Arterial and Venous Blood Gas Analysis Edward Omron MD, MPH

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Examines the utility of both arterial and venous blood gas analysis in critical illness
Edward Omron MD MPH FCCP
Pulmonary, Critical Care, and Internal Medicine
Morgan Hill, CA 95037
www.docomron.com

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  • There are 3 reasones why one should obtain a blood gas:
  • This is a typical printout from a blood gas report:
  • Recall altitiude Recall RS Allows to assess properly if the lungs are transferring loxygen appropriately into the blood
  • Arterial and Venous Blood Gas Analysis Edward Omron MD, MPH

    1. 1. Arterial And Venous BloodArterial And Venous BloodGas AnalysisGas AnalysisByByEdward Omron MD MPH FCCPEdward Omron MD MPH FCCPPulmonary Critical Care MedicinePulmonary Critical Care MedicineMorgan Hill, CAMorgan Hill, CAwww.docomron.comwww.docomron.com
    2. 2. INDICATIONSINDICATIONS• ABG– Oxygenation– Ventilation– Acid-Base Status• VBG– Ventilation and Acid-Base Status– Cardiac Output (venous arterial PCO2 difference)– Endpoint of resuscitation (PvO2 and ∆PCO2)
    3. 3. Blood Gas Report(Blood Gas Report(ArterialArterial))• pH (No Units) 7.35-7.45• PaCO2 (mm Hg) 35-45• PaO2 (mm Hg) 110 - 0.5(age)• HCO3-(mmol/L): calc. 22-26• B.E. (mmol/L) -2 to 2• O2 saturation: calc. >90%
    4. 4. Blood Gas ReportBlood Gas Report(mixed/central venous)(mixed/central venous)• pH = 7.32-7.42• PvCO2 = 40 - 50 (mm Hg)• PvO2 = 36 - 42 (mm Hg)• Oxygen Saturation > 70%• Base Excess = -2 to +2
    5. 5. ANALYSIS OF OXYGENATIONANALYSIS OF OXYGENATION• Alveolar Gas Equation– PAO2 = FIO2(PB - 47) - 1.2(PaCO2)– PAO2 defines upper limit of PaO2– FIO2 is 21% at all altitudes– Factor 1.2 determined by RQ– Water vapor pressure = 47 mm Hg– PAO2= 150 - 1.2(PaCO2) at room air– PAO2 = 102
    6. 6. Alveolar-Arterial Oxygen DifferenceAlveolar-Arterial Oxygen Difference• A-aDo2 = PAO2-PaO2(from ABG)• Insight in the patients state of gas exchange– If elevated, defect in gas exchange– Proper interpretation of the PaO2
    7. 7. Causes of a low PaOCauses of a low PaO22 and A-a Doand A-a Do22• P(B): Altitude• Alveolar Hypoventilation(Nl A-a Do2)• V/Q mismatch• Shunt• Diffusion Impairment• Decreased mixed / central venous O2content
    8. 8. Alveolar HypoventilationAlveolar Hypoventilation• No increase in P(A-a)O2 gradient• Muscle weakness• Neuromuscular Junction Disease• Reduced Respiratory Drive (drugs)• Chest wall elastic loads• Brainstem injury
    9. 9. Approach To HypoxemiaApproach To HypoxemiaPaO2 A-a Do2 pCO2NFIO2NAlv. Hypo.100% O2 Corrects V/Q Mis.No Correction ShuntDiffusion
    10. 10. Changes in P(A-a)O2 with FIO2
    11. 11. Acute Lung Injury• PaO2/FIO2 ratio– Normal = 90 mm Hg/0.2 = 450– Acute lung injury < 300– Acute Respiratory Distress syndrome<200
    12. 12. OXIMETRYOXIMETRY• Binding sites for O2 are heme groups• OXYGEN SATURATION– % of all heme sites saturated with O2• Measures the difference in the light absorbancecharaceteristics between Oxy Hb and Deoxy Hb• SpO2 = Oxy Hb x 100Oxy Hb + Deoxy Hb• ABG SaO2 is a calculated value from PaO2
    13. 13. Pulse OximetryPulse OximetryWhole BloodPaO2SaO2SpO2:Pulse Oximetry
    14. 14. OximetryOximetry• 54 yo WM with headaches, dyspnea anda Kerosene heater at home– ABG: PaO2 = 89, PaCO2 = 38, pH = 7.43– SaO2 = 98%– Whats the problem?
