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Arterial Blood Gas Analysis

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  1. Arterial Blood Gas Analysis Overview

  2. What is an ABG? • The Components • pH / PaCO2 / PaO2 / HCO3 / O2sat / BE • Desired Ranges • pH - 7.35 - 7.45 • PaCO2 - 35-45 mmHg • PaO2 - 80-100 mmHg • HCO3 - 21-27 • O2sat - 95-100% • Base Excess - +/-2 mEq/L

  3. Why Order an ABG? • Aids in establishing a diagnosis • Helps guide treatment plan • Aids in ventilator management • Improvement in acid/base management allows for optimal function of medications • Acid/base status may alter electrolyte levels critical to patient status/care

  4. Logistics • When to order an arterial line -- • Need for continuous BP monitoring • Need for multiple ABGs • Where to place -- the options • Radial • Femoral • Brachial • Dorsalis Pedis • Axillary

  5. Acid Base Balance • The body produces acids daily • 15,000 mmol CO2 • 50-100 mEq Nonvolatile acids • The lungs and kidneys attempt to maintain balance

  6. Acid Base Balance • Assessment of status via bicarbonate-carbon dioxide buffer system • CO2 + H2O <--> H2CO3 <--> HCO3- + H+ • ph = 6.10 + log ([HCO3] / [0.03 x PCO2])

  7. The Terms • ACIDS • Acidemia • Acidosis • Respiratory CO2 • Metabolic HCO3 • BASES • Alkalemia • Alkalosis • Respiratory CO2 • Metabolic HCO3

  8. Respiratory Acidosis • ph, CO2, Ventilation • Causes • CNS depression • Pleural disease • COPD/ARDS • Musculoskeletal disorders • Compensation for metabolic alkalosis

  9. Respiratory Acidosis • Acute vs Chronic • Acute - little kidney involvement. Buffering via titration via Hb for example • pH by 0.08 for 10mmHg  in CO2 • Chronic - Renal compensation via synthesis and retention of HCO3 (Cl to balance charges  hypochloremia) • pH by 0.03 for 10mmHg in CO2

  10. Respiratory Alkalosis • pH, CO2, Ventilation •  CO2  HCO3 (Cl to balance charges  hyperchloremia) • Causes • Intracerebral hemorrhage • Salicylate and Progesterone drug usage • Anxiety  lung compliance • Cirrhosis of the liver • Sepsis

  11. Respiratory Alkalosis • Acute vs. Chronic • Acute - HCO3 by 2 mEq/L for every 10mmHg  in PCO2 • Chronic - Ratio increases to 4 mEq/L of HCO3 for every 10mmHg  in PCO2 • Decreased bicarb reabsorption and decreased ammonium excretion to normalize pH

  12. Metabolic Acidosis • pH, HCO3 • 12-24 hours for complete activation of respiratory compensation • PCO2 by 1.2mmHg for every 1 mEq/L HCO3 • The degree of compensation is assessed via the Winter’s Formula  PCO2 = 1.5(HCO3) +8  2

  13. The Causes • Metabolic Gap Acidosis • M - Methanol • U - Uremia • D - DKA • P - Paraldehyde • I - INH • L - Lactic Acidosis • E - Ehylene Glycol • S - Salicylate • Non Gap Metabolic Acidosis • Hyperalimentation • Acetazolamide • RTA (Calculate urine anion gap) • Diarrhea • Pancreatic Fistula

  14. Metabolic Alkalosis • pH, HCO3 • PCO2 by 0.7 for every 1mEq/L  in HCO3 • Causes • Vomiting • Diuretics • Chronic diarrhea • Hypokalemia • Renal Failure

  15. Mixed Acid-Base Disorders • Patients may have two or more acid-base disorders at one time • Delta Gap Delta HCO3 = HCO3 + Change in anion gap >24 = metabolic alkalosis

  16. The Steps • Start with the pH • Note the PCO2 • Calculate anion gap • Determine compensation

  17. Sample Problem #1 • An ill-appearing alcoholic male presents with nausea and vomiting. • ABG - 7.4 / 41 / 85 / 22 • Na- 137 / K- 3.8 / Cl- 90 / HCO3- 22

  18. Sample Problem #1 • Anion Gap = 137 - (90 + 22) = 25  anion gap metabolic acidosis • Winters Formula = 1.5(22) + 8  2 = 39  2  compensated • Delta Gap = 25 - 10 = 15 15 + 22 = 37  metabolic alkalosis

  19. Sample Problem #2 • 22 year old female presents for attempted overdose. She has taken an unknown amount of Midol containing aspirin, cinnamedrine, and caffeine. On exam she is experiencing respiratory distress.

  20. Sample Problem #2 • ABG - 7.47 / 19 / 123 / 14 • Na- 145 / K- 3.6 / Cl- 109 / HCO3- 17 • ASA level - 38.2 mg/dL

  21. Sample Problem #2 • Anion Gap = 145 - (109 + 17) = 19  anion gap metabolic acidosis • Winters Formula = 1.5 (17) + 8  2 = 34  2  uncompensated • Delta Gap = 19 - 10 = 9 9 + 17 = 26  no metabolic alkalosis

  22. Sample Problem #3 • 47 year old male experienced crush injury at construction site. • ABG - 7.3 / 32 / 96 / 15 • Na- 135 / K-5 / Cl- 98 / HCO3- 15 / BUN- 38 / Cr- 1.7 • CK- 42, 346

  23. Sample Problem #3 • Anion Gap = 135 - (98 + 15) = 22  anion gap metabolic acidosis • Winters Formula = 1.5 (15) + 8  2 = 30  2  compensated • Delta Gap = 22 - 10 = 12 12 + 15 = 27  mild metabolic alkalosis

  24. Sample Problem #4 • 1 month old male presents with projectile emesis x 2 days. • ABG - 7.49 / 40 / 98 / 30 • Na- 140 / K- 2.9 / Cl- 92 / HCO3- 32

  25. Sample Problem #4 • Metabolic Alkalosis, hypochloremic • Winters Formula = 1.5 (30) + 8  2 = 53  2  uncompensated