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
- Metabolic acidosis (NAGMA due to HCO3 loss ie RTA)
- Urinary alkalization (salicylate poisoning)
- Tx Na Ch Blocker OD (TCA)
- 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