Hypertonic saline (20% concentrated)
This chapter is relevant to Section I2(i) of the 2017 CICM Primary Syllabus, which expects the exam candidates to develop or at least fake "an understanding of the pharmacology of colloids and crystalloids". The material being explored here is 20% saline, 200g of sodium chloride in every litre of sterile water. Its relevance to CICM trainees increases when they start studying for their Second part exam, where osmotherapy for management of raised intracranial pressure is a common topic for questions. In fact, a whole chapter dedicated to the various uses of hypertonic saline exists in that section. In the CICM First Part, the questions involving hypertonic saline are essentially limited to Question 10 from the first paper of 2015, where the candidates were expected to compare it to mannitol.
Name 20% saline Class Concentrated electrolyte Chemistry Monovalent cation salt Routes of administration IV only (specifically, can only be administered through CVC) Absorption Well absorbed in the small intestine. largely because of paracellular transport (high concentration). Good bioavailability Solubility pKa 3.09; good water solubility Distribution VOD=0.2L/kg, basically confined to the extracellular fluid
(thus: 25% remains intravascular, 75% becomes interstitial)Target receptor Mechanism of action does not involve receptor binding; you might say the target is the blood brain barrier Metabolism Does not undergo any metabolism; excreted unchanged in the urine Elimination Elininated renally, where specific reabsorption mechanisms in the renal tubule regulate the rate of sodium and chloride excretion Time course of action For control of intracranial pressure, onset of effect is very rapid (within tens of seconds of the infusion starting) Mechanism of action Increases the osmolality of the extracellular fluid, and therefore decreases the volume of the intracellular compartment by producing an osmotic shift of intracellular water of of the cells. This produces the desirable clinical effect of decreasing the volume of brain tissue, and therefore reducing the intracranial pressure. Clinical effects Increase in serum osmolality, which produces the desireable osmotic decrease in intracranial pressure.
A smaller increase in circulating volume than with mannitol, and without the diuresis.
Produced hypernatremia and hyperchloraemia, of which the latter may be associated with a metabolic acidosis
Concentrated saline, NaCl 20%
It is usually available in little plastic containers, each containing 10ml. I cannot think of a situation which would call for a litre of 20% NaCl, and if you encounter such an instance in your practice, please email me, so I can marvel at it. Now, AstraZeneca originally made this 20% stuff as a vein sclerosant. Needless to say, it remains a peripheral venous sclerosant, even when it is given with the noble intent of decreasing intracranial pressure or increasing serum sodium. It should only be given through a central line.
Given that it is a 20% solution, one might expect there to be 20g of salt in every 100ml of water; thus, every 10ml plastic vial contains 2g of salt. This contributes 34.22 mmol of sodium and chloride, each.
Lavoisier have a pdf describing the properties of their product, to which I referred when reading about this.
In some surreal world where litre bags of 20% saline are available, each litre bag of 20% saline contains 200g (3.422 moles) of sodium chloride.
References
From MIMS online, via CIAP; using Baxter Full PI data sheets. Those PI documents are word for word what you will find on the bags. Additionally, the anaesthesiauk website has this page, with a summary of the relevant details. To find out more about the pH of intravenous solutions, you could pay JAMA for this article.