Critical Care Fundamentals

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A review of fundamental critical care concepts
Edward Omron MD, MPH, FCCP
Pulmonary, Critical Care Medicine
Morgan Hill, CA 95037

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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:
  • Critical Care Fundamentals

    1. 1. Critical Care Concepts Edward Omron MD, MPH, FCCP Pulmonary and Critical Care
    2. 2. INDICATIONS <ul><li>ABG </li></ul><ul><ul><li>Oxygenation </li></ul></ul><ul><ul><li>Ventilation </li></ul></ul><ul><ul><li>Acid-Base Status </li></ul></ul><ul><li>VBG </li></ul><ul><ul><li>Ventilation and Acid-Base Status </li></ul></ul><ul><ul><li>Cardiac Output ( venous arterial PCO 2 difference) </li></ul></ul><ul><ul><li>Endpoint of resuscitation (ScvO 2 and  PCO 2 ) </li></ul></ul>
    3. 3. Blood Gas Report( Arterial ) <ul><li>pH (No Units) 7.35-7.45 </li></ul><ul><li>PaCO 2 (mm Hg) 35-45 </li></ul><ul><li>PaO 2 (mm Hg) 110 - 0.5(age) </li></ul><ul><li>HCO 3 - (mmol/L): calc. 22-26 </li></ul><ul><li>B.E. (mmol/L) -2 to 2 </li></ul><ul><li>O 2 saturation: calc. >90% </li></ul>
    4. 4. Blood Gas Report (mixed/central venous) <ul><li>pH = 7.32-7.42 </li></ul><ul><li>PvCO 2 = 40 - 50 (mm Hg) </li></ul><ul><li>PvO 2 = 36 - 42 (mm Hg) </li></ul><ul><li>Oxygen Saturation > 70% </li></ul><ul><li>Base Excess = -2 to +2 </li></ul>
    5. 6. ANALYSIS OF VENTILATON <ul><li>PaCO2 = V CO2 x K </li></ul><ul><li>V A </li></ul><ul><li>Hypercapnea > 45 mm Hg (Hypoventilation) </li></ul><ul><li>Respiratory Acidosis </li></ul><ul><li>Hypocapnea < 35 mm Hg (Hyperventilation) </li></ul><ul><li>Respiratory Alkalosis </li></ul>
    6. 7. Respiratory Acid-Base Status <ul><li>Respiratory Disturbances </li></ul><ul><ul><li>CO 2 +H 2 0 H 2 CO 3 H + + HCO 3 </li></ul></ul><ul><ul><li>Acute changes: </li></ul></ul><ul><ul><ul><li>Delta 10 mm Hg PaCO 2 , pH changes by 0.08 </li></ul></ul></ul><ul><ul><ul><li>Chronic change: 40 + B.E </li></ul></ul></ul><ul><ul><li>Alveolar Ventilation </li></ul></ul><ul><ul><ul><li>VA CO2 pH </li></ul></ul></ul><ul><li>Respiratory Acidosis pCO2 > 45 </li></ul><ul><li>Respiratory Alkalosis pCO2 < 35 </li></ul>
    7. 8. BASE EXCESS(B.E.) <ul><ul><li>Positive value, excess base, metabolic alkalosis </li></ul></ul><ul><ul><li>Negative value, excess acid, metabolic acidosis </li></ul></ul><ul><ul><li>Metabolic component of acid-base status </li></ul></ul><ul><ul><li>PCO 2 independent </li></ul></ul><ul><ul><li>Estimated by BE = (Total CO 2 – 24) </li></ul></ul>
    8. 9. <ul><li>Problem Solving </li></ul><ul><li>1. LOOK AT THE pH </li></ul><ul><ul><li>Whatever side of pH 7.4 is the primary disorder </li></ul></ul><ul><li>2. Look at pH, PCO 2 direction </li></ul><ul><ul><li>Both decrease or increase, then metabolic </li></ul></ul><ul><ul><li>If move in opposite directions, respiratory </li></ul></ul><ul><li>3. Respiration: acute or chronic? </li></ul><ul><ul><li>Acute: 10 mm Hg / 0.08 change in pH </li></ul></ul><ul><ul><li>Chronic: 40+Base Excess </li></ul></ul>
