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Interpreting ABGs

Interpreting ABGs. Suneel Kumar MD. Arterial Blood Gases. Written in following manner: pH/PaCO 2 /PaO 2 /HCO 3 pH = arterial blood pH PaCO 2 = arterial pressure of CO 2 PaO 2 = arterial pressure of O 2 HCO 3 = serum bicarbonate concentration. Oxygenation.

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Interpreting ABGs

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  1. Interpreting ABGs Suneel Kumar MD

  2. Arterial Blood Gases • Written in following manner: pH/PaCO2/PaO2/HCO3 • pH = arterial blood pH • PaCO2 = arterial pressure of CO2 • PaO2 = arterial pressure of O2 • HCO3 = serum bicarbonate concentration

  3. Oxygenation • Hypoxia: reduced oxygen pressure in the alveolus (i.e. PAO2) • Hypoxemia: reduced oxygen pressure in arterial blood (i.e. PaO2)

  4. Hypoxia with Low PaO2 • Alveolar diffusion impairment • Decreased alveolar PO2 • Decreased FiO2 • Hypoventilation • High altitude • R  L shunt • V/Q mismatch

  5. Hypoxia with Normal PaO2 • Alterations in hemoglobin • Anemic hypoxia • Carbon monoxide poisoning • Methemoglobinemia • Histotoxic hypoxia • Cyanide • Hypoperfusion hypoxia or stagnant hypoxia

  6. Alveolar—Arterial Gradient • Indirect measurement of V/Q abnormalities • Normal A-a gradient is 10 mmHg • Rises with age • Rises by 5-7 mmHg for every 0.10 rise in FiO2, from loss of hypoxic vasoconstriction in the lungs

  7. Alveolar—Arterial Gradient A-a gradient = PAO2 – PaO2 • PAO2 = alveolar PO2 (calculated) • PaO2 = arterial PO2 (measured)

  8. Alveolar—Arterial Gradient PAO2 = PIO2 – (PaCO2/RQ) • PAO2 = alveolar PO2 • PIO2 = PO2 in inspired gas • PaCO2 = arterial PCO2 • RQ = respiratory quotient

  9. Alveolar—Arterial Gradient PIO2 = FiO2 (PB – PH2O) • PB = barometric pressure (760 mmHg) • PH2O = partial pressure of water vapor (47 mmHg) RQ = VCO2/VO2 • RQ defines the exchange of O2 and CO2 across the alveolar-capillary interface (0.8)

  10. Alveolar—Arterial Gradient PAO2 = FiO2 (PB – PH2O) – (PaCO2/RQ) Or PAO2 = FiO2 (713) – (PaCO2/0.8)

  11. Alveolar—Arterial Gradient • For room air: PAO2 = 150 – (PaCO2/0.8) • And assume a normal PaCO2 (40): PAO2 = 100

  12. Acid-Base • Acidosis or alkalosis: any disorder that causes an alteration in pH • Acidemia or alkalemia: alteration in blood pH; may be result of one or more disorders.

  13. Six Simple Steps • Is there acidemia or alkalemia? • Is the primary disturbance respiratory or metabolic? • Is the respiratory problem acute or chronic? • For metabolic, what is the anion gap? • Are there any other processes in anion gap acidosis? • Is the respiratory compensation adequate?

  14. Henderson-Hasselbach Equation pH = pK + log [HCO3/PaCO2] x K (K = dissociation constant of CO2) Or [H+] = 24 x PaCO2/HCO3

  15. pH 7.20 7.30 7.40 7.50 7.60 [H+] 60 50 40 30 20 Henderson-Hasselbach Equation

  16. Step 1:Acidemia or Alkalemia? • Normal arterial pH is 7.40 ± 0.02 • pH < 7.38  acidemia • pH > 7.42  alkalemia

  17. Step 2:Primary Disturbance • Anything that alters HCO3 is a metabolic process • Anything that alters PaCO2 is a respiratory process

  18. Step 2:Primary Disturbance • If 6pH, there is either 5PaCO2 or 6HCO3 • If 5pH, there is either 6PaCO2 or 5HCO3

  19. Step 4:For Metabolic, Anion Gap? Anion gap = Na+ - (Cl- + HCO3-) • Normal is < 12

  20. Increased Anion Gap • Ingestion of drugs or toxins • Ethanol • Methanol • Ethylene glycol • Paraldehyde • Toluene • Ammonium chloride • Salicylates

  21. Increased Anion Gap • Ketoacidosis • DKA • Alcoholic • Starvation • Lactic acidosis • Renal failure

  22. Step 4:For Metabolic, Anion Gap? • If + AG, calculate Osm gap: Calc Osm = (2 x Na+) + (glucose/18) + (BUN/2.8) + (EtOH/4.6) Osm gap = measured Osm – calc Osm Normal < 10 mOsm/kg

  23. Nongap Metabolic Acidosis • Administration of acid or acid-producing substances • Hyperalimentation • Nonbicarbonate-containing IVF

