A comparison of bioimpedance and continuous thermodilution measurement of cardiac output in ICU patients

Objective

Bolus thermodilution is the standard bedside method of cardiac output measurement in the ICU. The Vigilance monitor (Baxter Healthcare Corporation) uses a modified thermodilution pulmonary artery catheter with a thermal filament (CCO/SvO₂/VIP Catheter model 746H8F) to give a continuous readout of cardiac output. This has been shown to correlate well with the gold standard dye dilution method with a mean difference of -0.01 and r² = 0.97 [1].

Bioimpedance cardiography using the NCCOM 3 monitor (Bomed Medical Mfg Ltd) offers a non-invasive means of continuous cardiac output measurement and has shown reasonable correlation with the bolus thermodilution method.

However, it is difficult to ensure contemporaneous measurement when comparing the continuous bioimpedance method with the intermittent bolus thermodilution method. We therefore investigated the agreement between the continuous bioimpedance and continuous thermodilution methods, thereby enabling simultaneous measurements of the two techniques.

Design

Prospective observational study.

Subjects

Seven patients requiring pulmonary artery catheterisation for cardiac output monitoring on the General ICU.

Methods

A continuous thermodilution pulmonary artery catheter was inserted into each patient and the bioimpedance electrodes were applied. Each patient was monitored for study purposes for approximately 6 h, with acquisition of simultaneous paired cardiac index (CI) data at 1 min intervals.

Results

A total of 2390 paired data points were analysed on SPSS for Windows 3.1 and a Bland-Altman plot [2] was constructed. There was poor correlation (r² = 0.0107) between the two methods of cardiac output measurement. The mean of the differences was -0.161 with a standard deviation of 1.158.

Conclusion

The NCCOM 3 bioimpedance monitor gives poor agreement with the well validated Vigilance continuous thermodilution monitor in a mixed group of General ICU patients and therefore the bioimpedance method cannot be recommended tor cardiac output monitoring in this situation.