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ARTERIAL BLOOD GAS ANALYSIS Module A. List the normal values for parameters found in a blood-gas analysis. List the normal values for parameters found.

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Presentation on theme: "ARTERIAL BLOOD GAS ANALYSIS Module A. List the normal values for parameters found in a blood-gas analysis. List the normal values for parameters found."— Presentation transcript:

1 ARTERIAL BLOOD GAS ANALYSIS Module A

2 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 P a CO 2 equation. Describe how alveolar minute ventilation is derived. Describe the relationship between P a CO 2, CO 2 production and Alveolar Minute Ventilation. Objectives

3 Describe the effects of altitude on partial pressure, barometric pressure and fractional concentrations. Given appropriate data, use Daltons 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 P I O 2 or P a CO 2 levels affect the P A O 2. State the formula for Oxygen Content and Oxygen Delivery.

4 Arterial Blood-Gas Analysis Two Components Acid Base Balance/Ventilation pH, P a CO 2, HCO 3 -, BE Electrolytes (primarily K + ) Oxygenation P a O 2, Hb, C a O 2, S a O 2, MetHb%, COHb% & any other abnormal Hemoglobin species. Oxygenation Indices: P a O 2 /F I O 2, A-aDO 2, s / t.

5 Acid-Base Balance Non-Respiratory Acid Base Component (Metabolic Indices) HCO 3 - BE Respiratory Indice (Respiratory Index) P a CO 2

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 O 2 CO 2 N 2 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

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10 Measured vs. Derived Most values are directly measured with various electrodes: Clark: P O 2 Severinghaus: P CO 2 Sanz: pH Some are calculated or derived Values are: HCO 3 - Base Excess (BE) C a O 2

11 Normal Values pH: 7.35 – 7.45 P a CO 2 : 35 – 45 torr P a O 2 : 80 – 100 torr S a O 2 : 97% HCO 3 - : mEq/L %MetHb: < 2% %COHb: < 2% Smokers: 5 – 10% BE: +/- 2 mEq/L C a O 2 : 18 – 20 vol% * Vol% = mL/100 mL of blood

12 Hemoglobin Saturation %S a O 2 + %COHb + %MetHb 100% Example of error: S a O 2 97%, %COHb 50%, MetHb% 0%

13 Interpretation of an ABG Three Areas of information are necessary Information about the patients immediate environment. Additional Lab Data. Clinical Information obtained through patient assessment.

14 Interpretation of an ABG Immediate Environment F I O 2 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 F I O 2 Barometric Pressure Age

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

19 Daltons Law of Partial Pressures All pressures in a gas mixture must add up to the total pressure (P BARO ). Dry Gas P gas = P BARO x F I O 2 Inspired Gas (ex. P I O 2 ) P gas = (P BARO - 47 torr) x F I O 2

20 Calculating Partial Pressures for dry gases P O 2 = 760 x mm Hg or torr P N 2 = 760 x mm Hg or torr P CO 2 = 760 x mm Hg or torr P Ar = 760 x mm Hg or torr NOTE: = 760

21 Altitudes Effect on Partial Pressure

22 High Altitude Response Increase Altitude P BARO P I O 2 P A O 2 P a O 2 To adapt to high altitudes Change the environment Airplanes are pressurized to feet. Increase F I O 2 (above 20,000 feet). Adapt Physiologically Hyperventilation. Collateral Circulation. Shift the oxygen dissociation curve. Increase Hemoglobin levels.

23 Calculating P Baro at High Altitudes P BARO falls 120 mm Hg per mile of altitude Example: Leadville is 2 miles above sea level. Calculate the P BARO & P O x 2 miles = 240 mm Hg decline = 520 mm Hg (P BARO ) P O 2 = 520 x mm Hg or torr (P O 2 )

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 P a CO 2 Equation Henderson Hasselbalch Alveolar Air Equation Oxygen Content (C a O 2 ) Oxygen Delivery Alveolar Ventilation Acid Base Oxygenation

26 P a CO 2 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 CO 2 produced, during metabolism, via ALVEOLAR ventilation.

27 Metabolism Steady State The amount of CO 2 added to the blood through metabolism = the amount of CO 2 excreted by the lungs. 200 mL/min

28 P a CO 2 Equation P a CO 2 = CO 2 production x Alveolar Minute Ventilation is a constant which equates dissimilar units. 40 mm Hg = 200 mL/min x L/min

29 P a CO 2 Equation If CO 2 production doubles (e.g. fever), alveolar minute ventilation must double to keep a normal P a CO 2 level. 40 mm Hg = 400 mL/min x L/min

30 Henderson-Hasselbalch Equation pH is defined as the negative log of the H + concentration pH = pK + Log HCO 3 - (Base) (P a CO 2 x 0.03) (Acid) pH = pK + Log 24.0 mEq/L 1.20 mEq/L Normal pH implies 20 times more base than acid

31 PAO2PAO2 P A O 2 = P BARO – 47 torr x F I O 2 – P a CO P A O 2 = P I O 2 - P a CO P A O 2 on room air = 100 – 104 mm Hg P A O 2 on 100% = 600s

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33 Effects of P a CO 2 on P A O 2 and P a O 2 A rise in the P a CO 2 will lower the P A O 2 and therefore the P a O 2. Hypoventilation is a cause of hypoxemia.

34 CaO2CaO2 C a O 2 = (S a O 2 x Hb x 1.34) + (P a O 2 x 0.003) With normal values: Oxyhemoglobin (attached) represents 19.7 vol%. Dissolved oxygen (P a O 2 ) 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 = C a O 2 x CO x 10 Oxygen Delivery = C a O 2 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 S a O 2 Hb P a O 2 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. F I O 2 stays the same regardless of changes in P Baro. P BARO decreases as altitude increases. Daltons Law. P O 2 is affected by F IO 2, P BARO and age.

39 P Airway = P BARO. 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. Summary of Important Points


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