Evaluation of Transthoracic Electrical Impedance in the Diagnosis of Pulmonary Edema

To evaluate the clinical usefulness of measuring transthoracic electrical impedance in patients with pulmonary edema, we studied 27 normal subjects and 33 patients. In normal subjects, impedance increased when body position changed from supine to standing (P < 0.01) and when lung volume increased from residual volume to total lung capacity (P < 0.01). Impedance values varied (±2.7 Ω) when measured on 4 separate days but were relatively constant (± 0.7 Ω) when measured hourly without removing the electrodes. In only five of 11 patients with clinically severe pulmonary edema and five of eight with radiologically severe pulmonary edema were impedance values outside the normal range. Changes in impedance values correlated with changes in clinical and radiographic indexes of pulmonary edema. We conclude that (1) single measurements of impedance are useless in diagnosing pulmonary edema, and (2) impedance changes as predicted as pulmonary edema increases and decreases, and these changes are readily detected by simpler methods of evaluation.