Symptoms of lung edema usually appear at a very late stage when the amount of fluid in the lungs has already sextupled.
At this stage, the patient typically has dyspnea (short breath) and hypoxia (low partial pressure of oxygen in the blood) because of impaired gas exchange due to the increased interstitial fluid volume. As a consequence, the patient is usually committed to an intensive care unit for medical treatment. Preclinical detection and continuous monitoring of the lung fluid volume during medical treatment would enhance medical care while reducing costs.
Unfortunately, conventional detection methods like radiographic imaging, monitoring pulmonary capillary wedge pressure or double indicator thermodilution are impractical for continuous monitoring. A promising alternative is the Bioimpedance Spectroscopy (BIS). Detection of lung edema using BIS is based on the fact that the amount of fluid in the lungs has significant impact on their electrical impedance. Normally, the lungs have about 5% of fluid and 95% of air, resulting in an electrical impedance in the range of about 10 to 20 Ohm. If the amount of fluid in the lungs increases, electrical impedance decreases because of the much lower electrical impedance of fluid (serum, for example, has a resistance of about 0.6 Ohm).
Compared to other measurement methods, BIS has two significant advantages: the measurement is non-invasive and it can easily be done at the patient’s home. In previous work, BIS was shown to be practical for detection of lung edema and, in a single-frequency version, it is used in some ICDs (implantable cardioverter-defibrillator).