    9. 10. <ul><li>Calculate the Adjusted Anion Gap </li></ul><ul><ul><li>High vs normal ANG differential </li></ul></ul><ul><ul><li>Adjusted ANG = ANG + 2.8(4.4 -Albumin) </li></ul></ul>
    10. 11. <ul><li>Arterial Draw : </li></ul><ul><li>pH = 7.28, PaCO 2 = 34, HCO 3 = 16 </li></ul><ul><li>Na = 153 Cl = 106 Total CO 2 = 17 </li></ul><ul><li>Alb = 3 g/dL, Saturation = 84% </li></ul><ul><li>Primary Acid-Base Disturbance? </li></ul><ul><li>Adjusted ANG </li></ul><ul><li>Metabolic Acid-Base Status </li></ul>74 yo male found unresponsive and pulseless
    11. 12. <ul><li>Primary Disorder </li></ul><ul><ul><li>Acidosis and acidemia (pH < 7.4) </li></ul></ul><ul><li>pH and PCO 2 direction </li></ul><ul><ul><li>Both down: Metabolic Acidosis </li></ul></ul><ul><li>Base Excess </li></ul><ul><ul><li>16 – 24 = -8 mmols/L </li></ul></ul><ul><li>Adjusted Anion Gap </li></ul><ul><ul><li>ANG + 2.8*(4.4 - 3) or 30 + 4 = 34 </li></ul></ul><ul><ul><li>Anion Gap Acidosis </li></ul></ul><ul><li>Compensation? </li></ul><ul><ul><li>40 + BE or 32 mm Hg </li></ul></ul>
    12. 13. <ul><li>Venous Draw </li></ul><ul><li>pH = 7.08, pCO2 = 75, HCO3 = 21 </li></ul><ul><li>Na = 145, Cl = 103, Total CO 2 =22 </li></ul><ul><li>Alb = 3 g/dL, Saturation = 20% </li></ul><ul><li>Primary Acid-Base Disorder? </li></ul><ul><li>Adjusted ANG? </li></ul><ul><li>Metabolic Acid-Base Status? </li></ul>
    13. 14. <ul><li>Primary Disorder </li></ul><ul><ul><li>pH < 7.4, acidosis and acidemia </li></ul></ul><ul><li>pH and PCO2 direction </li></ul><ul><ul><li>Opposite therefore RESPIRATORY acidosis </li></ul></ul><ul><li>Base Excess </li></ul><ul><ul><li>22 – 24 = -2 mmol/L </li></ul></ul><ul><li>Adjusted Anion Gap </li></ul><ul><ul><li>ANG + 2.8(4.4 -3) = 20+4 =24 </li></ul></ul><ul><ul><li>Anion Gap Acidosis </li></ul></ul><ul><li>40 + BE rule  Comp in VBG </li></ul>
    14. 15. 74 yo male found unresponsive and pulseless <ul><li>Why a metabolic acidosis in arterial bed and respiratory acidosis in venous bed? </li></ul><ul><ul><li>Venous arterial PCO2 difference? </li></ul></ul><ul><ul><li>PvCO2 (75) - PaCO2 (34) = 41 </li></ul></ul><ul><ul><li>PvCO2 – PaCO2  1 / cardiac index </li></ul></ul><ul><ul><li>Normal ≤ 6 mm Hg </li></ul></ul><ul><li>Venous vs Arterial saturation difference? </li></ul><ul><ul><li>PaO2 = 50 mm Hg, saturation = 84% </li></ul></ul><ul><ul><li>PvO2 =18, Venous Saturation = 20% </li></ul></ul><ul><ul><li>Increased oxygen extraction from circulatory failure </li></ul></ul>
    15. 16. PaO 2 vs PvO 2 in Cardiogenic Shock Arterial Venous Saturation Difference SHOCK
    16. 17. Effects of Cardiac Output on Arterio-venous Difference <ul><li>V O 2 = 1.34*Hgn*10*C.O.*(SaO 2 –SvO 2 ) </li></ul><ul><li>V O 2 = 1.34*Hgn*10* C.O.* (SaO 2 –SvO 2 ) </li></ul><ul><ul><li>A decrement in C.O. must be accompanied by an increase in the arteriovenous difference at constant oxygen consumption </li></ul></ul>
    17. 18. A fall in venous saturation from 70% to 50% represents A fall in cardiac index of 42%
    18. 19. Paradoxical Respiratory Acidosis of Cardiopulmonary Arrest Venous Arterial CO2 Difference
