Measurement of relative lung perfusion with electrical impedance and positron emission tomography: an experimental comparative study in pigs

Abstract

Background: Electrical impedance tomography (EIT) with indicator dilution may be clinically useful to measure relative lung perfusion, but there is limited information on the performance of this technique.

Methods: Thirteen pigs (50-66 kg) were anaesthetised and mechanically ventilated. Sequential changes in ventilation were made: (i) right-lung ventilation with left-lung collapse, (ii) two-lung ventilation with optimised PEEP, (iii) two-lung ventilation with zero PEEP after saline lung lavage, (iv) two-lung ventilation with maximum PEEP (20/25 cm H2O to achieve peak airway pressure 45 cm H2O), and (v) two-lung ventilation under unilateral pulmonary artery occlusion. Relative lung perfusion was assessed with EIT and central venous injection of saline 3%, 5%, and 10% (10 ml) during breath holds. Relative perfusion was determined by positron emission tomography (PET) using 68Gallium-labelled microspheres. EIT and PET were compared in eight regions of equal ventro-dorsal height (right, left, ventral, mid-ventral, mid-dorsal, and dorsal), and directional changes in regional perfusion were determined.

Results: Differences between methods were relatively small (95% of values differed by less than 8.7%, 8.9%, and 9.5% for saline 10%, 5%, and 3%, respectively). Compared with PET, EIT underestimated relative perfusion in dependent, and overestimated it in non-dependent, regions. EIT and PET detected the same direction of change in relative lung perfusion in 68.9-95.9% of measurements.

Conclusions: The agreement between EIT and PET for measuring and tracking changes of relative lung perfusion was satisfactory for clinical purposes. Indicator-based EIT may prove useful for measuring pulmonary perfusion at bedside.

Keywords: electrical impedance tomography; indicator dilution; positron emission tomography; regional pulmonary perfusion.

Copyright © 2019 British Journal of Anaesthesia. Published by Elsevier Ltd. All rights reserved.

Figures

Fig. 1

Time course of interventions. At each ventilation/perfusion condition (1–5), EIT scanning was performed continuously. Perfusion scans were performed with EIT and boluses of saline with different concentrations (3%, 10%, and 5%; orange bars), followed by PET imaging (red bars). The signal (Δz) shown in the lower subplot represents the global impedance measured with EIT during one-lung ventilation; swings with higher amplitude represent tidal ventilation, whilst the flatter parts of the signal correspond to the impedance variation during injection of saline boluses during breath holds. acCT, CT-based attenuation correction; CO, cardiac output; EIT, electrical impedance tomography; PET, positron emission tomography; a.u., arbirtrary unit.

Fig. 2

Images of lung perfusion of one representative animal under different conditions recorded with EIT (Qrel,EIT) and PET (Qrel,PET) during i.v. saline boluses at breath hold. Left three rows represent different saline concentrations used for bolus injection. The corresponding PET projection based on 68Gallium is shown in the right row. The colour scale was consistent for all figures. White solid lines in the right upper perfusion images represent the distribution of lung regions of equal ventro-dorsal height created from corresponding CT scans, whereas white dashed lines represent dorsal lung borders based on CT-based attenuation correction. EIT, electrical impedance tomography; NaCl, sodium chloride; PA occlusion, block of a main pulmonary artery; PET, positron emission tomography.

Fig. 3

Mean regional distribution of lung perfusion of all animals along the ventro-dorsal gradient recorded with PET (red lines) and EIT using saline 10% (blue lines). Standard deviation is represented by line thickness. EIT, electrical impedance tomography; PA occlusion, block of a main pulmonary artery; PET, positron emission tomography.

Fig. 4

Correlation and Bland–Altman plot for EIT-based perfusion estimation using saline 3% and PET, with lung images divided into eight lung regions of equal ventro-dorsal height. Five conditions of different lung perfusion are included. Dashed lines represent limits of agreement (95% confidence interval). Lighter dots represent non-dependent ROI; darker dots represent dependent ROI. Compared with PET, EIT reveals proportionally higher perfusion signals in non-dependent ROI, but lower signals in dependent ROI (compare solid regression line with slope and intercept). EIT, electrical impedance tomography; LoA, limit of agreement; PET, positron emission tomography; ROI, region of interest; sd, standard deviation.

Fig. 5

Scatterplots of changes in relative perfusion for selected conditions of ventilation/perfusion matching. Values were obtained with EIT using saline 3% (ΔQrel,EIT) and PET (ΔQrel,PET). The predictive ability (concordance) was determined for ΔQrel,PET and ΔQrel,EIT <2% only (closed dots). Dashed lines represent linear regression, and respective regression equations are given. EIT, electrical impedance tomography; OLV, one-lung ventilation; PA occlusion, block of a main pulmonary artery; PET, positron emission tomography.