Accurate monitoring of blood loss: thoracic electrical impedance during hemorrhage in the pig

Background: Cardiovascular variables are closely regulated in that they remain relatively stable during minor hemorrhage, We considered that such stability would make these variables less accurate for monitoring a blood loss. In contrast, thoracic electrical impedance would be unlikely to be a regulated variable and could serve as a non-invasive monitor of a volume deficit. Methods: In 10 pigs bled (0-24 ml kg(-1)) and retransfused (to 28 ml kg(-1)) during halothane anesthesia, the magnitude of the electrical impedance, cardiovascular, blood gas and temperature variables, atrial natriuretic peptide and near infrared spectroscopy of the leg muscles were recorded. Results: During hemorrhage and retransfusion, the median correlations between changes in the magnitude of the thoracic impedance and the external blood loss ranged from 0.97 to 0.98 with an individual range from 0.80 to 1.0. These correlation coefficients were higher and their ranges were lower than correlations established for any other measured parameter. Conclusion: During hemorrhage and retransfusion in the halothane anesthetized pig, a change in the magnitude of thoracic electrical impedance appears to be an accurate and also noninvasive monitor of a blood volume deficit. (C) Acta Anaesthesiologica Scandinavica 44 (2000).