any of several instruments for measuring and recording various body functions, as the velocity or volume of blood flow, heart rate or breathing rate, changes in the size of organs or limbs, etc.
See plethysmograph in American Heritage Dictionary 4
(plĕ-thĭzˈmə-grăfˌ, plə-)
noun
An instrument that measures variations in the size of an organ or body part on the basis of the amount of blood passing through or present in the part.