Intrathoracic blood volume measurement: comparison of transpulmonary lithium indicator dilution with indocyanine green indicator dilution

Introduction

Intrathoracic blood volume (ITBV) is thought to be a superior measure of cardiac preload compared with intravascular pressure [1]. Transpulmonary indocyanine green (ICG) indicator dilution is regarded as the most reliable method of ITBV measurement but is no longer commercially available. Our previous work suggests lithium indicator dilution could be used to measure the ITBV [2].

Methods

Patients undergoing cardiac surgery with cardiopulmonary bypass who met inclusion criteria were enrolled into a single-centre, observational study. Perioperative care was standardised. Comparative ITBV measurements were performed 1, 2, 4 and 6 hours after surgery, using lithium indicator dilution via a radial artery catheter (LiDCOplus; LIDCO Ltd, UK) and ICG indicator dilution via a femoral artery catheter (COLD-Z; Pulsion, Germany). Data were compared by Bland–Altman analysis.

Results

Seventeen patients were recruited (age 69 (54–87) years; Parsonnet score 10 (0–29)), providing a total of 68 paired measurements. Sixteen ICG measurements were excluded because of poor-quality indicator dilution curves, leaving 52 paired comparisons. The mean ITBV measured by lithium dilution was 2,522 ml (± 691) and measured by ICG dilution was 1,708 ml (± 432). The mean bias between paired measurements was 813 ml (limits of agreement (Bland–Altman analysis) ± 1,248; P < 0.001). For the cardiac index, however, the bias between techniques was only 0.39 l/min/m² (limits of agreement (Bland–Altman analysis) ± 0.9 l/min/m²; P < 0.0001). The discrepancy between the techniques therefore related to differences in the measurement of the mean indicator transit time. There was a decreasing trend in the mean differences in ITBV and mean indicator transit time (Li-ICG) from 1,014 ml and 16.1 seconds at hour 1 to 466 ml and 10.6 seconds at hour 6 (P = not significant).

Conclusion

Poor agreement between ITBV measurements taken using ICG and lithium indicator dilution appears to be due to inaccurate measurement of the mean indicator transit time. This may relate to the use of a radial as opposed to a femoral artery catheter in patients with poor peripheral perfusion.