  24. Nongap Metabolic Acidosis • GI loss of HCO3 • Diarrhea • Pancreatic fistulas • Renal loss of HCO3 • Distal (type I) RTA • Distal (type IV) RTA • Proximal (type II) RTA

  25. Nongap Metabolic Acidosis • Calculate urine anion gap: Urine AG = (Na+ + K+) – Cl- • Positive gap indicates renal impaired NH4+ excretion • Negative gap indicates normal NH4+ excretion and nonrenal cause

  26. Nongap Metabolic Acidosis • Urine Cl- < 10 mEq/l is chloride responsive and accompanied by “contraction alkalosis” and is “saline responsive” • Urine Cl- > 20 mEq/l is chloride resistant, and treatment is aimed at underlying disorder

  27. Step 5: Any other process with elevated AG? • Calculate rgap, or “gap-gap”: rGap = Measured AG – Normal AG (12)

  28. Step 5: Any other process with elevated AG? • Add rgap to measured HCO3 • If normal (22-26), no other metabolic problems • If < 22, then concomitant metabolic acidosis • If > 26, then concomitant metabolic alkalosis

  29. Step 6: Adequate respiratory compensation? Winter’s Formula Expected PaCO2 = (1.5 x HCO3) + 8 ± 2 • If measured PaCO2 is higher, then concomitant respiratory acidosis • If measured PaCO2 is lower, then concomitant respiratory alkalosis

  30. Step 6: Adequate respiratory compensation? • In metabolic alkalosis, Winter’s formula does not predict the respiratory response • PaCO2 will rise > 40 mmHg, but not exceed 50-55 mmHg • For respiratory compensation, pH will remain > 7.42

  31. Clues to a Mixed Disorder • Normal pH with abnormal PaCO2 or HCO3 • PaCO2 and HCO3 move in opposite directions • pH changes in opposite direction for a known primary disorder

  32. Case 1 • A 24 year old student on the 6 year undergraduate plan is brought to the ER cyanotic and profoundly weak. His roommate has just returned from a semester in Africa. The patient had been observed admiring his roommate's authentic African blowgun and had scraped his finger on the tip of one of the poison darts (curare).

  33. Case 1 138 100 26 7.08/80/37

  34. Case 1 • What is the anion gap? AG = 138 – (100 + 26) AG = 12

  35. Case 1 • Acute respiratory acidosis

  36. Case 2 • A 42 year old diabetic female who has been on insulin since the age of 13 presents with a 4 day history of dysuria which has progressed to severe right flank pain. She has a temperature of 38.8ºC, a WBC of 14,000, and is disoriented.

  37. Case 2 135 99 4.8 12 7.23/25/113

  38. Case 2 • What is the A-a gradient? A-a = [150 – 25/0.8] – 113 = 6 • Acidemia or alkalemia? • Primary respiratory or metabolic? • What is the anion gap? AG = 135 – (99 + 12) = 24

  39. Case 2 • What is the rgap? rGap = 24 – 12 = 12 rGap + HCO3 = 12 + 12 = 24 • No other metabolic abnormalities • Is the respiratory compensation appropriate? Expected PCO2 = (1.5 x 12) + 8 ± 2 = 24 ± 2 • It is appropriate

  40. Case 2 • Acute anion gap metabolic acidosis (DKA)

  41. Case 3 • A 71 year old male, retired machinist, is admitted to the ICU with a history of increasing dyspnea, cough, and sputum production. He has a 120 pack-year smoking history, and quit 5 years previously. On exam he is moving minimal air despite using his accessory muscles of respiration. He has acral cyanosis.

  42. Case 3 135 93 30 7.21/75/41

  43. Case 3 • What is the A-a gradient? A-a = [150 – 75/.8] – 41 = 15 • Acidemic or alkalemic? • Primary respiratory or metabolic? • Acute or chronic? • Acute 5PCO2 by 35 would 6pH by 0.28 • Chronic 5PCO2 by 35 would 6pH by 0.105 • Somewhere in between

  44. Case 3 • What is the anion gap? AG = 135 – (93 + 30) = 12

  45. Case 3 • Acute on chronic respiratory acidosis (COPD)

  46. Case 3b • This same patient is intubated and mechanically ventilated. During the intubation he vomits and aspirates. He is ventilated with an FiO2 of 50%, tidal volumes of 850cc, PEEP of 5, rate of 10. One hour later his ABG is 7.48/37/215.

  47. Case 3b • What is the A-a gradient? A-a = [FiO2 (713) – 37/.8] – 215 A-a = 310 – 215 = 95 • Why is he alkalotic with a normal PCO2? • Chronic compensatory metabolic alkalosis and acute respiratory alkalosis

  48. Case 4 • A 23 year old female presents to the Emergency Room complaining of chest tightness and light-headedness. Other symptoms include tingling and numbness in her fingertips and around her mouth. Her medications include Xanax and birth control pills, but she recently ran out of both.

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