In order to measure small volumes displaced (1-700 μl) by contracting airways without changing the load (pressure) we developed a new device: the microplethysmograph (MP).
Methods: The cannulated airway (A) is connected with the measurement cone (M) of a MP. The pressure difference between cone M and the identical reference cone (R) is measured using a differential pressure transducer (D) (LCVR 0-2 cm H2O, Celesco, USA). The fluid volume in both cones can be increased to increase its full scale sensitivity from 700 to only 10 μl. The height (H) of the MPG can be varied to set the pressure (P) in the airway.
Measurement: As the airway contracts, fluid will move into cone M inducing a pressure rise. This pressure rise is compensated by lowering the MPG to keep conditions isobaric. The pressure difference between cone M and R is measured by transducer D.
Conclusion: We developed a highly sensitive and stable microplethysmograph for measurement of extremely small volume changes in isobaric conditions. We think that this system is applicable to pediatric research in many fields.