Display Screen or Portion Thereof with a Transitional Graphical User Interface for a Medical Device

FIG. 1 is a front view of a display screen or portion thereof, with a transitional graphical user interface for a medical device; and,

FIG. 2 is a front view of a display screen or portion thereof with the transitional graphical user interface for a medical device.

The ornamental design which is claimed is shown in solid lines in the drawings. The appearance of the transitional graphical user interface sequentially transitions between the images shown in FIGS. 1 - 2 . The process or period in which one image transitions to another image forms no part of the claimed design. The outermost broken line rectangle showing the outer border of the display screen or portion thereof and the interior broken lines showing portions of the transitional graphical user interface in the drawings forms no part of the claimed design.