Static measures of intravascular volume

To reiterate, as opposed to its more conventional meaning, a static measure of volume assessment does not mean that the signal itself does not vary with time, patient position, or stimulation. Rather, static measures are those whose values when measured are intended to provide information relevant for decision making. By way of a clarifying analogy, consider measuring the fullness of a car tire with via the pressure of air within it. Over the course of a day the overall pressure within the tire will vary as temperatures rise and fall. Even as one drives, the localized pressure distributions will change as one region of the tired experiences loading by making contact with the road. Even with all this variation, a single static measurement of pressure can be useful to tell us whether or not we should add more air to our tires. So it goes with static measurements of volume status.

Static measures can be subdivided into either those utilizing pressures or those utilizing volumes. The principle pressure measurements that have been used for volume assessment include central venous pressure and pulmonary artery occlusion pressure. The volume measurements include right ventricular end-diastolic volume, left-ventricular end-diastolic area, inferior vena cava diameter, and bioimpedance vector analysis. Though not each of these reports a volume (one reports an area, another a diameter, the third a vector of resistance), they each represent an underlying volume of interest and are best thought of as doing so. Though there is a relationship between pressures and volumes, in the complex environments of the body (especially the broken body), they are not directly related nor can they necessarily be converted from one to the other. Hence, for examination of them we will consider their measurements separately here.