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Evaluating the Hypoxic Patient

Evaluating the Hypoxic Patient. Catherine J. Markin MD Pulmonary and Critical Care Noon Conference 2004. Goals. Discuss mechanisms of hypoxia Explore clinical/lab tests for hypoxia Provide a “framework” for evaluation of patients with hypoxia.

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Evaluating the Hypoxic Patient

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  1. Evaluating the Hypoxic Patient Catherine J. Markin MD Pulmonary and Critical Care Noon Conference 2004

  2. Goals • Discuss mechanisms of hypoxia • Explore clinical/lab tests for hypoxia • Provide a “framework” for evaluation of patients with hypoxia

  3. You are called by a 7CVA nurse who is taking care of patient LB. LB is a 52 y/o woman with rheumatoid arthritis and a TKA 3 days ago. The nurse reports that LB is requiring 3L/min nc oxygen to maintain a SpO2 of 89%, but is otherwise not distressed. The nurse comments that the only thing the patient is complaining of is a sore throat for which prn cetacaine spray is being used. The nurse thinks that the patient might benefit from a bronchodilator and asks if you would give her a verbal order.

  4. You are in the middle of dinner and are tempted to give the order. But instead you decide to evaluate the patient yourself. As you are walking across the bridge, you are thinking about the following clinical questions:

  5. Is the patient in respiratory distress and in need of immediate assistance in breathing? • What is the differential diagnosis of hypoxia in this patient? • What are the appropriate clinical tests that can help sort out the cause of hypoxia?

  6. Is the patient in respiratory distress and in need of immediate assistance in breathing? • What is the differential diagnosis of hypoxia in this patient? • What are the appropriate clinical tests that can help sort out the cause of hypoxia?

  7. Hypoxia: • Inadequate utilization of oxygen by cells Hypoxemia: • Abnormally low oxygen in the blood

  8. Clinical Signs of Hypoxia • Cyanosis • Restlessness/agitation • Tachypnea • Tachycardia • Confusion • +/- Low SpO2

  9. Indications for Endotracheal Intubation • Airway protection • Relief of airway obstruction • Shock • Facilitation of suctioning/pulm toilet • Reducing work of breathing • Respiratory failure • Hypercapnic • Hypoxic

  10. Upon your arrival, pt is breathing 14/min and with complaints of vague chest constriction but no pain. The nurse hands you an EKG that is significant only for ST 120 bpm. You note the patients cigarettes next to the bed and she has an epidural in place. Lungs with faint bibasilar crackles. The patient is obese and you are unsure of her JVP.

  11. Is the patient in respiratory distress and in need of immediate assistance in breathing? • What is the differential diagnosis of hypoxia in this patient? • What are the appropriate clinical tests that can help sort out the cause of hypoxia?

  12. Mechanisms of Hypoxia • Hemoglobin/histotoxic • Hypoventilation • Low alveolar pressure • Low FIO2 • R to L Shunt (V>Q) • Dead Space Ventilation (V<<Q) • Diffusion Impairment

  13. airways alveolus surfactant/air-water interface pneumocyte interstitial space endothelial cell serum RBC membrane hgb R L R cell membrane cytoplasm mitochondria cell

  14. Mechanisms of Hypoxia Hypoxia without hypoxemia • Hemoglobin/histotoxic • Low atm pressure • Low FIO2 • Hypoventilation • R to L Shunt (V<Q) • Dead Space Ventilation (V > Q) • Diffusion Impairment Hypoxemia

  15. Diagnosis of Hypoxia without Hypoxemia • Physical or laboratory signs of hypoxia • Adequate PaO2 (>60 mmHg) • Normal or mildly abnormal SpO2

  16. • DO2 = Qt • (CaO2 ) • 10 Qt = Cardiac output (liters/m) CaO2 = arterial content of oxygen (ml/dl) CaO2 = Hgb bound O2 + dissolved O2

  17. CaO2 = Hgb bound O2 + dissolved O2 Hgb bound O2 = (Hgb g/dl • 1.34ml/g Hgb • SaO2) Dissolved O2 = (PaO2 torr • .003 ml O2/dl/torr) PaO2= partial pressure of arterial oxygen SaO2 = oxygen saturation arterial blood

  18. Oxygen Saturation % Hb saturation = O2 bound to Hg x 100% O2 capacity of Hg

  19. Transport of Oxygen by Blood Dissolved oxygen + oxygen bound to Hgb Partial Pressure of O2 (PaO2)

  20. Hemoglobin Dissociation Curve

  21. Pulse Oxygen Saturation Meter(SpO2) • Comparison of peak and trough absorption 2 wavelengths of light (660 and 940 nm) Soft tissue bone blood photodiode detector

  22. pulsatile arterial blood • non-pulsatile arterial blood • venous and capillary blood • tissue

  23. R = ratio AC = pulsatile DC = non pulsatile

  24. 2+ O2 Heme Molecule in Ferrous State (reduced) Oxyhemogobin

  25. 3+ Heme Molecule in Ferrous State (MetHgb)

  26. Methemoglobin • Heme in ferric (3+) state • “Muddy brown” blood • MetHgb saturation is 85% , SpO2 89% • Absorption at 660 nm and 940 nm are equal • Peak absorption 631 nm • Causes: Congenital defect in enzyme metHgb reductase, high level or chronic exposure to anoxidizing agent (benzocaine, dapsone, fava beans)

