The morbidity and mortality associated with the use of PACs is well recognized and must be weighed against the potential benefits for the individual patient of gaining valuable information regarding the cardiovascular system and oxygen delivery . Bioimpedance cardiography offers an apparently attractive noninvasive way of estimating cardiac output and obtaining derived haemodynamic parameters. However, this is of no benefit if the information acquired is unreliable, leading to inappropriate management. We investigated the use of bioimpedance in the critically ill to assess whether it can be a reliable method of cardiac output measurement for this group of patients.
The thermodilution method is an indirect measure of cardiac output, but has been shown to correlate well with the gold standard dye dilution method. Moreover, it is the method most commonly used to measure cardiac output in patients in the ICU and upon which much of our understanding of the cardiovascular changes in critical illnesses is based.
Bioimpedance cardiography has been validated in some patient groups , and newer systems with improved software for advanced signal processing may be valid in critically ill patients . However, there have been concerns raised as to its accuracy and reliability in ICU patients [9,10]. Correct placement of the eight electrodes is important in obtaining accurate information; in ICU patients this may be hampered by dressing covering internal jugular line sites, thoracotomy wounds and chest drain sites. In this study, the directions for electrode placement detailed on the NCCOM 3 monitor were followed as a closely as practically possible, aiming to reproduce the conditions that would pertain to routine clinical use of the monitor.
The use of positive pressure ventilation with PEEP, and the presence of endotracheal tubes, chest drains and sternal wires may affect bioimpedance measurements by affecting the rate of change of thoracic impedance . However, cardiological studies in patients with pacemakers have shown bioimpedance to be a useful technique . The presence of the thermal filament in the modified PAC used by the Vigilance monitor may also affect bioimpedance measurements; standard PACs may also affect measurements by the presence of the thermistor wire. In any case, if the bioimpedance data are affected by foreign material in the thorax, this makes the use of the Bomed in ICU patients very problematic.
The design of this study is novel in two ways, giving major advantages over previous studies. Firstly, by comparing two 'continuous' methods of cardiac output measurement, the inevitable errors of synchronization using intermittent methods are virtually eliminated. (Previous studies comparing bioimpedance with thermodilution have needed to average several bolus measurements before or after the acquisition of bioimpedance data.) Secondly, we were able to collect a very large number of simultaneous paired data points from the two methods, averaged over the whole respiratory cycle, enabling a more accurate comparison.
This study shows there is essentially no relationship between cardiac index as measured by the Bomed NCCOM 3 and the coninuous thermodilution method (r
2 = 0.01); this is surprising as the two methods claim to measure the same variable. There is extremely poor agreement between the methods according to the Bland–Altman method. The lack of precision is quite unacceptable. From our data, a cardiac index of 4.0 l/min/m2 would be subject to an error of up to +54% or -62% at the 95% limits of agreement. Importantly, we were able to assess the changes in measured cardiac output in individual patients and these also showed poor agreement with variable precision and bias. This would indicate that the use of the NCCOM 3 is unlikely to be of value even if a subgroup of patients, in whom the precision is acceptable, could be identified.
The two methods cannot therefore be used interchangeably to monitor cardiac output. Furthermore, therapeutic interventions to improve cardiovascular function that have been shown to be beneficial in patients monitored by the thermodilution technique cannot be similarly applied to patients monitored by bioimpedance cardiography. Bioimpedance cardiography cannot be recommended for use in critically in patients such as this group from a general ICU.