[Contrast medium-induced expansion of circulating blood volume: Comparison between nonionic and ionic contrast media]

We assessed whether the osmotic expansion of circulating blood volume (CBV) induced by nonionic contrast medium (NCM) is less than that induced by ionic contrast medium (ICM). Iohexol (Io) (NCM: 795 mO sm/kg H2O), 1.28 g iodine/kg, was injected intravenously into 5 mongrel dogs and blood samples were drawn at certain times. One week later, meglumine iothalamate (MI) (ICM: 1470 mOsm/kgH2O), 1.28 g iodine/kg, was injected into the same dogs. Another 5 dogs received MI first and Io one week later. Colloid oncotic pressure (COP) of the blood samples was measured by a needle osmometer, and changes in CBV were calculated from the COP values. The injection of Io or MI resulted in an immediate decrease in COP, and an increase in CBV at 1 min. MI induced significantly more severe and longlasting changes in COP and CBV than Io. Neither MI nor Io modified COP when they were added to the control blood samples. Thus, although NCM considerably expanded CBV, the magnitude of expansion induced by NCM was less than that induced by ICM. This may explain one of the reasons why NCM causes fewer adverse reactions than ICM.