Monday, December 23, 2013
NICaS (noninvasive cardiac system)
Physiol. Meas. 27 (2006) 817–827 doi:10.1088/0967-3334/27/9/005 Impedance cardiography revisited G Cotter1, A Schachner2, L Sasson2, H Dekel2 and Y Moshkovitz3 1 Divisions of Clinical Pharmacology and Cardiology, Duke University Medical Center, Durham, NC, USA 2 Angela & Sami Sharnoon Cardiothoracic Surgery Department, Wolfson Medical Center, Israel 3 Department of Cardiac Surgery, Assuta Hospital, Petah Tikva, Israel E-mail: email@example.com Received 7 February 2006, accepted for publication 9 June 2006 Published 5 July 2006 Online at stacks.iop.org/PM/27/817 Abstract Previously reported comparisons between cardiac output (CO) results in patients with cardiac conditions measured by thoracic impedance cardiography (TIC) versus thermodilution (TD) reveal upper and lower limits of agreement with two standard deviations (2SD) of approximately ±2.2 l min−1, a 44% disparity between the two technologies. We show here that if the electrodes are placed on one wrist and on a contralateral ankle instead of on the chest, a configuration designated as regional impedance cardiography (RIC), the 2SD limit of agreement between RIC and TD is ±1.0 l min−1, approximately 20% disparity between the two methods. To compare the performances of the TIC and RIC algorithms, the raw data of peripheral impedance changes yielded by RIC in 43 cardiac patients were used here for software processing and calculating the CO with the TIC algorithm. The 2SD between the TIC and TD was ±1.7 l min−1, and after annexing the correcting factors of the RIC formula to the TIC formula, the disparity between TIC and TD further declined to ±1.25 l min−1. Conclusions: (1) in cardiac conditions, the RIC technology is twice as accurate as TIC; (2) the advantage of RIC is the use of peripheral rather than thoracic impedance signals, supported by correcting factors. Keywords: cardiac output measurements, thoracic bioimpedance, whole-body bioimpedance, impedance cardiography Introduction Three basic technologies are currently in use for impedance cardiography (ICG): (1) the thoracic ICG (TIC), where the electrodes are placed on the root of the neck and the lower part of the chest, being the dominant method in the market (Patterson et al 1964, Kubicek 0967-3334/06/090817+11$30.00 © 2006 IOP Publishing Ltd Printed in the UK 817 818 G Cotter et al et al 1966, 1974); (2) the whole-body ICG (ICGWB), where four pairs of electrodes are used, one pair on each limb (Tischenko 1973, Koobi et al 1999); (3) the regional ICG (RIC), a technology which is used by the NICaS (noninvasive cardiac system). In this technology, which is the subject of this report, only two pairs of electrodes are used, performing best when placed on one wrist and on the contralateral ankle (Cohen et al 1998, Cotter et al 2004, Torre-Amione et al 2004). Two comprehensive reviews of the literature on clinical experience in measuring the cardiac output (CO) by TIC determined that in patients with cardiac conditions the TIC-CO results are unreliable (Raaijmakers et al 1999, Handelsman 1991). According to Patterson (1985) andWang et al (2001), a number of sources in the chest, such as the lungs, vena cava, and systemic and pulmonary arterial vasculatures, generate systolic impedance reductions, while the heart generates signals of increased impedance. In addition to thesemultifarious sources of Z,4 variations in the electrical conductivities between the sources of impedance changes and the TIC electrodes (Kim et al 1988, Kauppinen et al 1998), and between the various impedance sources (Wtorek 2000) have been described. These model experimentations indicated that the thoracic Z is not a reliable signal for calculation of the SV due to the multiple sources of dZ/dt (Kim et al 1988, Wang and Patterson 1995, Kauppinen et al 1998, Wtorek 2000), providing the explanations for the above-mentioned unsatisfactory clinical results obtained by TIC (Raaijmakers et al 1999, Handelsman 1991). In this report, an attempt is made to define the differences between the peripheral and thoracic impedance signals, and based on this, to explain the differences in the performance of RIC and TIC. As we are capable of saving raw data from the wrist–ankle (peripheral) impedance signals, we were able to use the peripheral impedance waveforms and base impedance values to calculate stroke volumes, using various algorithms that have been associated with TIC calculations. This enabled us to prove that (1) the performance of RIC is twice as accurate as reported TIC results; (2) the reasons for this are as follows: (a) the impedance changes which are yielded by the limb electrodes are more suitable than the impedance changes of the thoracic electrodes for calculating the stroke volume and (b) the use of properly designed coefficients improved the accuracy of the CO results by at least an additional 25%. Methods The data for this project were gathered from two patient series. In both, comparisons were made between cardiac output results measured by the NICaS versus thermodilution. One series, which was studied in hospital A, consisted of 30 patients who were studied immediately upon arrival at the ICU following an open heart operation. In 11 (36%), despite the intravenous administration of adrenalin, cardiac index (CI) was lower than 2.5 l min−1 m−2. The second series included 13 cases of acute heart failure that were studied in hospital B. CI was lower than 2.5 l min−1 m−2 in seven (54%), and in the combined group of 43 cases of the two hospitals, it was lower than 2.5 l min−1 m−2 in 18 (43%). The purpose of this study was to use peripheral impedance waveforms to calculate stroke volume by means of four different ICG algorithms and to compare each of these SV values with the thermodilution SV result. Of the 55 and 31 studies conducted in hospitals A and B, respectively, raw data were successfully retrieved from only the last 30 consecutive patients of hospital A and the last 13 4 In the ICGWB and RIC, where the impedance changes are depicted in the periphery, the impedance value is automatically converted into the real parts (R0 and R) of the measured impedance signals (Lamberts et al 1984).