    19. 20. Central Venous-Arterial PCO2 Gradient
    20. 21. Fick Equation for CO 2 production <ul><li>V CO2 =Carbon dioxide production (200 mL/min) </li></ul><ul><li>V CO2 = 10*C.O.*(PvCO 2 – PaCO 2 ) </li></ul><ul><li>If cardiac output decreases and V CO2 remains constant, what must happen to venous-arterial CO2 difference? </li></ul><ul><li>V CO2 = 10* C.O.* (PvCO 2 – PaCO 2 ) </li></ul><ul><li>Respiratory Quotient V CO2 /V O2 = 200/250 = 0.8 </li></ul>
    21. 22. Venous Arterial CO 2 Difference <ul><li>Circulatory Failure </li></ul><ul><ul><li>Associated with Tissue Hypercarbic Acidosis </li></ul></ul><ul><ul><li>Hypovolemia, sepsis, shock … </li></ul></ul><ul><li>Cardiac Index = e (1.787 – 0.151(v-a CO2)) </li></ul><ul><ul><li>Endpoint of Resuscitation </li></ul></ul><ul><li>PvO2 or SvO2 from VBG </li></ul><ul><ul><li>Enpoint of Resuscitation </li></ul></ul>
    22. 23. <ul><li>Cardinal Rules of Critical Care </li></ul><ul><ul><li>GOLDEN HOUR </li></ul></ul><ul><ul><ul><li>Time dependent function </li></ul></ul></ul><ul><ul><li>Maximize Oxygen Delivery </li></ul></ul><ul><ul><ul><li>Improve Cardiac Performance </li></ul></ul></ul><ul><ul><ul><ul><li>Maintain MAP > 65 mm Hg (arbitrary) </li></ul></ul></ul></ul><ul><ul><ul><ul><ul><li>Preload Augmentation (crystalloid/colloid) </li></ul></ul></ul></ul></ul><ul><ul><ul><ul><ul><li>Inotropes (norepinephrine) </li></ul></ul></ul></ul></ul><ul><ul><ul><ul><ul><li>Peripheral vasoconstrictor (vasopressin) </li></ul></ul></ul></ul></ul><ul><ul><ul><li>Transfuse Packed Red Blood Cells </li></ul></ul></ul><ul><ul><ul><li>Assume control of ventilation/oxygenation </li></ul></ul></ul><ul><ul><ul><ul><li>FIO 2 factors </li></ul></ul></ul></ul>
    23. 24. Cardinal Rules of Critical Care <ul><ul><li>Minimize oxygen consumption </li></ul></ul><ul><ul><ul><li>Sedation, analgesia, rarely neuromuscular blockade </li></ul></ul></ul><ul><ul><ul><li>Work of breathing </li></ul></ul></ul><ul><ul><ul><li>Fever </li></ul></ul></ul><ul><ul><ul><li>Rigors </li></ul></ul></ul><ul><ul><ul><li>Pain </li></ul></ul></ul><ul><ul><ul><li>Anxiety </li></ul></ul></ul>
    24. 25. Endpoint of Resuscitation <ul><li>Conventional Endpoints are lagging indicators of inadequate oxygen delivery </li></ul><ul><ul><li>Blood Pressure </li></ul></ul><ul><ul><li>Heart Rate </li></ul></ul><ul><ul><li>Urine Output </li></ul></ul><ul><ul><li>Mental Status Changes </li></ul></ul><ul><ul><li>Central Venous Pressure (poor surrogate of filling pressures) </li></ul></ul>
    25. 26. <ul><li>More effective endpoints of resuscitation </li></ul><ul><ul><ul><li>Continuous Cardiac Index </li></ul></ul></ul><ul><ul><ul><ul><li>Pulse pressure variation </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Systolic pressure variation </li></ul></ul></ul></ul><ul><ul><ul><li>SvO2 or ScvO2 </li></ul></ul></ul><ul><ul><ul><li>Serum lactate (Tissue hypoxia </li></ul></ul></ul><ul><ul><ul><li>Venous – arterial PCO2 </li></ul></ul></ul><ul><ul><ul><ul><li>Directly correlates with cardiac index </li></ul></ul></ul></ul><ul><ul><ul><li>Metabolic acid-base status (SBE) </li></ul></ul></ul>
