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  • 1. ABG Interpretation ยิ่งศักดิ์ ศุภนิตยา นนท์
  • 2. Case 1 FiO2 0.21 pH 7.21 PaCO2 64 BE 2 PaO2 48 SpO2 80 • Acute respiratory acidosis with moderate hypoxemia
  • 3. Normal composition of clean, dry air near sea level Nitrogen 78.084 Oxygen 20.9476 Argon 0.934 Carbon dioxide 0.0314 Neon 0.001818 Helium 0.000524 Krypton 0.000114 Xenon 0.000087 Hydrogen 0.00005 Methane 0.0002 Nitrous oxide 0.00005
  • 4. Dalton’s Law of Partial Pressure GAS PARTIAL PRESSURE = % GAS CONC x 7.13 mmHg
  • 5. ABG DATA pH 7.40 ( 7.35 - 7.45 ) H+ 40 nM/L PaO2 97 ( 90- 100 ) torr PaCO2 40 ( 35 - 45 ) torr Actual HCO3 24 ( 22 - 26 ) mEq/L Base excess +/- 2 mEq/L Hb 15 gm% O2 sat 95-100% O2 content 15-23%
  • 6. IMPLICATIONS OF ABG • LUNG FUNCTION • OXYGENATION, VENTILATION, ACID BASE STATUS • EARLY DIAGNOSIS OF ARF • DIAGNOSIS OF SECONDARY POLYCYTHEMIA DUE TO PULMONARY DISEASE • HEART AND CIRCULATORY FUNCTION • KIDNEY FUNCTION • METABOLISM • THE USE OF SOME MEDICATIONS • QUALIFY PATIENTS FOR HOME OXYGEN USE • DETECT EXPOSURE TO CARBON MONOXIDE AND OTHER CHEMICALS
  • 7. TECHNIQUES SITES: radial, brachial, femoral arteries. Allen’s test VOLUME: 1 mL
  • 8. ERRORS • PLASTIC SYRINGE • AIR MIXTURE • OVER HEPARINIZATION (acid) • VENOUS SAMPLE • PAIN • DELAY OR UNCOOL SPECIMEN • ERROR OF GAS ANALYSER
  • 9. PaCO2 • 80% plasma NaHCO3 • 10% carboxy Hb + 2% carbamino comp • 8% dissolved in plasma
  • 10. EXCRETORY RATE OF CO2 = VA x Pa CO2 Pa CO2 = 1 / VA VA = ( VT - VD ) x f INTERPRETATION OF PaCO2 PaCO2 > 45 mmHg = alveolar hypoventilation = resp acidosis PaCO2 < 35 mmHg = alveolar hyperventilation = resp alkalosis
  • 11. PaO2 Interpretation > 100 mmHg hyperoxemia 90-100 normal 60-80 mild hypoxemia 40-60 moderate hypoxemia < 40 severe hypoxemia
  • 12. Oxygenation and external respiration Causes of hypoxemia Low FIO2 Hypoventilation Diffusion defect Ventilation perfusion mismatch Dead space and shunting Venous admixture
  • 13. Oxygen transport and internal respiration Arterial oxygen content Volume of dissolved oxygen + Volume of combined oxygen with hemoglobin ------------------------------------- Total oxygen content --------------------------------------
  • 14. volume of dissolved O2 = PaO2 x CsO2 = 0.3 vol%O2 volume of combined O2 = Hb x SaO2 x 1.34 = 19.7vol% CaO2 = dissolved O2 + combined O2 = 20 vol% CaO2 - CvO2 = 20 - 15.2 = 4.8 vol% O2 consumption = Q x C(a - v) O2 = 250 mL O2/min total O2 transport = cardiac output x CaO2 = 1000mLO2/min
  • 15. Oxygenation ratio (PaO2/%FiO2) Pulmonary status O2 ratio normal 4.0 - 5.0 moderate pulmonary dysfunction 2.0 - 3.9 substantial pulmonary dysfunction < 2.0
  • 16. pH Henderson’s equation Kc = [H] [HCO3] / [H2CO3] Hasselbalch’s equation pH = pKc + log [HCO3] / [H2CO3] pH = pKc + log [HCO3 / dissolve CO2] pH = 6.1 + log 24 / 1.2 pH = 7.4
  • 17. Determination of primary problem pH > 7.4 Alkalosis is primary; acidosis is compensatory < 7.4 Acidosis is primary; alkalosis is compensatory
  • 18. Severity of generalized acid-base disturbances pH Degree of impairment < 7.20 severe acidemia 7.20-7.29 moderate acidemia 7.30-7.34 mild acidemia 7.35-7.45 normal pH 7.46-7.50 mild alkalemia 7.51-7.55 moderate alkalemia > 7.55 severe alkalemia
  • 19. Calculated bicarbonate • Actual bicarbonate • It is a calculated value based on the Henderson-Hasselbalch equation. • Henderson’s equation Kc = [H] [HCO3] / [H2CO3] Hasselbalch’s equation pH = pKc + log [HCO3] / [H2CO3] pH = pKc + log [HCO3 / dissolve CO2] pH = 6.1 + log 24 / 1.2
  • 20. Base excess of blood BE = Observed BB - normal BB
  • 21. Classification of laboratory metabolic acid-base compensation Classification BE HCO3 normal metabolic component 0 +/-2 24+/-2 metabolic acidosis < - 2 < 22 metabolic alkalosis > + 2 > 26
  • 22. Stepwise approach to diagnosing acid-base disorders • Step1: Acidemic or alkalemic? • Step2: Is the primary disturbance respiratory or metabolic? • Step3: For a respiratory disturbance, determine whether it is acute or chronic. • Step4: For a metabolic acidosis, determine whether an anion gap is present. • Step5: Determine whether other metabolic disturbances coexist with an anion gap acidosis. • Step6: Assess the normal compensation by the respiratory system for a
  • 23. Step1: Acidemic or Alkalemic? Normal arterial blood pH = 7.40 +/- 0.05 Acidemic: pH < 7.35 Alkalemic: pH > 7.45
  • 24. Step2: Is the primary disturbance respiratory or metabolic? A respiratory disturbance alters the arterial PaCO2 (normal value 40, range 38-42). Go to step 3. A metabolic disturbance alters the serum HCO3 (normal value 24, range 22-26) • If HCO3 < 22, metabolic acidosis is present. Go to step 4. • If HCO3 > 26, metabolic alkalosis is present, is respiratory compensation adequate? Go to step 6.
