TRAUMATIC SHOCK. XII. HEMODYNAMIC EFFECTS OF ALTERATIONS OF BLOOD VISCOSITY IN NORMAL DOGS AND IN DOGS IN SHOCK 1

Abstract

The hemodynamic effects of alteration in blood viscosity were studied in normal dogs and in dogs in shock. The viscosity was altered by exchanging packed red cells for whole blood or plasma for whole blood. Normal dogs compensate for increased viscosity by lowering peripheral resistance, i.e., by vasodilatation. Dogs in hemor-rhagic shock cannot compensate for increased viscosity by lowering peripheral resistance because blood pressure must be maintained. Cardiac output falls. The % drop in cardiac output with increased viscosity is much higher at reduced blood volume than at normal blood volume. If the viscosity is abnormally high in hemorrhagic shock, cardiac output falls to critically low levels at blood pressures higher than those observed when blood viscosity is not altered. The higher level of blood pressure in these circumstances produces a deceptively favorable impression of the state of the circulation. Irreversibility to transfusion sets in at relatively high blood pressures (80 mm. Hg or higher), as is the case in tourniquet shock. Restoring normal viscosity improves cardiac output. Lowering viscosity to below normal improves cardiac output still further. In tourniquet shock, the sharp drop in cardiac output observed shortly after removal of tourniquets and while the blood pressure is still above ordinary shock levels is due to the increase in blood viscosity as well as the fall in blood vol. This explains the onset of irreversibility to transfusion in tourniquet shock at higher levels of blood pressure than is the case for hemorrhagic shock. Restoration of blood vol. alone, however, does more to restore cardiac output than does restoration of normal viscosity. A normal cardiac output can be restored in tourniquet shock by merely lowering the hematocrit to very low levels. But recovery will not occur unless normal blood volume is restored. Therefore, if an abnormally high viscosity exists in the presence of a lowered blood vol., the need for restoration of blood vol. becomes all the more urgent.