Fig 1.
Temporal evolution of the experimental protocol consisting of the thirteen tasks (T1 to T13) illustrated on top and the expected changes in SV shown below.
The protocol comprises different postures such as sitting (T1 and T12), lying in supine position (T2, T6 and T13), lying with legs up (T3 and T7), cycling in supine position (T4, T8 and T10) and the subsequent recovery periods (T5, T9 and T11).
Fig 2.
(a) Placement of the 32 gel electrodes used for EIT: two planes of 16 electrodes each are placed above and below the nipple line. (b) Example image of the 3D camera and (c) the resulting 3D subject-specific model of the thorax including the electrodes (green circles). L1 to L5 illustrate the five planes on which EIT data was reconstructed.
Fig 3.
Block diagram of the measurement setup and the first processing steps resulting in ECG-gated EIT images (Imgs) and the regions of heart (ROIH) and lung (ROIL).
Fig 4.
Block diagram of the data analysis.
To test the four hypotheses (H1 to H4), different features were extracted from the EIT images and potential outliers with high noise or unstable heart and lung ROIs were removed. Then, the ability to estimate SV with these features via a linear model was evaluated by means of Bland-Altman analysis (absolute SVEIT in H1 and H2) or four-quadrant plot analysis (relative ΔSVEIT in H3 and H4).
Fig 5.
3D EIT images showing the respiration by means of standard deviation (SD) images on the five planes L1 (highest) to L5 (lowest) for the nine volunteers (a) to (i), in supine position.
The images of each subject (each column) were scaled to an individual color scale and show the last minute in the first recovery sequence (task T5). Prior to SD calculation the images were filtered using a 2nd-order Butterworth bandpass with fc = {0.04, 0.5} Hz.
Fig 6.
ECG-gated 3D EIT images showing the conductivity difference (end systole vs end diastole) on the five planes L1 (highest) to L5 (lowest) for the nine volunteers (a) to (i), in supine position.
The images of each subject (each column) were scaled to an individual color scale and show the average of the last minute in the first recovery sequence (task T5).
Fig 7.
ECG-gated 3D EIT images showing the conductivity difference (end systole vs end diastole) on the five planes L1 (highest) to L5 (lowest) for a selection of ten measurements (a) to (j) of volunteer S05.
All images are shown in a common color scale. Note that the last line in the title below each task name indicates the time range (relative to the start of the corresponding task) over which the one-minute average was performed to obtain one representative cardiac cycle.
Table 1.
Overall performance for a selection of features and the four hypotheses.
(H1) absolute SV via subject-independent calibration, (H2) absolute SV via subject-specific calibration, (H3) relative SV via subject-independent calibration, and (H4) relative SV via subject-specific calibration. (H1) and (H2) are evaluated in terms of absolute error ϵAbs and correlation coefficient r between SVEIT and SVRef. (H3) and (H4) are evaluated in terms of angular error ϵα and angular concordance rate CR between ΔSVEIT and ΔSVRef. The (†) indicates unrealistic solutions with calibrations coefficients not having identical sign for all subjects. Cell shadings indicate whether the acceptance criteria (see methods section) are met (green), not met (red), or met but with unrealistic calibration coefficients (yellow). The errors ϵAbs and ϵα are given as Mean ± Std and the 95% limits of agreement correspond to [Mean − 1.96 ⋅ Std, Mean + 1.96 ⋅ Std].
Fig 8.
SVEIT vs SVRef for a subject-specific calibration in hypothesis (H2) with the features (a) ΔσH or (b) ΔσH and .
Fig 9.
(Top) Example of temporal evolution of reference stroke volume (black) and heart rate (red) for the entire protocol comprised of tasks T1 to T13 (see Fig 1) for subject S03. The beginning of each task is marked with a dashed vertical line and the particular tasks considered for analysis are shaded in light (lying) or dark gray (recovery). (Middle) One-minute averages used for analysis showing SVRef and two EIT features: the systolic heart amplitude (ΔσH) and the temporal standard-deviation of the heart signal during one full cardiac cycle (tStdH). (Bottom) Tidal volume VR (blue) measured by the reference device (MetaMax 3B) and the one-minute averages of the global conductivity feature σG (green).
Fig 10.
Trending ability of ΔSVEIT vs ΔSVRef shown by means of four-quadrant plots for the combination of the two features ΔσL and and (a) a subject-independent calibration for hypothesis (H3) or (b) a subject-specific calibration for hypothesis (H4).
The exclusion zone was set to ±30%.