Transthoracic electrical impedance at 1 and 100 kHz--a means for separating thoracic fluid compartments?

The electrical impedance of biological tissues varies with their water and electrolyte contents. Alternating current above 5-10 kHz passes both intra- and extracellular fluid, and lower frequency current preferentially extracellular fluid. In an attempt to evaluate thoracic fluid in different compartments, transthoracic electrical impedance (TEI) was measured at 1 and 100 kHz in 15 consecutive patients, without overt left heart failure and under haemodynamic surveillance, during the first two days of myocardial infarction. To achieve different states of hydration the patients were given i.v. furosemide, either 40 mg t.i.d. (high dose group--HDG) or 20 mg once daily (low dose group--LDG). Effects of altered body position and the respective furosemide injections, were evaluated on both days. Mean 24-h-diuresis was 3.9 l in the HDG and 2.5 l in the LDG. After 24 h mean pulmonary arterial diastolic pressure had decreased by 30% in the HDG but remained almost unchanged in the LDG. By then mean basal TEI was increased only in the HDG, by 17% at 1 kHz and 13% at 100 kHz. On Day 1, within one hour after furosemide, TEI increased temporarily by 3-5%, at both frequencies and in both groups. On Day 2, this short-term increase was similar in the LDG at both frequencies and in the HDG at 100 kHz, but not at 1 kHz, consistent with a major extracellular fluid loss in the HDG. TEI at 1 and 100 kHz may thus reflect extracellular and total thoracic fluid, respectively.