Influence of 7% NaCl on the mechanical properties of the systemic circulation in the hypovolemic dog.

We examined the effects of hypertonic saline (7%) administration during hypovolemia in the anesthetized dog on the mechanical properties of the systemic circulation that constitute the major determinants of venous return. By using a right-heart bypass preparation in which venous return from the splanchnic and non-splanchnic vascular beds was isolated and drained into a common reservoir, venous resistance, venous compliance, and blood flow distribution measurements were made during control conditions and during a period of lowered systemic blood flow (30 min at a mean arterial pressure of 50 mm Hg). These measurements were repeated following hypertonic saline administration at the reduced and control levels of systemic blood flow. Hypertonic saline administration (8 ml/kg) produced an average increase in reservoir volume of 23 ml/kg and osmolality of 25.5 mOsm/kg. Changes in venous compliance, venous resistance, and blood flow distribution in response to hypertonic saline accounted for no more than 5% of the reservoir volume increase. Furthermore, plasma volume expansion estimated from hematocrit dilution suggests that hypertonic saline does not alter unstressed vascular volume. The most likely mechanism by which hypertonic saline enhances venous return is by plasma volume expansion and not by alterations of the mechanical properties of the systemic circulation.