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ACID BASE DISORDERS

ACID BASE DISORDERS. It aiN’T All that Simple Dr alex Hieatt Consultant ED. What is an ABG?. The Components pH / PaCO 2 / PaO 2 / HCO 3 / O 2 sat / BE Desired Ranges pH - 7.35 - 7.45 PaCO 2 – 4.5 – 6 kPa PaO 2 – 10.5 – 13.5 kPa HCO 3 - 21-27 O 2 sat - 95-100%

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ACID BASE DISORDERS

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  1. ACID BASE DISORDERS It aiN’T All that Simple DralexHieatt Consultant ED

  2. What is an ABG? • The Components • pH / PaCO2 / PaO2 / HCO3 / O2sat / BE • Desired Ranges • pH - 7.35 - 7.45 • PaCO2 – 4.5 – 6 kPa • PaO2 – 10.5 – 13.5 kPa • HCO3 - 21-27 • O2sat - 95-100% • Base Excess - +/-2 mEq/L

  3. 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 • Buffering also occurs in the liver through ammonia metabolism to urea / glutamate

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

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

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

  7. Respiratory Acidosis • Acute vs Chronic • Acute - little kidney involvement. Buffering via titration via Hb for example • pH by 0.1 for 1.25 kPa  in CO2 • Chronic - Renal compensation via synthesis and retention of HCO3 (Cl to balance charges  hypochloremia) • pH by approx 0.05 for 1 kPa in CO2

  8. Respiratory Alkalosis • pH, CO2, Ventilation •  CO2  HCO3 (Cl to balance charges  hyperchloremia) • Causes CHAMPS • C – CNS Disease e.g. Intracerebral hemorrhage/ Cirrhosis • H – Hypoxia • A – Anxiety • M – Over ventilation • P – Progesterone • S – Salicylate/Sepsis

  9. Respiratory Alkalosis • Acute vs. Chronic • Acute - HCO3 by 1.5 mEq/L for every 1 kPa  in PCO2 • Chronic - Ratio increases to 3 mEq/L of HCO3 for every 1 kPa  in PCO2 • Decreased renal bicarb reabsorption and decreased ammonium excretion to normalize pH

  10. Metabolic Acidosis • pH, HCO3 • 12-24 hours for complete activation of respiratory compensation • PCO2 by 0.15 kPa for every 1 mEq/L HCO3 • The degree of compensation is assessed via the Winter’s Formula  PCO2 = {1.5(HCO3) +8  2 } x 0.133 [converts to kPa]

  11. The Causes • Metabolic Gap Acidosis • M - Methanol • U - Uremia • D – DKA - AKA • P - Paraldehyde • I – Isoniazid / Iron • L - Lactic Acidosis • E - Ethylene Glycol • R- Rhabdomyolysis • S - Salicylate • Non Gap Metabolic Acidosis • H - Hyperalimentation • A - Acetazolamide • R - RTA • D - Diarrhoea • U - Uretero-pelvic shunt • P - Pancreatic Fistula • S – Spironolactone

  12. Osmolar Gap • OG = Measured osmolality – calculated osmolality • OG = 2 x [ Na mmol/L] + [glucose mmol/L] + [urea mmol/L] + (1.25 x [Ethanol mmol/L]) • Should be <10 • Causes: • Methanol Glycine (TRUP) • Ethylene Glycol Propylene Glycol • Sorbitol Polyethylene Glycol • Mannitol Maltose (IV IG)

  13. OG • For raised AG Metabolic Acidocis • Common Causes: • Ketones • Lactate • Renal Failure • NO – • Ingestion possible • YES – Measure OG • Raised – Then likely Ethylene Glycol / Methanol • Normal – Salicylate, Paraldehyde, Iron + Isoniazid

  14. Metabolic Alkalosis • pH, HCO3 • PCO2 by 0.1 for every 1mEq/L  in HCO3 • Causes – CLEVER PD • C- Contraction • L - Liquorice • E - Endocrine: Conn’s / Cushing’s / Bartter’s • V - Vomiting / NG Suction • E - Excess Alkali • R - Refeeding Alkalosis • P - Post Hyper-capnoea • D - Diuretics and Chronic diarrhoea

  15. Mixed Acid-Base Disorders • Patients may have two or more acid-base disorders at one time • Corrected Bicarbonate = AG – 12 + Serum HCO3- • If > 30 then there is also underlying metabolic alkalosis • If < 23 then there is an underlying non-AG metabolic acidocis

  16. The Steps • Start with the pH – acidaemia or alkalaemia • Note the PCO2 • Look for disorders revealed by failure of compensation • Calculate anion gap • Calculate Corrected Bicarbonate

  17. Sample Problem #1 • An ill-appearing alcoholic male presents with nausea and vomiting. • ABG - 7.4 / 5.4 / 11.3 / 22 • Na- 137 / K- 3.8 / Cl- 90 / HCO3- 22

  18. Sample Problem #1 • Winter’s Formula = {1.5(22) + 8  2} x 0.133 = {39  2} x 0.133 = 5.3 kPa  compensated • Anion Gap = 137 - (90 + 22) = 25 •  anion gap metabolic acidosis • Corrected Bicarbonate = 25 - 12 = 13 13 + 22 = 35  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 / 2.5 / 15.7 / 14 • Na- 145 / K- 3.6 / Cl- 109 / HCO3- 17 • ASA level - 38.2 mg/dL

  21. Sample Problem #2 • Winters Formula = {1.5 (17) + 8  2} x 0.133 • = 4.65 kPa •  uncompensated • Anion Gap = 145 - (109 + 17) = 19  anion gap metabolic acidosis • Corrected HCO3- = 19 - 12 = 7 7 + 17 = 24  no metabolic alkalosis

  22. Sample Problem #3 • 47 year old male experienced crush injury at building site. • ABG - 7.3 / 4.2 / 12.8 / 15 • Na- 135 / K-5 / Cl- 98 / HCO3- 15

  23. Sample Problem #3 • Winters Formula = {1.5 (15) + 8  2} x 0.133 = 4 kPa  compensated • Anion Gap = 135 - (98 + 15) = 22 •  anion gap metabolic acidosis • Corrected Bicarb = 22 - 12 = 10 10 + 15 = 25 expected no additional deficit

  24. Sample Problem #4 • 1 month old male presents with projectile vomiting for x 2 days. • ABG - 7.49 / 5.33 / 13 / 30 • Na- 140 / K- 2.9 / Cl- 92 / HCO3- 32

  25. Sample Problem #4 • Metabolic Alkalosis, hypochloremic • Winters Formula = {1.5 (30) + 8  2} x 0.133 • = 53  2 • = 7.3 kPa •  uncompensated

  26. Questions • Practice makes perfect • MD Calc App has Winters and ABG analysis with SI units. • Josh Steinberg MD App – ABG eval (but US units.)

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