Transpulmonary lithium indicator dilution: a new method of intrathoracic blood volume measurement

Introduction

Extravascular lung water (EVLW) measurement may improve outcome. Double indicator dilution, which is the most accurate method of EVLW measurement, is no longer commercially available. Lithium indicator dilution could be used to measure the intrathoracic blood volume index (ITBVI) and therefore EVLW.

Method

A single-centre, observational study. Consent was sought from patients aged over 50 years undergoing elective cardiac surgery with cardiopulmonary bypass (CPB). Exclusion criteria included significant valvular regurgitation and lithium therapy. Anaesthetic, CPB, blood transfusion, ventilation and sedation practices were standardised. Indicator dilution measurements were performed following induction of anaesthesia, after CPB and then 2, 4 and 24 hours following surgery, using existing technology (LiDCO Ltd, London, UK). Data are presented as the median (IQR).

Results

Twenty patients were recruited (age 70 years (64–75 years); Parsonnet score 10 (1–14)). No difficulties were encountered with the new method of ITBVI measurement. Absolute values and the changes in the ITBVI were close to those anticipated. Linear regression analysis did not indicate mathematical coupling between the cardiac index (CI) and the ITBVI (R² = 0.22; P < 0.001). The relationship between the pulmonary blood volume index (PBVI) and the ITBVI was not constant.

Conclusion

Lithium indicator dilution may be a valuable new method of ITBVI measurement, and therefore EVLW measurement.

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