    26. 27. Resuscitation Endpoints <ul><li>A single set of data points is useless </li></ul><ul><ul><li>C.I., SvO2, PaCO2-PvCO2, SBE, Lactate </li></ul></ul><ul><li>Construct multiple data points to assess trends and response to interventions! </li></ul><ul><li>REAL time bedside interventions </li></ul>
    27. 28. Pulse Pressure Variation
    28. 29. Systolic Pressure Variation
    29. 30. Preload Augmentation RVEDV or LVEDV Stroke Volume Normal Abnormal (Cardiogenic or septic shock) 50 mL 100 mL 150 mL
    30. 31. Preload Augmentation s/p fluid bolus RVEDV or LVEDV Stroke Volume Normal Abnormal (Cardiogenic or septic shock) 50 mL 100 mL 150 mL 200 mL
    31. 32. Stroke Volume Ventricular preload The lower the ventricular preload, the more likely the preload-dependency preload-dependence preload-independence
    32. 33. . Stroke volume Ventricular preload normal heart failing heart preload-dependence preload-independence
    33. 34. <ul><li>Preload Augmentation </li></ul><ul><li>DO 2 (oxygen delivery in mL O 2 /min) </li></ul><ul><li>DO 2 = CO x CaO 2 x 10 </li></ul><ul><li>DO 2 = HR x SV x CaO 2 x 10 </li></ul><ul><li>DO 2 = HR x SV x Hb x SaO 2 x 13.8 </li></ul>
    34. 35. Central Venous Oxygen Saturation ScvO 2 <ul><li>Allows separation of early and late shock </li></ul><ul><li>Easily measured with venous blood gas </li></ul><ul><li>Surrogate measurement of mixed venous oxygen sat. </li></ul><ul><ul><li>5-18% higher </li></ul></ul><ul><ul><li>A low ScvO2 always means a low SvO2! </li></ul></ul><ul><li>Normal ScvO 2  68-76% </li></ul><ul><ul><li>25% extraction coefficient of normal physiology </li></ul></ul>
    35. 37. Fick Equation for Oxygen Consumption <ul><li>V O 2 = Oxygen Consumption (250 mL/min) </li></ul><ul><li>V O 2 = 10*C.O.*(CaO2 –CvO2) </li></ul><ul><li>V O 2 = 10 * C.O. * (1.34*Hgn*SaO2 -1.34*Hgn*SvO2) </li></ul><ul><li>V O 2 = 1.34*Hgn*10*C.O.*(SaO 2 – SvO 2 ) </li></ul><ul><li>Solve for SvO 2 ? </li></ul>
    36. 38. Four Determinants of Central Venous Oximetry ScvO 2  SvO 2 = SaO 2 - (VO 2 / C.O. x Hgb x 1.34) ScvO 2 = Central venous saturation (%) SvO 2 = Mixed venous saturation (%) SaO 2 = Arterial oxygen saturation (%) VO 2 = Oxygen consumption mL (O 2 /min) Hgb = Hemoglobin concentration (g/dL) Cardiac Output (C.O.) = dL/min
    37. 39. Master Equation ScvO2  SvO2 = SaO2 - (VO2 / C.O. x Hgb x 1.34) <ul><li>Acute Illness or Post-op Surgery </li></ul><ul><ul><li>SaO 2 , VO 2 , Cardiac Output, and Hgb are dynamically changing concurrently </li></ul></ul><ul><ul><li>Optimize each parameter then recheck ScvO 2 to assess response to intervention </li></ul></ul>
    38. 44. REFERENCES <ul><li>Current Opinion Critical Care 2001; 7: 204-211 </li></ul><ul><li>NEJM 2001; 345: 1368-1377 </li></ul><ul><li>Critical Care Medicine 2002; 30: 1686-1692 </li></ul><ul><li>Circulation 1969; 40: 165 </li></ul><ul><li>Thorax 2002; 57: 170-177 </li></ul><ul><li>Academic Emer Med 1999; 6: 421 </li></ul>

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