An extracorporeal reservoir device for continuous monitoring of tissue volume change

The conventional use of a gravimetric or plethysmographic method for continuous monitoring of tissue volume changes has some inherent problems related to organ exteriorization, denervation, and tissue handling. A new method using an extracorporeal reservoir is devised for indirect measurement of tissue volume changes. When the organ is perfused with constant flow and the venous outflow is collected in a reservoir, the reservoir volume reaches a steady level by pumping out equal amount of blood entering the reservoir. Under this condition, every perturbation causing a change in tissue volume will result in a reservoir volume change of the opposite direction. Two different pressure-regulating systems are also designed for the control of arterial perfusion pressure. Because volume change is not allowed to occur inside the regulating system, the device still guarantees that the reverse change in reservoir volume reflects the tissue volume change on alteration of arterial perfusion from constant-flow to constant-pressure condition. The method has been tested in six canine ileum segments by comparing the reservoir volume with the tissue volume recorded with direct gravimetric measurement. Experimental trials include venous pressure elevation, arterial perfusion changes, and intra-arterial infusion of vasoactive agents. The tests indicate that the tissue volume changes can be faithfully measured from the reverse change in reservoir volume. The technical advantages and possible applications of the reservoir method in the realm of hemodynamic and microcirculatory studies are also discussed.