Bioreactance: A New Method for Non-invasive Cardiac Output Monitoring
Although measurement of oxygen consumption in the clinical setting is hampered by numerous technical difficulties, the objective of hemodynamic resuscitation is to ensure that oxygen consumption balances the metabolic needs of the body [1 ]–[3 ]. When the
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Bioreactance: A New Method for Non-invasive Cardiac Output Monitoring P. Squara
Introduction Although measurement of oxygen consumption in the clinical setting is hampered by numerous technical difficulties, the objective of hemodynamic resuscitation is to ensure that oxygen consumption balances the metabolic needs of the body [1 – 3]. When the hemoglobin level is stable, oxygen consumption is determined by three interrelated variables: Cardiac output, arterial oxygen saturation, and mixed venous oxygen saturation (SvO2). This explains the considerable efforts that have been devoted to develop tools to measure and monitor each of these three variables. Such tools are important for diagnosis, optimization of treatment, and tracking progress of patients with hemodynamic compromise [4]. The gold standard for cardiac output measurement in clinical practice is the thermodilution technique which requires invasive placement of a right heart, pulmonary artery catheter (PAC) [5 – 7]. Recognition of the risks associated with PAC insertion and with the long catheter dwell times typically encountered in the intensive care setting have contributed to the more restricted use of invasive monitoring over recent years [8 – 10]. Therefore, cardiac output is only measured in a small proportion of patients in whom it could be helpful. Consequently, clinicians have long sought accurate non-invasive methods for measuring cardiac output. Several less invasive monitoring methods have been proposed recently, including arterial pulse contour analysis via an arterial catheter [11 – 14]; an intra-tracheal tube for lung capillary blood flow derivation using partial CO2 re-breathing [15], and an intra-esophageal probe for continuous Doppler velocity flow assessment [16]. Thoracic bioimpedance is currently the most widely used truly non-invasive method for cardiac output monitoring [17, 18]. Although early studies in controlled settings yielded interesting results, and despite the development of progressively more complex algorithms [19, 20], recent studies have yielded mixed results concerning the accuracy of this approach. This is especially true in more complex cardiac patients, as typically found in the intensive care and post-operative settings [21, 22]. The variability and lack of consistent accuracy identified in these studies may be in part due to the inherently low signal-to-noise ratio of this approach. Consequently, although available for use, this technique is not widely used in the intensive care unit (ICU). Standard bioimpedance systems apply a high frequency electrical current of known amplitude and frequency across the thorax and measure changes in voltage. The ratio between voltage and current amplitudes is a measure of transthoracic direct current resistance (more generically referred to as impedance, Zo) and this varies in proportion to the amount of fluid in the thorax. In addition to changing
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the amplitude of an electrical signal passing through the thorax, changes in thoracic blood volume also produce
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