Correlation of cardiac output measured by non-invasive continuous cardiac output monitoring (NICOM) and thermodilution in patients undergoing off-pump coronary artery bypass surgery

Our study shows that NICOM is convenient and safe in a clinical setting, as it only requires the placement of two double electrodes on the patient’s thorax. Correlation coefficient of PCO and NCO (r = 0.77) is better than those of PCO with BCO (r = 0.63). Although NICOM up to now cannot replace the use of PAC, it provides consistent continuous non-invasive measurements of CO and CI and seems to be able to track in real time the trends and the change with the time. There is literature that suggests that real-time tracking of the trends of change in CO could be more important than the ability of the monitor to deliver a highly accurate single measurement under stable conditions [10]. In the current study, the trends of change in CO during the surgery are similar among the three devices, and the trends by NICOM are closer to PAC than by BioZ.com (Fig. 2). From Fig. 3a we can deduce that for values of CO between 4 and 6 NICOM and PAC values have a good correlation. For values of CO below 4, NICOM overestimates values of CO as compared to PAC. For values of cardiac output above 6, NICOM underestimates values of CO as compared to PAC. Similarly, from Fig. 3b we can deduce that for values of CO between 4 and 6, BioZ.com and PAC values have a good correlation. For values of CO below 4, BioZ.com overestimates values of CO as compared to PAC. For values of CO above 6, BioZ.com underestimates values of CO as compared to PAC.

As an invasive hemodynamic monitor, PAC was regarded as a gold standard of reference because we have no better reference for continuous CO monitoring [11‐14]. We therefore compared the relationship between non-invasive cardiac output monitor (NICOM) with PAC. In the current study, we are still not sure if NICOM can replace the PAC as a means to measure CO. A meta-analysis study literature also showed that none of the four tested alternative methods (pulse contour analysis, esophageal Doppler, thoracic electrical bioimpedance) achieved agreement with PAC [15]. However, considering that the scope of PAC’s applicability is limited, and that it also brings a lot of risks and complications (e.g., infection) [20], the clinical hemodynamic monitoring is moving towards less invasive or even non-invasive methods.

Bioimpedance is based on the fact that the conductivity of a high-frequency, low-magnitude alternating current passed through the thorax changes as blood flow varies with each cardiac cycle. These changes can be measured using electrodes placed on a patient’s chest and used to generate a waveform from which cardiac output can be calculated. However, its accuracy is always in doubt, as the technique has its own limitation [16].

Bioreactance is developed out of bioimpedance and measures changes in the frequency of the electrical currents traversing the chest, rather than changes in impedance, potentially making it less sensitive to noise [17, 18]. Also, bioreactance-based NICOM has acceptable accuracy in challenging clinical environments [19]. Furthermore, one study suggests that NICOM can be clinically valid when used in ICU patients [21, 22], but the accuracy of NICOM and Bioz.com have not been confirmed, especially compared with the PAC technique in patients undergoing off-pump coronary artery bypass surgery. This is why we compare the three devices to evaluate the accuracy of CO measurements in patients undergoing off-pump coronary artery bypass surgery. The measurement points in this study were defined according to clinical importance of surgical step, as they are also very common in cardiac surgery.

The limitations of the current study are as follows: Firstly, all the data for the patient group were collected after induction of anesthesia. PAC values before anesthesia are unavailable, because we abstained from placing a PAC in the patient who is awake so as not to further increase discomfort before surgery. Secondly, the study was limited only to patients undergoing cardiac surgery and did not take into account post-surgical patients in the ICU. Thirdly, the current study did not evaluate the ability of the NICOM to predict fluid responsiveness.

In conclusion, the NICOM is a safe, convenient device, and reliable to continuously and non-invasively monitoring cardiac output and cardiac index, and could track the trend of changes under dynamic conditions. Although the device cannot replace PAC for patients undergoing off-pump coronary artery bypass surgery at present stage, it may potentially be used in a wide range such as during laparoscopic surgery or for the patients with heart diseases undergoing non-cardiac surgery, when invasive monitoring is not an option or not desired.