Quantification of left ventricular modifications in weightlessness conditions from the spatio-temporal analysis of 2D echocardiographic images

Two-dimensional echocardiography (2DE) performed during flights with parabolic trajectory to simulate weightlessness provide a unique means to study left ventricular (LV) modifications in order to prevent post-flight orthostatic intolerance in astronauts. However, conventional analysis of 2DE is based on manual tracings, which depends on readers' experience. Accordingly, our aim was to objectively quantify from 2DE images the LV modifications relevant to different gravity levels, by applying a semi-automated level-set border detection technique. The algorithm validation was performed by comparing manual tracing results, obtained by two independent observers on twenty images, with the semi-automated measurements. To quantify LV modifications, three consecutive cardiac cycles were analyzed for each gravity phase (1Gz, 1.8Gz, 0Gz). The level-set procedure was applied frame-by-frame to detect the LV endocardial contours and obtain LV area versus time curves, from which end-diastolic (EDA) and end-systolic (ESA) areas were computed and averaged to compensate for respiratory variations. Linear regression (y=0.91x+1.47, r=0.99, SEE:0.80cm2) and Bland-Altman analysis (bias=–0.58cm2, 95% limits of agreement=±2.14cm2) showed excellent correlation between semi-automatic and manually traced values. Interobserver variability was 5.4%, while the inter-technique variability resulted in 4.1%. Modifications in LV dimensions during the parabola were found: compared to 1Gz values, EDA and ESA were significantly reduced at 1.8Gz of 8.8±5.5% and of 12.1±10.1% respectively, while during 0Gz EDA and ESA increased of 13.3±7.3% and of 11.6±5.1% respectively, due to abrupt changes in venous return. The proposed method resulted in fast and reliable estimations of LV dimensions, whose changes caused by different gravity conditions were objectively quantified.