In vitro validation of a new respiratory ultrasonic plethysmograph


Johannes Schramel, Division of Anaesthesiology and perioperative Intensive Care, University of Veterinary Medicine Vienna, Veterinaerplatz 1, 1210 Vienna, Austria. E-mail:


Objective  The in-vitro validation of a novel Respiratory Ultrasonic Plethysmography (RUP) system designed to detect circumference changes of rib cage and abdominal compartments in large and small animals.

Study design  Experimental in vitro study.

Methods  The experimental system includes two compliant fluid-filled rubber tubes functioning as ultrasonic waveguides. Each has an ultrasonic transmitter and a detector at the opposing ends. Sensor length can be individually adapted in the range of 0.15–2 m. Data are downloaded to a computer at a sampling rate of 10 or 100 Hz. Measurements have a resolution of 0.3 mm. Baseline stability, linearity and repeatability were investigated with dedicated experiments. The base line drift was tested measuring a fixed distance for 2 hours continuously and then 18 hours later. A hand-operated horse thorax dummy (elliptically shaped, circumference 1.73 m) was used to compare waveforms of RUP with a respiratory inductive plethysmograph (RIP). The electromagnetic interference was tested by approaching metallic objects.

Results  Baseline drift and repeatability (10 repeated steps of 1.6% and 6.6% elongations and contractions) were within ± 0.3 mm. The response of the system for tube stretching up to 11% of total length was linear with a coefficient of determination for linearity of 0.998. In contrast to RIP, electromagnetic interference could not be observed with RUP.

Conclusions and clinical relevance  The low baseline drift and the lack of electromagnetic interference favours the use of RUP compared to an RIP device when studying the breathing pattern and end expiratory lung volume changes in conscious and anaesthetized animals.