Reliability and precision of a new thoracic electrical bioimpedance monitor in a lower body negative pressure model.

OBJECTIVE

To evaluate the reliability and precision of measurement in a new thoracic electrical bioimpedance (TEB) monitor.

DESIGN

Prospective clinical trial using healthy volunteers.

SETTING

Military tertiary care teaching hospital.

SUBJECTS

Seventy-five healthy adult volunteers taking no medications.

INTERVENTIONS

Induction of severe preload reduction using a standardized lower-body negative pressure protocol. Measurement of hemodynamic variables using a TEB monitor before, during, and immediately after application of negative pressure.

MEASUREMENTS AND RESULTS

Seventy-five subjects were enrolled and completed the study. Pulse, blood pressure, stroke index, cardiac index, systolic time ratios (STR), and index of contractility were obtained on all subjects undergoing monitoring with the lower body negative pressure (LBNP) device. Hemodynamic measurements were recorded at 15-sec intervals during incremental application of 0, -10, -20, -40, and -60 mm Hg pressure for 10 mins at each pressure. Maximal tolerated LBNP produced reductions in cardiac, stroke, and contractility indices of 50%, 65%, and 45%, respectively. Pulse and STRs increased 44% and 113%, respectively. The precision of measurement (mean +/- 2 SD) for TEB-derived cardiac and stroke index was 16% and 10%, respectively. Repeatability of measurement was assessed by measuring hemodynamic changes after the abrupt cessation of maximal LBNP. There were significant increases in stroke index (p < .001) and decreases in STRs (p < .001) and pulse (p < .001) 3 mins after LBNP. There was no significant difference between initial and post-LBNP cardiac index (p > .05). Regression equations were applied to scattergram plots of stroke index vs. STRs and index of contractility vs. body mass. The use of these plots allowed elimination of values that appeared to be spurious (stroke index vs. STRs) and also raised the question whether the Sramek-Bemstein equation (stroke volume = left ventricular ejection time x volume of electrically participating tissue x dZ/dt/Zo) fully explained all the factors affecting the TEB waveform.

CONCLUSIONS

This new monitor appears to overcome many of the signal processing problems encountered with previous devices. The results clearly demonstrate that accurate and reliable measurement of bioimpedance waveforms is possible and suggest that the monitor is capable of generating precise hemodynamic data across a wide spectrum of hemodynamic alterations. However, the evidence also indicates that new algorithms may be needed to more fully explain the multiple factors affecting this waveform.