    15. 15. OximetryOximetry• Carboxyhemoglobin: Hb +CO– Does not affect PaO2 only SaO2– Pulse oximetry reads CO-HB as OxyHb• Follow Up:– PaO2 = 79, PaCO2 = 31, SpO2 = 53%, pH =7.36– CO-Hb46%
    16. 16. Correlation of symptoms and signs with carbon monoxide levelPercent of CO in inspiredairPercent of HbCO in blood Signs and symptoms0.007 10Common in cigarette smokers; dyspneaduring vigorousexertion; occasionaltightness in forehead; dilation of cutaneousblood vessels0.012 20Dyspnea during moderate exertion;occasional throbbing headache in temples0.022 30Severe headache; irritability; easyfatigability; disturbed judgment; possibledizziness and possible dimness of vision0.035 40+ Headache; confusion; fainting on exertion0.080 60+Unconsciousness; intermittent convulsions;respiratory failure; death if exposureprolonged0.195 80 FatalModified from Winter, P.M., and Miller, J.N.: JAMA 236:1503, 1976, Copyright 1976, American MedicalAssociation.
    17. 17. ProblemProblem• 42 yo HIV pt with fevers,chills, SOB,cough– Taking Dapsone for PCP prophylaxis– ABG: PaO2 82.5, PaCO2 35.2, pH 7.43, SaO289%– PCP Pneumonia, started onPrimaquine, Clinda,and Prednisone– ABG: PaO2 378, PaCO2 of 35, pH 7.42, SaO280%– Whats Happening?
    18. 18. MethemoglobinMethemoglobin• Oxidation of Fe++ to Fe+++ state• Unlike CO-Hb, Met-Hb does depress theSpO2 reading• Both Dapsone and Primaquine areoxidants• Met-Hb depresses the SpO2 to 80’s– Further increaeses in Met-Hb do not depressSpO2• Methylene Blue administration is Rx
    19. 19. Some drugs implicated in causing methemoglobinemiaGeneric name UseDapsone Skin protectantBenzocaine Local anestheticMetoclopramide Gastric stasisNitroglycerin AnginaPhenazopyridine Urinary tract analgesicPrilocaine Local anestheticPrimaquine Malaria prophylaxis and treatmentTrimethoprim Urinary antibacterialAmyl nitriteRarely used clinically; often used by drugabusers
    20. 20. ANALYSIS OF VENTILATONANALYSIS OF VENTILATON• PaCO2 = VCO2 x KVAHypercapnea > 45 mm Hg (Hypoventilation)Respiratory AcidosisHypocapnea < 35 mm Hg (Hyperventilation)Respiratory Alkalosis
    21. 21. Respiratory Acid-Base Status• Respiratory Disturbances– CO2+H20 H2CO3 H++ HCO3– Acute changes:• Delta 10 mm Hg PaCO2, pH changes by 0.08• Chronic change: 40 + B.E– Alveolar Ventilation• VA CO2 pH• Respiratory Acidosis pCO2 > 45• Respiratory Alkalosis pCO2 < 35
    22. 22. BASE EXCESS (B.E.)BASE EXCESS (B.E.)• Base Excess• Quantity of acid or base needed to restore plasmapH to 7.4, at a PCO2 equillibrated to 40 mm Hg• Positive value, excess base, metabolic alkalosis• Negative value, excess acid, metabolic acidosis• Metabolic component of acid-base status• PCO2 independent• Estimated by BE = (Total CO2 – 24)
    23. 23. Problem Solving1. LOOK AT THE pH– Whatever side of pH 7.4 is the primary disorder2. Look at pH, PCO2direction– Both decrease or increase, then metabolic– If move in opposite directions, respiratory3. Respiration: acute or chronic?– Acute: 10 mm Hg / 0.08 change in pH– Chronic: 40+Base Excess
    24. 24. • Calculate the Adjusted Anion Gap– High vs normal ANG differential– 2.8 mmol of acid /gram serum albumin– Law of Electrical Neutrality• Positive charges = negative charges or• Positive charges - negative charges = 0– [Na+] - [Cl-] -[HCO3-] - [Albumin-] = 0– [Na+] - [Cl-] -[HCO3-] = [Albumin-]– 140 - 104 -24 = 4.4 gm/dL* 2.8 ≅ 12– Normal ANG = 12 = 2.8 * [Albumin-]– Adjusted ANG = ANG + 2.8(4.4 -Albumin)
    25. 25. Arterial Draw:• pH = 7.28, PaCO2 = 34, HCO3 = 16• Na = 153 Cl = 106 Total CO2 = 17• Alb = 3 g/dL• Primary Acid-Base Disturbance?• ANG and Adjusted ANG• Metabolic Acid-Base Status74 yo male found unresponsive and pulseless
    26. 26. • Primary Disorder– Acidosis and acidemia (pH < 7.4)• pH and PCO2direction– Both down: Metabolic Acidosis• Base Excess– 16 – 24 = -8 mmols/L• Adjusted Anion Gap– ANG + 2.8*(4.4 - 3) or 30 + 4 = 34– Anion Gap Acidosis• Compensation?– 40 + BE or 32 mm Hg
    27. 27. Venous Draw• pH = 7.08, pCO2 = 75, HCO3 = 21• Na = 145, Cl = 103, Total CO2 =22• Alb = 3 g/dL• Primary Acid-Base Disorder?• ANG and expected ANG?• Metabolic Acid-Base Status?
