Estimation of Stroke Volume and Stroke Volume Changes by Electrical Impedance Tomography

BACKGROUND: Electrical impedance tomography (EIT) is a noninvasive imaging method that identifies changes in air and blood volume based on thoracic impedance changes. Recently, there has been growing interest in EIT to measure stroke volume (SV). The objectives of this study are as follows: (1) to evaluate the ability of systolic impedance variations (&Dgr;Zsys) to track changes in SV in relation to a baseline condition; (2) to assess the relationship of &Dgr;Zsys and SV in experimental subjects; and (3) to identify the influence of body dimensions on the relationship between &Dgr;Zsys and SV. METHODS: Twelve Agroceres pigs were instrumented with transpulmonary thermodilution catheter and EIT and were mechanically ventilated in a random order using different settings of tidal volume (VT) and positive end-expiratory pressure (PEEP): VT 10 mL·kg−1 and PEEP 10 cm H2O, VT 10 mL·kg−1 and PEEP 5 cm H2O, VT 6 mL·kg−1 and PEEP 10 cm H2O, and VT 6 mL·kg−1 and PEEP 5 cm H2O. After baseline data collection, subjects were submitted to hemorrhagic shock and successive fluid challenges. RESULTS: A total of 204 paired measurements of SV and &Dgr;Zsys were obtained. The 4-quadrant plot showed acceptable trending ability with a concordance rate of 91.2%. Changes in &Dgr;Zsys after fluid challenges presented an area under the curve of 0.83 (95% confidence interval, 0.74–0.92) to evaluate SV changes. Conversely, the linear association between &Dgr;Zsys and SV was poor, with R2 from linear mixed model of 0.35. Adding information on body dimensions improved the linear association between &Dgr;Zsys and SV up to R2 from linear mixed model of 0.85. CONCLUSIONS: EIT showed good trending ability and is a promising hemodynamic monitoring tool. Measurements of absolute SV require that body dimensions be taken into account.