  27. 85% = 1

  28. AcetanilidPhenolsHydroxylamineAlloxansPhenylenediamineKiszkaAminophenolsPhenylhydroxylamineMentholAmyl nitratePiperazineMethylacetanilidAnilinoethanolPrilocaineMonochloroanilineArsinePropitocaineNaphthylaminesBismuth subnitratePyrogallol NitritesChloroanilinesQuinonesNitrogen oxideChloronitrobenzeneShoe dyeNitroglycerinCorning extractSulfonalNitrosobenzeneDiaminodiphenylsulfoneDimethylamineTetralanPara-chloroanilineDinitrobenzeneToluidinePara-toluidineDinitrotolueneTrionalHydroxyacetanilide AcetophenetidinPhenylazopyridineInks, markingAlpha naphthylaminePhenylhydrazineLidocaineAmmonium nitratePhenytoinMeta-chloroanilineAniline dyesChloroquineMethylene blueAntipyrinePrimaquineMoth ballsBenzocainePyridiumNitratesChloratesPyridine NitrobenzeneChlorobenzeneResorcinolNitrofuransCobaltSpinachNitrophenolDapsoneSulfonamidesSulfonesPara-bromoanilineDimethyl anilineToluenediaminePara-nitroanilineDinitrophenolTolylhydroxylaminePhenacetin

  29. CO Carboxy Hemoglobin

  30. Carboxyhemoglobin • Heme bound to CO • “Cherry red” blood • Absorbtion co-efficient similar to oxyhemoglobin—falsely positive SpO2 • Causes: Exposure to high levels of inhaled carbon monoxide • Normal levels: 0-3% non-smokers, up to 15% smokers

  31. 630 nm Co-oximetry = 4 or more wavelengths

  32. Mitochondrial Hypoxia • Cyanide poisoning • Binds to ferric iron in cytochrome C oxidase • Inhibits electron transport chain • Low affinity for hgb • Cellular anoxia, anaerobic metabolism, lactic acidosis • Exposures: Occupational exposure to hydrogen cyanide (electroplating, photography, jewelry making), combustion of household materials, nitroprusside

  33. Mitochondrial Hypoxia, cont. • Congenital disorders • Mitochondrial genetic disorders • Sepsis • HAART therapy • Nucleoside reverse transcriptase inhibitors (stavudine, lamvudine, zidovudine)

  34. Limitations of O2 Saturation • +/- 2% between 70-100% • Inaccuracies with: • Poor perfusion • Venous pulsations • Nail polish • Hyperbilirubinemia • Methylene blue • Indigo and indocyanine green • Onchomycosis • Carboxy hemoglobin • Methemoglobin

  35. Mechanisms of Hypoxia Hypoxia without hypoxemia • Hemoglobin/histotoxic • Low atm pressure • Low FIO2 • Hypoventilation • R to L Shunt (V<Q) • Dead Space Ventilation (V>Q) • Diffusion Impairment Hypoxemia Abnormal Aa gradient

  36. How do I calculate an Aa gradient? PAO2 - PaO2 Alveolar Gas Equation Arterial Blood Gas

  37. airways alveolus surfactant/air-water interface pneumocyte interstitial space endothelial cell serum RBC membrane hgb PAO2 R L R cell membrane cytoplasm mitochondria cell

  38. Alveolar Gas Equation PAO2 = (Patm – PH20)(FI02) - PaCO2 0.8 PAO2 = partial pressure of oxygen in the alveoli (total) Patm = atmospheric pressure (760 mmHg) PH20 = partial pressure of water (47 mmHg) FI02 = fraction inspired oxygen (21% RA) PaCO2 = partial pressure of CO2 in blood O.8 = respiratory quotient

  39. PAO2 = (Patm – PH20)(FI02) - PaCO2 0.8 1 = 1.2 (760 – 47)(.21) = 150 RA = 1.0.8

  40. Remember this Formula! PAO2 (RA)= 150 - 1.2(PaCO2)

  41. What causes a Low Alveolar Oxygen (Nl A-a)? PAO2 = (Patm – PH20)(FI02) - PaCO2 0.8

  42. Mechanisms of Hypoxia • Hemoglobin/histotoxic • Low atm pressure • Low FIO2 • Hypoventilation • R to L Shunt (V<Q) • Dead Space Ventilation (V>Q) • Diffusion Impairment Hypoxemia Abnormal Aa gradient

  43. What is a Normal Aa Gradient? • Increases with age • Can be calculated at any FI02 • I can only interpret 21% and 100% Aa • A formula adjusting for age (RA): Normal Aa = 2.5 + .25 (age)

  44. Mechanisms of Hypoxia • Hemoglobin/histotoxic • Low atm pressure • Low FIO2 • Hypoventilation • R to L Shunt (V<Q) • Dead Space Ventilation (V>Q) • Diffusion Impairment Hypoxemia Abnormal Aa gradient

  45. V < Q V  Q V > Q R L L R Dead Space Ventilation Shunt Physiologic Anatomic Normal Lung

  46. Shunt (V>Q) • Anatomic • • AV malformation • • Uncorrected congenital heart disease • • Patent foraman ovale • Hepatopulmonary syndrome • Physiologic • • Pneumonia • • Pulmonary hemorrhage • Pulmonary edema • Mucous plugging (OLD)

  47. V < Q V  Q V > Q R L L R Dead Space Ventilation Shunt Physiologic Anatomic Normal Lung

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