    28. 28. • Primary Disorder– pH < 7.4, acidosis and acidemia• pH and PCO2 direction– Opposite therefore RESPIRATORY acidosis• Base Excess– 22 – 24 = -2 mmol/L• Adjusted Anion Gap– ANG + 2.8(4.4 -3) = 20+4 =24– Anion Gap Acidosis• 40 + BE rule ≠ Comp in VBG
    29. 29. 74 yo male found unresponsive and pulseless• Why a metabolic acidosis in arterial bedand respiratory acidosis in venous bed?– Venous arterial PCO2 difference?– PaCO2 = 34 and PvCO2 = 75– PvCO2 – PaCO2 ∝ 1 / cardiac index• Venous vs Arterial saturation difference?– PaO2 = 50 mm Hg, saturation = 84%– PvO2 =18, Venous Saturation = 20%– Increased oxygen extraction from circulatoryfailure
    30. 30. Central Venous Oxygen SaturationScvO2• Easily measured with venous blood gas• Surrogate measurement of mixed venous oxygen sat.– 5-18% higher– A low ScvO2 always means a low SvO2!• Normal ScvO2 ≅ 68-76%– 25% extraction coefficient of normal physiology– Peripheral venous gas as a screen for further investigation
    31. 31. PaO2 vs PvO2 in Cardiogenic Shock
    32. 32. Paradoxical Respiratory Acidosis of Cardiopulmonary Arrest
    33. 33. Four Determinants of Central Venous OximetryScvOScvO22 ≅≅ SvOSvO22 = SaO= SaO22 - VO- VO22 / C.O. x Hgb x 1.36/ C.O. x Hgb x 1.36ScvO2 = Central venous saturation (%)SvO2 = Mixed venous saturation (%)SaO2 = Arterial oxygen saturation (%)VO2 = Oxygen consumption mL (O2/min)Hgb = Hemoglobin concentration (g/dL)Cardiac Output (C.O.) = dL/min
    34. 34. Effect of changes in PaO2 on SvO260020010080604000.10.20.30.40.50.60.70.80.910 100 200 300 400 500 600 700PaO2 (mm Hg)SvO2SvO2
    35. 35. Effect of changes in Hgn on SvO213107.5500.10.20.30.40.50.60.70.80.90 2 4 6 8 10 12 14Hemoglobin (g/dL)SvO2SvO2
    36. 36. The Effects of Cardiac Output on SvO20.870.830.730.660.550.3100.10.20.30.40.50.60.70.80.910 2 4 6 8 10 12Cardiac Output (L/min)SvO2SvO2
    37. 37. Effect of Oxygen Consumption (VO2) on SvO20.850.790.740.680.570.4600.10.20.30.40.50.60.70.80.90 100 200 300 400 500 600Oxygen Consumption (VO2)SvO2SvO2
    38. 38. Venous Arterial CO2 Difference• Circulatory Failure– Associated with Tissue Hypercarbic Acidosis– Hypovolemia, sepsis, shock …• Cardiac Index = e (1.787–0.151(v-aCO2))– Endpoint of Resuscitation• PvO2– Enpoint of Resuscitation
    39. 39. SvO2 vs SaO2 response in Septic Shock "Golden Hour"27516873100 100 989101020304050607080901001 2 3 4DaysPercentageSvO2SaO2
    40. 40. REFERENCES• Current Opinion Critical Care 2001; 7: 204-211• NEJM 2001; 345: 1368-1377• Critical Care Medicine 2002; 30: 1686-1692• Circulation 1969; 40: 165• Thorax 2002; 57: 170-177• Academic Emer Med 1999; 6: 421

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