When red blood cells are introduced into water or sodium chloride solution containing less than 0.9% of the solute, they swell and often burst due to the diffusion of water into the cells and the fact that the cell wall is not strong enough to resist the pressure.
This phenomenon is called haemolysis. Parenteral products are made isotonic with the body fluids otherwise they may penetrate the red blood cells and cause haemolysis.
Additives present in parenteral solutions contribute to the overall hemolytic character of the solution. If the solutions are hypotonic, the osmotic pressure of the solutions can be increased by the addition of either sodium chloride or dextrose.
Compounds which contribute to the isotonicity of a product also reduce pain at the site of injection in areas with nerve endings. A 1.8% solution of urea has the same osmotic pressure as 0.9% sodium chloride solution but the urea solution produces haemolysis.
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Hence a product should not be considered isotonic until it has been tested in the biological system.
Administration of hypertonic solutions into the blood stream may cause crenulation of cells which may return to normal with equalization of osmotic pressure. In such cases a slow injection into a vein, in which circulation is rapid, is indicated as the solution is thereby rapidly diluted and swept away.
Parenteral products are sometimes required to possess specific gravity matching with that of the body fluids and hence such adjustments may also be called for.
The intraspinal injections should have predetermined specific gravity as compared to body fluid into which it is to be injected. This is of particular importance in spinal anesthesia.
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In case the upper part of the patient’s body is raised by slopping the operating table, the solution of lower specific gravity than the spinal fluid will tend to rise on injection and that of higher specific gravity will tend to sink.
For operation on the lower part of the body where the patient is titled head downwards the opposite effects will occur. Hence a careful choice must be made of the specific gravity of the solution and the position of the patient.
The average specific gravity of spinal fluid at 37°C may be taken as 1.0059. The specific gravity of injection solution with respect to that of spinal fluid is expressed as isobaric (i.e. of equal) hypobaric (i.e. lower) and hyperbaric (i.e. higher).
A 1 in 5000 solution of cinchocaine hydrochloride in 0.5% saline is a hypobaric solution having a specific gravity of 1,0036 at 37°C. A 1 in 200 solution of cinchocaine hydrochloride in 6% dextrose gives a hyperbaric solution having specific gravity of 1.02.