Haemodynamic and rheological effects of contrast media: the role of viscosity and osmolality.

All currently available iodinated contrast media (CM) produce some form of haemodynamic or rheological change when injected. These effects are thought to be due primarily to the osmolality and viscosity of the CM. The new hexa-iodinated contrast agent Visipaque has the advantage of being iso-osmotic with blood at all clinically relevant concentrations, but is more viscous than the monomers at equivalent iodine concentrations. The purpose of this review is to discuss the relative importance of osmolality and viscosity in the clinical situation.