Abstract
Currently commercial devices exist that monitor pulmonary edema by measuring the electrical impedance between a lead in the right ventricle (RV) and the device can. Studies have shown up to a 40% false alarm rate with these devices. We have published, using a computer model, about a three fold improvement in lung sensitivity to edema by measuring the impedance between a left ventricle lead in the coronary vein and can, which has been confirmed by others with animal experiments. New research has shown a further improvement with a new lead configuration requiring only right side heart measurements.
An electrical impedance based finite difference model of the thorax with 3.8 million elements was constructed based on 43 MRI slices to study the impedance response to simulated lung edema. For the impedance measurements the excitation current was between the RV apex and the device can. The voltage sensing electrodes were in the superior vena cava (SVC) with spacing of 1.5, 5 and 7 cm. The lung resistivity was decreased from the normal value of 1400 ohm-cm to 200 ohm-cm in 200 ohm-cm steps reflecting increasing lung edema. The ventricles were increased in volume approximately 50% to simulate an enlarged heart. The decrease in impedance for lungs from 1400 to 400 ohm-cm, showed -39.9%, -37.4%, and -36% change for SVC spacings of 1.5, 5 and 7 cm respectively. In comparison, the decrease with the same condition for the RV to can was 8.9%. The simulated enlarged heart alone resulted in a significantly smaller impedance change for the SVC lead compared to the RV to can lead. The improved lead configuration shows an approximate fourfold improvement compared to the RV to can lead configuration and is less influenced by heart enlargement.