Venous Circulation: A Few Challenging Concepts in Goal-Directed Hemodynamic Therapy (GDHT)

A decrease in circulating blood volume means a decrease in stressed volume (Vs). Any vasodilation (veno-dilation), iatrogenic (i.e. spinal or epidural anesthesia) or resulting from pathologic processes (e.g., sepsis), might lead to an increase in unstressed volume (Vu) and a decrease in Vs. There are two ways to increase Vs. One is to infuse fluid. If the infused fluid does not exert transmural pressure (Ptm) above 0, it is still Vu. This might be the reason for the lack of fluid responsiveness. When/if infused fluid increases Ptm above zero, this would increase Ptm, increasing BP and/or CO. The second way to increase Vs is to administer a small dose of a vasopressor to constrict veins, decrease venous capacity, squeeze blood out of compliant veins, and increase Vs. Combining an alpha-1 adrenergic agonist with a beta-2 adrenergic agonist might be useful, because that would speed up the conversion of Vu to Vs (mainly within the splanchnic system). Therefore, if small doses increase BP and/or CO, it is reasonable to assume that this was the result of conversion of part of the Vu into Vs. This might have the benefit of preventing excessive fluid load.