Fig. 4

Polar plot assessing trending ability of EELV_MBNW to track changes in EELV_CT. The radial axis joining 0 to 180° is a 45° clockwise rotation of the line of identity in the four-quadrant plot and represents perfect agreement. The better the agreement between ΔEELV measurements, the closer data pairs will lie along the horizontal radial axis. The distance from the center of each plot represents the mean change in EELV between methods (mean ΔEELV) at each consecutive time points. Data points located between 315 and 45° refer to time points in which both EELV_CT and EELV_MBNW increased (upper right quadrant of the four-quadrant plot), while data points located between 135 and 225° refer to consecutive time points in which both EELV_CT and EELV_MBNW decreased (lower left quadrant of the four-quadrant plot). Data points located between 45 and 135° or 225 and 315° correspond to disagreement in the directional change of EELV between both techniques. Continuous line represents angular bias, while dashed lines represent radial limits of agreement. EELV_MBNW = end-expiratory lung volume assessed with the multiple breath nitrogen washin–washout technique; EELV_CT = end-expiratory lung volume assessed by computed tomography; ΔEELV = change in EELV between consecutive measurements