCO (Base)
(PaCO2 x 0.03) (Acid)
pH = pK + Log mEq/L
mEq/L
“Normal” pH implies 20 times more base than acid

31. PAO2 PAO2 = PBARO – 47 torr x FIO2 – PaCO2 0.8 PAO2 = PIO2 - PaCO2
PAO2 on room air = 100 – 104 mm Hg
PAO2 on 100% = 600’s

33. 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.

34. 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%.

35. Vol %
mL of oxygen/100 mL of blood Or
mL of oxygen/dL of blood

36. 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.

37. Factors that Influence Oxygen Delivery to the Tissues
SaO2 Hb
PaO2
Stroke Volume
Heart Rate

38. 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.

39. 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.