Most non-ionic agents are placed into the LOCM category while all ionic agents are in the HOCM category. An injection of contrast media, especially ionic HOCM, results in a big increase in the number of particles contained in the vascular system. This process is known as osmosis. The blood vessels dilate in an attempt to compensate from the increased fluid volume. Sometimes the fluid shift may be too dramatic for the vessels to handle therefore, fluid actually extravasates into the surrounding tissues.
Extravasation is the leaking of a fluid from the vessel into surrounding tissue.
Hypervolemia is the term that is used to describe an abnormal increase in the volume of circulating fluids or blood. The rapid fluid movement, especially water, throughout the vascular system is believed to contribute to pain associated with vessel dilation, flushing, damage to the vascular endothelium, red cell changes, nausea, vomiting, and dehydration. The osmotic effect can cause the arteries of the kidneys to expand. When the arteries expand vasoconstrictors are released to compensate for the artery expansion. The vasoconstrictors constrict the arteries resulting in a rapid opening and closing action of the arteries.