Definition of Parameters and Maneuvers Used in Generic Plethysmography for Venous Insufficiency

Venous insufficiency causes the three venous systems in the lower extremity to misdirect venous blood volume. Therefore, the goal of this testing is to characterize misdirection of venous blood volume, if present.

Our test subject is placed in the supine position. This will lower venous pressure in the lower extremities to a value only slightly above right atrial pressure (~0 mmHg). Using a plethysmograph an operator can obtain a baseline volume in the segment of interest. When the subject is placed in the erect position, lower extremity venous pressure increases due to the hydrostatic column of blood extending from the right atrium to the segment of interest. Since veins are compliant (i.e., increase volume with increased internal pressure), vein blood volume in the segment of interest increases. This volume increase is displayed on a graph from which measurements may be taken. The Y-axis is volume, and the X-axis is time. Since all measurements are either times or ratios, the Y-axis is not required to be strictly calibrated as volume. However, its display on the graph must correlate with volume change (see Figure 5.2).

The measurement between the supine baseline volume and the erect volume plateau is known as the Venous Volume (VV). From this same curve the operator can determine the Venous Refilling Time (VRT). This is the time measured from when the baseline volume begins to increase to its plateau. With the subject in the erect position, the operator instructs the subject to perform a single brisk ankle flexion. This will produce a momentary reduction in Y-axis volume. This change in volume is called the Ejection Volume (EV). To calculate the Ejection Fraction (EF), the operator divides EV by VV. The subject is then instructed to perform 10 brisk ankle flexions. This will produce a reduction in Y-axis volume, which will be larger than the volume reduction experienced with one flexion. This allows the operator to measure Residual Volume (RV). This is defined as the difference between the volume after 10 flexions and the baseline volume. Finally, the operator can calculate the Residual Volume Fraction (RVF) by dividing RV by VV.15-17

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