Acid Base & Metabolites

expected [HCO3-] = 24 * 0.1 * (PaCO2 – 40)

expected [HCO3-] = 24 * 0.35 * (PaCO2 – 40)

expected [HCO3-] = 24 – 0.2 * (40 – PaCO2)

expected [HCO3-] = 24 – 0.5 * (40 – PaCO2)

expected PaCO2 = 1.5 * [HCO3-] + 8 – short cut: PaCO2 = last 2 digits of pH. For 10 mmol ¯ in HCO3, ¯ PaCO2 by 12.5

expected PaCO2 = 0.6 * [HCO3-] + 40 - short cut: PaCO2 = last 2 digits of pH. For 10 mmol ­ in HCO3, ­ PaCO2 by 7

Na – Cl – HCO3 Normal 8 - 12

• * Is it acidosis or alkalosis
• * Respiratory or Metabolic
• * Is compensation appropriate: PaCO2 and HCO3 will move in the same direction
• * Check Extent of compensation.
• * Calculate AGc
• * If High: measure osmolality.
• * If Normal: measure urinary osmolality.
• * If Low: lab error, lithium, IgG myeloma

• * Hyperchloraemia
• * GI losses of SID enteric fluid
• * Renal tubular acidosis
• * Acid loads: TPN and NH4Cl administration
• * Drug induced ­ K with renal insufficiency
• * Li toxicity

• * Lactate: L-Lactate acidosis, Short bowel syndrome (D-lactate)
• * Ketoacidosis
• * Renal failure: sulphate and other organic anions, ­ PO4
• * Poisoning: Ethylene glycol, Methanol, Salicylates, Iron
• * Pyroglutamic acidosis
• * Myeloma IgA bands

d AG / d HCO3

Done in high AG disorders to diagnoses and additional acid-base problem

• * Loop or thiazide diuretics: decr ECF leads to incr­ aldosterone + high distal flow rates of Na leading to K resorption/H loss
• * Steroid excess: Corticosteroids, Cushing’s * Hypercalcaemia, Milk Alkali syndrome (­ Ca ingestion leads to ­ incr Na excretion + decr PTH leads to HCO3 retention)
• * Mg deficiency
• * Loss of H+: vomiting, suctioning
• * Gain of high SID fluid: NaHCO3, Na citrate (> 8 units stored blood, plasma exchange), renal replacement with high SID fluid

• Acute:
• * Hypoxaemia
• * Sepsis
• * PE
• * Asthma
• * Drugs: salicylates, SSRIs
• * Pain, anxiety

• Chronic:
• * Pregnancy
• * Altitude
• * Chronic lung or liver disease

(>285 mOsm/kg):

• Impermeant solutes:
• * Glucose
• * Hypertonic infusions: mannitol, TURP syndrome (absorption of gylcine from irrigation solution)

(285 – 295 mOsm/kg): ­incr lipids or incr­ protein leading to factitious hyponatraemia (error of measurement)

• Urinary sodium > 20:
• * Acute and chronic renal failure
• * Hyperaldosteronism from any cause (eg Steroids, Cushings)

• Urinary sodium < 20:
• * Liver failure/cirrhosis
• * CCF (leading to 2ndary hyperaldosteronism)
• * Nephrotic syndrome
• * Excess 5% dextrose

• Urinary Na < 20 mEq/L:
• * Water intoxication
• * Pain (eg post-operative)

• Urinary Na > 20 mEq/L:
• * Renal failure
• * Hypothyroidism
• * Adrenal insufficiency
• * Medications: oxytocin for induced labour, SSRIs in the elderly, thiazides, omeprazole
• * SIADH (eg pulmonary infections, SCLC)

• Urinary Na < 20 mEq/L (ie non-urinary loss. Treat with slow resuscitation)
• * Vomiting
• * Diarrhoea, fistulas/stomas
• * Skin losses (sweating, Cystic fibrosis)
• * Third spacing (eg burns, pancreatitis)

• Urinary Na > 20 (ie urinary loss - salt wasting states):
• * Diuretics
• * Adrenal insufficiency
• * Renal tubular acidosis
• * Cerebral salt wasting

• Decr Intake: anorexia, malabsorption
• Incr Excretion:
• * Renal:
• - Steroids: Conn’s, Cushing’s, Ectopic ACTH
• - Drugs: diuretics, corticosteroids, carbapenems, gentamicin
• - RTA
• * Vomiting: loss of HCl
• * Diarrhoea, laxatives

• Redistribution:
• * Insulin
• * Alkalosis
• * b 2 adrenergics
• * Delayed following blood transfusion

• Artifact: haemolysis
• Incr­ Intake:
• * Blood transfusion
• * Exogenous

• Decr output:
• * Renal:
• - Failure
• - K sparing diuretics
• - Hypoaldosteronism, including drug induced (ACEI, ARB)

• Redistribution:
• * Cell lysis
• * Acidosis
• * Digoxin overdose

• Excretion:
• * Decr Vitamin D
• * Decr PTH
• * Loop diuretics

• Redistribution:
• * Alkalosis ® ­ protein binding
• * Citrate from transfusion
• * decr PTH leading to incr­ bone uptake

• Intake (rare):
• * Vitamin D intoxication
• * Milk-alkali syndrome

• Decr excretion:
• * Renal failure
• * Thiazides, Li
• * Incr Vitamin D

• Altered redistribution (generally incr resorption from bone):
• * Any cause of ­ incr Vitamin D (eg granulomatous disease)
• * Any cause of incr­ PTH: primary hyperparathyroidism, PTH secreting tumours, hyperthyroidism
• * Bony infiltrates by infection, Paget’s, malignancy
• * Dehydration

• Incr PTH
• Decr Vitamin D
• RTA
• Alkalosis
• Alcoholism
• Refeeding Syndrome

• * Rhabdomyolysis
• * Renal failure
• * Vitamin D toxicity
• * Acidosis
• * Tumour lysis
• * Decr PTH