ACID BASE DISORDERS IT AINT ALL THAT SIMPLE

ACID BASE DISORDERS IT AIN’T ALL THAT SIMPLE DR ALEX HIEATT CONSULTANT ED

WHAT IS AN ABG? The Components • p. H / Pa. CO 2 / Pa. O 2 / HCO 3 / O 2 sat / BE Desired Ranges • p. H - 7. 35 - 7. 45 • Pa. CO 2 – 4. 5 – 6 k. Pa • Pa. O 2 – 10. 5 – 13. 5 k. Pa • HCO 3 - 21 -27 • O 2 sat - 95 -100% • Base Excess - +/-2 m. Eq/L

ACID BASE BALANCE The body produces acids daily • 15, 000 mmol CO 2 • 50 -100 m. Eq Nonvolatile acids The lungs and kidneys attempt to maintain balance Buffering also occurs in the liver through ammonia metabolism to urea / glutamate

ACID BASE BALANCE Assessment of status via bicarbonate-carbon dioxide buffer system • Henderson-Hasselbalch • p. H= p. K + log ([HCO 3 -] / [H 2 CO 3 ]) • CO 2 + H 2 O <--> H 2 CO 3 <--> HCO 3 - + H+ • ph = 6. 10 + log ([HCO 3] / [0. 03 x PCO 2])

THE TERMS ACIDS • Acidemia • Acidosis • Respiratory CO 2 • Metabolic HCO 3 BASES • Alkalemia • Alkalosis • Respiratory CO 2 • Metabolic HCO 3

RESPIRATORY ACIDOSIS ph, CO 2, Ventilation Causes • • • CNS depression Pleural disease COPD/ARDS Musculoskeletal disorders Compensation for metabolic alkalosis

RESPIRATORY ACIDOSIS Acute vs Chronic • Acute - little kidney involvement. Buffering via titration via Hb for example • p. H by 0. 1 for 1. 25 k. Pa in CO 2 • Chronic - Renal compensation via synthesis and retention of HCO 3 ( Cl to balance charges hypochloremia) • p. H by approx 0. 05 for 1 k. Pa in CO 2

RESPIRATORY ALKALOSIS p. H, CO 2, Ventilation CO 2 HCO 3 ( 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

RESPIRATORY ALKALOSIS Acute vs. Chronic • Acute - HCO 3 by 1. 5 m. Eq/L for every 1 k. Pa in PCO 2 • Chronic - Ratio increases to 3 m. Eq/L of HCO 3 for every 1 k. Pa in PCO 2 • Decreased renal bicarb reabsorption and decreased ammonium excretion to normalize p. H

METABOLIC ACIDOSIS p. H, HCO 3 12 -24 hours for complete activation of respiratory compensation PCO 2 by 0. 15 k. Pa for every 1 m. Eq/L HCO 3 The degree of compensation is assessed via the Winter’s Formula PCO 2 = {1. 5(HCO 3) +8 2 } x 0. 133 [converts to k. Pa]

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

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)

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

METABOLIC ALKALOSIS p. H, HCO 3 PCO 2 by 0. 1 for every 1 m. Eq/L in HCO 3 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

MIXED ACID-BASE DISORDERS Patients may have two or more acid-base disorders at one time Corrected Bicarbonate = AG – 12 + Serum HCO 3 If > 30 then there is also underlying metabolic alkalosis If < 23 then there is an underlying non-AG metabolic acidocis

THE STEPS Start with the p. H – acidaemia or alkalaemia Note the PCO 2 Look for disorders revealed by failure of compensation Calculate anion gap Calculate Corrected Bicarbonate

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 / HCO 3 - 22

SAMPLE PROBLEM #1 Winter’s Formula = {1. 5(22) + 8 2} x 0. 133 = {39 2} x 0. 133 = 5. 3 k. Pa compensated Anion Gap = 137 - (90 + 22) = 25 anion gap metabolic acidosis Corrected Bicarbonate = 25 - 12 = 13 13 + 22 = 35 metabolic alkalosis

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.

SAMPLE PROBLEM #2 ABG - 7. 47 / 2. 5 / 15. 7 / 14 Na- 145 / K- 3. 6 / Cl- 109 / HCO 3 - 17 ASA level - 38. 2 mg/d. L

SAMPLE PROBLEM #2 Winters Formula = {1. 5 (17) + 8 2} x 0. 133 = 4. 65 k. Pa uncompensated Anion Gap = 145 - (109 + 17) = 19 anion gap metabolic acidosis Corrected HCO 3 - = 19 - 12 = 7 7 + 17 = 24 no metabolic alkalosis

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 / HCO 3 - 15

SAMPLE PROBLEM #3 Winters Formula = {1. 5 (15) + 8 2} x 0. 133 = 4 k. Pa compensated Anion Gap = 135 - (98 + 15) = 22 anion gap metabolic acidosis Corrected Bicarb = 22 - 12 = 10 10 + 15 = 25 expected no additional deficit

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 / HCO 3 - 32