Contrast dye is injected into a vein in the foot, allowing visualization of the vascular tree as the dye follows the circulatory pathway. A filling defect in a vein lumen indicates the presence of a thrombus. The dye may be contraindicated in patients with renal problems. The examiner places electrodes on the calf and inflates a pneumatic cuff around the middle of the patient's thigh. The electrodes measure electrical impedance as he deflates the cuff.
Impedance is resistance to the transmission of electrical energy.
A rapid decrease in impedance after the cuff is deflated indicates normal venous flow. A slow decrease in impedance after the cuff is deflated indicates the diminished blood flow associated with a thrombus. The IPG, which usually requires serial testing, isn't used as much as it once was.