It aiN’T All that Simple Dr alex Hieatt Consultant ED ACID BASE DISORDERS It aiN’T All that Simple Dr alex Hieatt Consultant ED

What is an ABG? The Components Desired Ranges 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

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

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])

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

Respiratory Acidosis ph, CO2, 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 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

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

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

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]

The Causes Metabolic Gap Acidosis Non Gap Metabolic 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 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

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

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

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

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

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 / HCO3- 17 ASA level - 38.2 mg/dL

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

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

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

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

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

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