  • 25. Step3: For a respiratory disturbance, determine whether it is acute or chronic. • Ac resp acid: pH decrease = 0.08*(PaCO2- 40)/10 • Ch resp acid: pH decrease = 0.03*(PaCO2- 40)/10 • Ac resp alka: pH increase = 0.08*(40 - PaCO2)/10 • Ch resp alka: pH increase = 0.017*(40 - PaCO2)/10
  • 26. Step4: For a metabolic acidosis, determine whether an anion gap is present. • Anion gap = Na - (Cl + HCO3) • Anion gap metabolic acidosis, anion gap > 12 • Normal or non anion gap acidosis, anion gap </= 12
  • 27. Anion gap reflects the unmeasured anion and cation. Unmeasured Anions Proteins, mostly albumin 15 mEq/L Organic acids 5 mEq/L Phosphates 2 mEq/L Sulfates 1 mEq/L Total: 23 mEq/L Measured Anions Chloride 104 mEq/L Bicarbonate 24 mEq/L Total: 128 mEq/L Unmeasured Cations Calcium 5 mEq/L Potassium 4.5 mEq/L Magnesium 1.5 mEq/L Total: 11 mEq/L Measured Cations Sodium 140 mEq/L Total: 140 mEq/L
  • 28. Step5: Determine whether other metabolic disturbances coexist with an anion gap acidosis. Corrected HCO3 = measured HCO3 + (anion gap - 12) If the corrected HCO3 varies significantly above or below 24, then a mixed or more complex metabolic disturbance exists. To be more specific, if the corrected HCO3 is greater than 24, a metabolic alkalosis coexists. If the corrected HCO3 is less than 24 then a non anion gap acidosis coexists.
  • 29. Step6: Assess the normal compensation by the respiratory system for a metabolic disturbance. Winter’s Formula Expected PaCO2 = (1.5*HCO3) +(8+/-2) Winter’s Formula does not predict the resp response to a metabolic alkalosis. Two general rules • a pt will increase PaCO2 above 40 but not greater than 50-55 to compensate for a metabolic alkalosis. • a pt will be alkalemic if the PaCO2 is elevated to compensate for a met alk ( If the patient is acidemic,PH < 7.38, then an additional resp acid is present).
  • 30. Steps in evaluation and classification of acid-base compensation • Evaluate for the presence of compensation. • Determine the probable primary problem. • Classify the degree of compensation.
  • 31. Alerts to mixed acid-base disturbances • If respiratory and metabolic parameters change proportionately, pH remains unchanged. • Both parameters are altered in fashion that changes the pH in the same direction. • Fails to compensate in the expected manner for a primary disorder after sufficient time has elapse. • A metabolic alkalosis is accompanied by an increase in the anion gap. • Absent of compensation. • Long standing pulmonary or renal disease. • Excessive compensation. • Respiratory assistance. • Settings conducive to mixed disturbances. • Triple disorders may also be encountered.
  • 32. Case 2 FiO2 0.21 pH 7.22 PaCO2 25 HCO3 10 PaO2 96 SaO2 95 creatinine 11 mg/dL •Simple metabolic acidosis with normoxemia
  • 33. Case 4 FiO2 0.21 pH 7.35 PaCO2 22 HCO3 12 PaO2 41 SaO2 75 •Mixed respiratory alkalosis and metabolic acidosis.
  • 34. Case 5 FiO2 0.21 pH 7.10 PaCO2 95 BE - 5 HCO3 29 PaO2 60 SpO2 78% • Partially compensated respiratory acidosis.
  • 35. Case 6 FiO2 0.21 pH 7.53 PaCO2 49 HCO3 39 PaO2 92 SaO2 98 • Partially compensated metabolic alkalosis with normoxemia.
  • 36. Case 7 FiO2 0.21 pH 7.58 PaCO2 31 HCO3 28 PaO2 65 SaO2 96 • Combined respiratory alkalosis and metabolic alkalosis with mild hypoxemia.
  • 37. Case 9 FiO2 0.21 pH 7.04 PaCO2 15 BE -22 PaO2 125 SaO2 95 •Partially compensated metabolic acidosis with hyperoxemia.
  • 38. Case 10 FiO2 0.21 pH 7.25 PaCO2 80 HCO3 34 PaO2 39 SaO2 52 •Partially compensated respiratory acidosis with severe hypoxemia.