Thoracic impedance as an index of body fluid balance during cardiac surgery

Thoracic impedance at 2.5 (TI2.5) and 100 kHz (TI100), central venous pressure (CVP), and body fluid balance were recorded together with rectal temperature and arterial haematocrit in 15 consecutive patients subjected to coronary artery bypass grafting. I.v. fluid and blood were administered in an excess of 3.18 (1.38-9.35) 1 during the operation. TI2.5 decreased from 51.7 (39.2-66.4) to 34.9 (21.1-45.7) ohm (P = 0.001), while TI100 decreased from 41.9 (31.4-55.0) to 30.3 (18.3-40.8) ohm (P = 0.002). CVP, 6 (3-11) mmHg [0.8 (0.4-1.5) kPa], was the same before and after surgery. Temperature decreased during cardiopulmonary bypass from 35.4 (34.1-36.6) to 26.7 (22.9-31.0) degrees C and haematocrit from 39 (34-46)% to a lowest value of 27 (23-32)% (P = 0.0001). A close linear correlation between TI and body fluid balance was observed (TI2.5: r = -0.96, TI100: r = -0.95, P = 0.0001). Corrections of TI for temperature and/or haematocrit improved the correlation between TI and fluid balance to 0.99 (TI2.5) and 0.98 (TI100). The data indicate that changes in thoracic impedance can be used to monitor body fluid balance during cardiac surgery.