[Whole body and thoracic impedance changes induced by cardiopulmonary bypass]

Impedance is defined as resistance to alternating electrical current. It is inversely proportional to the volume of tissues traversed by the current. Cardiopulmonary bypass, as used in cardiac surgery, results in extravascular fluid overload manifested by a decrease in whole body and thoracic impedance. This non-invasive method has been used in 18 adult patients before, and in the days immediately following cardiopulmonary bypass for non-mitral cardiac surgery. Right and left thoracic impedances were reduced by 32 and 37 percent respectively during the first postoperative hour and returned to preoperative values after the second postoperative day. Whole body impedance was reduced by 14 percent in the first post-operative day, and this was accompanied by 17 percent and 16 percent decreases in the PaO2/PAO2 and PaO2/FIO2 ratios respectively. There was no correlation between impedance and these ratios which reflect pulmonary gases exchanges. Impedance appears to be a simple method to evaluate whole body and thoracic water contents, thereby providing guidance in post-cardiac surgery management.