Hemodilution, Tonicity, and Blood Viscosity.

Since the introduction of intravenous therapy, fluids containing glucose or saline in relatively isotonic concentrations have far exceeded others in use. Within the last decade new techniques in many areas of diagnosis and treatment have required that fluids of various tonicity and viscosity be introduced into the blood stream with such rapidity or in such great volume as to preclude adequate dilution with the circulating cells and plasma. Several in vivo studies (1-4) have demonstrated the effect of hypertonic solutions on blood flow. Some authors (5-7) have noted circulatory changes, especially in the pulmonary vascular bed, secondary to the injection of angiographic contrast media, which are both hypertonic and highly viscous. Others (8-11) have commented on the benefits of gross dilution of perfused blood during extracorporeal support for open-heart surgery, and some data have been presented concerning the flow properties of blood diluted with solutions of dextrose (12) or dextran (13, 14). At any given shear rate or temperature it is the concentration of red cells which primarily affects the viscosity of whole blood. Viscosity rises with an increasing hematocrit. While investigating the effects of various intravenous solutions on the hematocrit and viscosity of whole blood in vitro, we have noted that under certain conditions a drop in hematocrit has been accompanied by a rise in viscosity, whereas under other circumstances an increase in hematocrit has been accompanied by a fall in viscosity. The significance of these findings in respect to the flow of blood exposed to diluents of varying osmolality and viscosity appears to be of sufficient importance to warrant reporting at this time.