Impedance phlebography is a noninvasive method for monitoring the changes in venous blood volume which normally occur with deep breathing. Since these respiratory venous volume changes are diminished by venous thrombosis, impedance phlebography provides an indirect method for the diagnosis of this condition. Over 3,000 impedance examinations have now been carried out on 710 subjects in many different clinical settings. As later confirmed by venography, impedance phlebograms have been used to detect venous thrombosis in asymptomatic patients, and to rule out this diagnosis in patients whose signs and symptoms suggested venous thrombosis. The incidence of abnormal impedance phlebograms has conformed to predictions in various patient groups, ranging from 0% in normal volunteers (0 of 50 limbs) to 91% in patients with pulmonary emboli documented by lung scan and pulmonary angiogram (20 of 22 patients).
H. Brownell Wheeler, Daniel Pearson, Daniel O'Connell, Subhas C. Mullick. Impedance PhlebographyTechnique, Interpretation, and Results. Arch Surg. 1972;104(2):164–169. doi:10.1001/archsurg.1972.04180020044008