I2i / 21A14: Describe the pharmacology of sodium bicarbonate

21A14: Exam Report

Describe the pharmacology of sodium bicarbonate

29% of candidates passed this question.

This question was best answered with a structured approach as per any pharmacology question. It nonetheless required good understanding of various aspects of physiology. Many candidates failed to gain marks by omitting to mention facts which could have been prompted by a defined structure. A good response mentioned the pharmaceutic features including formulation and the hypertonicity of IV bicarbonate, pharmacodynamics including indications for use, mode of action, adverse effects (systemic and local), pharmacokinetics and dose. Pleasingly a few candidates stated that sodium bicarbonate’s mechanism of action to cause alkalosis involved increasing the strong ion difference in plasma. Credit was also given for stating the mechanism of action as providing bicarbonate ions to augment the extracellular buffer system.

Sodium Bicarbonate

Sodium Bicarbonate

Chemical

Sodium Bicarbonate

Use

  1. Metabolic acidosis (NAGMA due to HCO3 loss ie RTA)
  2. Urinary alkalization (salicylate poisoning)
  3. Tx Na Ch Blocker OD (TCA)
  4. Hyperkalaemia

Presentation

NaHCO3 8.4%

  • Contains ImmoI/ml of sodium and Immol/ml bicarbonate ions
  • Dissociates into a 2osmole solution \has a calculated osmolarity of 2000mosmol/l.

Dose

{Base deficit (mEq/L) x body weight (kg)} / 3

Or 1mmol/kg IV in cardiac arrest as a rapid bolus

Route

IV

Onset

Immediate

MoA

High Na+ load (100mmol) = Hypertonic

High HCO3- load (100mmol) = Alkalinising

PD

Normal ECF [Na] = 140mmol/L (x19L) = 2660mmol Na

Normal ECF [HCO3] = 24mmol/L (x19) = 456mmol HCO3

Therefore will have

  • volume effects
  • osmotic effects
  • change [Na]
  • change [HCO3

Increases [HCO3]

Normal ECF [HCO3] = 24mmol/L (x19) = 456mmol HCO3

Adding 100mmol HCO3 = 556mmol HCO3 = 29mmol/L

Extra HCO3 will be converted into CO2 and expired in approximately 30 seconds!


Increases [Na] ECF

Na will remain in ECF

Adding 100mmol Na = 2760mmol Na in 19.4L = 142mmol/L

→ Accounts for the volume & osmotic shifts

→ You have also just added 100mmol Na without any Chloride

→ If you are a Stewardite, you have just increased your SID

→ therefore altering the state of dissociation of Plasma H2O such that less plasma H2O is dissociated

→ because of that your pH goes up, you have alkalanised the plasma


 Osmotic effects & Volume Effects

Normal osmolality = 290 mOsm/L

∴ ECF osmolality = 290/L x19 = 5510 mOsm/L ECF

∴ ICF osmolality 290/L x 23 = 6670 mOsm/L ICF

adding 200mOsm to ECF = 5710 mOsm/L

New total osmolality = 5710 + 6670 = 12341 mOsm in 42L = 294mOsm/L

ECF 5710 osmoles/294 = 19.4L

∴ ECF expansion with 400ml water from increased osmolality

Osmolarity increases 1.3% \ stimulates osmoreceptors → ­ADH secretion from PPG

PK

A

NA

D

NA

M

NA

E

NA

Adverse Effects

Local tissue necrosis w extravasation

Metabolic alkalosis

Hypernatraemia

Fluid overload

Hypocalcaemia

Hypokalaemia

Hypercapnia

Author: Krisoula Zahariou