Electrical impedance tomography compared to positron emission tomography for the measurement of regional lung ventilation: an experimental study

J C Richard, C Pouzot, A Gros, C Tourevieille, D Lebars, F Lavenne, I Frerichs, C Guérin, J C Richard, C Pouzot, A Gros, C Tourevieille, D Lebars, F Lavenne, I Frerichs, C Guérin

Abstract

Introduction: Electrical impedance tomography (EIT), which can assess regional lung ventilation at the bedside, has never been compared with positron-emission tomography (PET), a gold-standard to quantify regional ventilation. This experiment systematically compared both techniques in injured and non-injured lungs.

Methods: The study was performed in six mechanically ventilated female piglets. In normal lungs, tidal volume (VT) was randomly changed to 6, 8, 10 and 15 ml/kg on zero end-expiratory pressure (ZEEP), then, at VT 10 ml/kg, positive end-expiratory pressure (PEEP) was randomly changed to 5, 10 and 15 cmH2O. Afterwards, acute lung injury (ALI) was subsequently created in three animals by injecting 3 ml/kg hydrochloric acid into the trachea. Then at PEEP 5 cmH2O, VT was randomly changed to 8 and 12 ml/kg and PEEP of 10 and 15 cmH2O applied at VT 10 ml/kg. EIT and PET examinations were performed simultaneously. EIT ventilation (VTEIT) and lung volume (VL) were measured in the anterior and posterior area of each lung. On the same regions of interest, ventilation (VPET) and aerated lung volume (VAatten) were determined with PET.

Results: On ZEEP, VTEIT and VPET significantly correlated for global (VTEIT = VPET - 2E-13, R2 = 0.95, P < 0.001) and regional (VTEIT = 0.81VPET+7.65, R2 = 0.63, P < 0.001) ventilation over both conditions. For ALI condition, corresponding R2 were 0.91 and 0.73 (P < 0.01). Bias was = 0 and limits of agreement were -37.42 and +37.42 ml/min for global ventilation over both conditions. These values were 0.04 and -29.01 and +29.08 ml/min, respectively, for regional ventilation. Significant correlations were also found between VL and VAatten for global (VL = VAatten+1E-12, R2 = 0.93, P < 0.0001) and regional (VL = 0.99VAatten+0.92, R2 = 0.65, P < 0.001) volume. For ALI condition, corresponding R2 were 0.94 (P < 0.001) and 0.54 (P < 0.05). Bias was = 0 and limits of agreement ranged -38.16 and +38.16 ml for global ventilation over both conditions. These values were -0.24 and -31.96 to +31.48 ml, respectively, for regional ventilation.

Conclusions: Regional lung ventilation and volume were accurately measured with EIT in healthy and injured lungs and validated by simultaneous PET imaging.

Figures

Figure 1
Figure 1
Description of one given experimental condition. During the first five minutes the experimental step, either change in tidal volume or positive end-expiratory pressure (PEEP), is applied without any measurement and continued up to the end of this phase. Then positron emission tomography (PET) transmission scan is taken for 10 minutes followed by a five-minute wash-in phase. Afterwards, 13N-N2 positron-emitting tracer is washed-out for five minutes. In-between the amount of the tracer entering the lung is measured (entry function). PET emission scans are then performed at tracer equilibrium and during tracer wash-out. The electrical impedance tomography signals used in present analysis are recorded for one minute at the end of both transmission and emission periods (black squares). Each step lasts 30 minutes.
Figure 2
Figure 2
Global lung ventilation. (a) Relationship of global lung ventilation measured with electrical impedance tomography (VTEIT predicted) and positron emission tomography (VPET) in the first part of the experiment. The regression line was drawn over all experimental points pertaining to both normal (open circles) and acute lung injury (closed circles) conditions. (b) Relationship of the difference to the mean of global lung ventilation measured with electrical impedance tomography (VTEIT predicted) and positron emission tomography (VPET) in the first part of the experiment. Horizontal continuous line and horizontal broken lines are the mean and the upper (mean + 2 standard deviations) and lower (mean - 2 standard deviations) values of the difference, respectively.
Figure 3
Figure 3
Regional Lung Ventilation. (a) Relationship of regional lung ventilation measured with electrical impedance tomography (VTEIT predicted) and positron emission tomography (VPET) in the first part of the experiment. The regression line was drawn over all experimental points pertaining to normal and acute lung injury conditions in each quadrant. (b) Relationship of the difference to the mean of regional lung ventilation measured with electrical impedance tomography (VTEIT predicted) and positron emission tomography (VPET) in the first part of the experiment. Horizontal continuous line and horizontal broken lines are the mean and the upper (mean + 2 standard deviations) and lower (mean - 2 standard deviations) values of the difference, respectively.
Figure 4
Figure 4
Global lung volume. (a) Relationship of global lung volume measured with electrical impedance tomography (VLEIT predicted) and positron emission tomography (VAatten) in the second part of the experiment. The regression line was drawn over all experimental points pertaining to both normal (open circles) and acute lung injury (closed circles) conditions. (b) Relationship of the difference to the mean of global lung volume measured with electrical impedance tomography (VLEIT predicted) and positron emission tomography (VAatten) in the second part of the experiment. Horizontal continuous line and horizontal broken lines are the mean and the upper (mean + 2 standard deviations) and lower (mean - 2 standard deviations) values of the difference, respectively.
Figure 5
Figure 5
Regional lung volume. (a) Relationship of regional lung volume measured with electrical impedance tomography (VLEIT predicted) and positron emission tomography (VAatten) in the second part of the experiment. The regression line was drawn over all experimental points pertaining to normal and acute lung injury conditions in each quadrant. (b) Relationship of the difference to the mean of regional lung volume measured with electrical impedance tomography (VLEIT predicted) and positron emission tomography (VAatten) in the second part of the experiment. Horizontal continuous line and horizontal broken lines are the mean and the upper (mean + 2 standard deviations) and lower (mean - 2 standard deviations) values of the difference, respectively.
Figure 6
Figure 6
Plots of the residuals to the predicted values. (a) Regional ventilation (VTEIT) and (b) volume (VL EIT).

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