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Impedance Phlebography: Critical Evaluation of its Relation to Inspiratory Capacity

Impedance phlebography, a noninvasive approach to the diagnosis of venous thrombosis, depends on decreasing calf electrical impedance in normal subjects during inspiration. In order to assess the relationship between impedance changes and inspiratory volume, normal subjects were studied at various depths of inspiration. The mean inspiration volume required for a result indicating no evidence of venous thrombosis was over 1500 ml. There was significant variation between subjects and between limbs in the same subject. Routine postoperative patients were not generally able to achieve these inspiration volumes. It is concluded that insufficient tidal volume is responsible for many false positive results. Impedance phlebography is not reliable without simultaneous spirometric evidence of high inspiratory volume.