Impedance cardiography to assess hemodynamic status: a comparison with transpulmonary thermodilution

Introduction

Measurement of extravascular lung water (EVLW) obtained with transpulmonary thermodilution (PiCCO system) can help the physician to guide fluid management of critically ill patients [1]. The thoracic fluid content (TFC) is a parameter deriving from the electric conductivity of the thorax, determined from intravascular, alveolar and interstitial fluids [2, 3]. To the author's knowledge, there is no clinical study comparing PiCCO EVLW and the TFC provided by the impedance cardiac output (ICG) system. The aim of the study was to compare measurements of cardiac index (CI) obtained with PiCCO (P) and ICG before and after fluid challenge (FC), to evaluate whether the TFC can provide a noninvasive estimate of lung fluid balance, compared with PiCCO EVLW, in 10 critically ill patients.

Methods

We studied 10 patients (eight males), aged 16–76 years (mean 22 ± SD 38), admitted to our ICU for head injury (three patients), septic shock (four patients), ARDS (one patient), and postsurgical (two patients).

The APACHE II score was 26–36 (30 ± 5). They were all monitored with the P system (PiCCO, V4.12; Pulsion Medical Systems AG) and the ICG system (Solar ICG module; GE Medical Systems Technology, Milwaukee, USA, 2001), to evaluate the CI. All patients received FC to optimize the haemodynamic status. Haemo-dynamic measurements were made before and after FC with colloids (5 ml/kg in 30 min). Statistical analysis was performed with Spearman nonparametric correlation and the Bland–Altman test.

Results

Twenty samples of data were collected. The CI P mean ± SD was 3.91 ± 0.83 l/min/m² before FC and 3.32–8.52 l/min/m² after FC. The mean CI ICG value before FC was 3.44 ± 0.99 l/min/m² (2.10–5.50) and was 4.56 ± 1.37 l/min/m² after FC. The correlation coefficient found was 0.526 (P < 0.05) and 0.588 after. The 95% CI was 0.149–0.804. The overall mean CI P – CI ICG difference was 0.70 l/min/m², with ± 1.96 SD of -2.53 and 3.23, respectively. One measurement (5%) extended beyond the lower SD limit. The EVLW index ranges from 3.3 to 13.7 (7.86 ± 3.27) ml/kg before FC and 3.4 to 15.1 (8.58 ± 3.52) ml/kg after FC (P = 0.015). The TFC before FC was 34–60 (43 ± 10) ml/kg and 32–64 (46 ± 10) ml/kg after FC (P = 0.011). The correlation coefficient found before FC is 0.798 (P = 0.007) with 95% CI 0.656–0.940. The correlation coefficient found after FC is 0.802 (P = 0.005) with 95% CI 0.661–0.943.

Conclusion

The main findings in this study are the great discrepancy between the two methods. CI measurements obtained with the ICG system underestimated CI when compared with the P system, particularly after FC. The TFC and EVLW index trends derived from FC appear similar; TFC measurements obtained with the ICG system show good correlation when compared with the EVLW index of the P system and it may be a useful index of pulmonary overloading, if supported by further randomized clinical trials.