Decreases in electric thoracic impedance during transurethral resection of the prostate: an index of early water intoxication.

Thoracic impedance of 18 patients undergoing transurethral resection of the prostate was measured 15 minutes preoperatively, and 30 and 60 minutes after the start of the operation. Impedance variations were compared to variation in 1) cardiac output, 2) serum sodium osmolality and 3) alveolar-arterial oxygen tension difference. Thoracic impedance, initially 24.6 plus or minus 0.3, decreased to 23.8 plus or minus 0.5, 30 minutes after the onset of the operation (p less than 0.0005) and 22.9 plus or minus 0.5, 30 minutes later (p less than 0.01 from previous reading and 0.0005 from control). Patients in whom impedance had decreased 10 per cent or more from control values received 10 mg. furosemide intravenously 60 minutes after the onset of the operation. This therapy permitted the restoration of impedance values, cardiac output, alveolar-arterial oxygen tension difference and serum sodium osmolality to values statistically similar to those found in the 10 patients who had not sustained such precipitous decreases in impedance within 60 minutes. We believe that the measurement of thoracic impedance during transurethral prostatic surgery offers the most sensitive index of early water intoxication. Measurements can be obtained without delay in the operating room and, thus, permit immediate correction of the condition.