Fluid Dynamics in Applied Physiology: Instrumentation and Validation of a Venturi Airflow Sensor for Expired Gas Analysis Indirect Calorimetry

This thesis covers three studies that investigated the design, instrumentation, and application of a novel pulmonary ventilation measurement device, based on the Venturi effect, a fluid dynamic principle theorised in the 1700’s. The device is called a Venturi airflow sensor (fV), which uses a Venturi tube and differential pressure transducer that quantifies fluid flow based on the differential pressure that occurs between two sections of pipe that are of different diameters. This method of fluid flow measurement has never been validated for use in pulmonary ventilation applications. Collectively, the aim of this thesis was to provide a thorough investigation that would lead to acquisition of the scientific evidence needed to validate the application of fV technology to measurements of human pulmonary ventilation during exercise.

Study 1 looked at the instrumentation processes involved in designing a fV device suitable for human pulmonary ventilation conditions during exercise, that is, measurement of pulmonary ventilation with airflow rates between 0.1 L·s-1 to 10 L·s-1. Therefore, the main features that were focused on, were the effects of fV throat radius and differential pressure input/output specifications on measurement range and sensitivity. This was investigated via the application of the Bernoulli Principle for theoretical computations of airflow based on a fV configuration and transducer specification, which were then compared to actual measurements taken from a fV airflow sensor manufactured to the same specifications. The results indicate that the theoretical airflow measurements were in close agreement with actual airflow measurements. Thus, a technician can use this computational method to design a fV to suit their application.

Study 2 looked at components of airflow sensor performance, which included signal quality, response time and frequency response, and accuracy and reliability of volume measurements. The aim of this study was to see if the fV device had similar performance to two industry accepted criterion devices, the Turbine, and Pneumotachograph. Results indicate the fV device has close agreement with both criterions for their respective advantages. The findings conclude the fV device has adequate signal quality, dynamic response, and acceptable error in volume measurement following calibration, which allow for reliable measurements of pulmonary ventilation from rest to exercise conditions.

Study 3 looked at a clinical application of the fV device, where it was used to quantify pulmonary ventilation during expired gas analysis indirect calorimetry. The aim of the study was to compare differences in simultaneous pulmonary ventilation measurements between the fV and 2 criterion methods, during exercise to volitional exhaustion on a stationary cycle ergometer. Results indicate all three devices had close agreement, which had minimal effect on subsequent computations in indirect calorimetry, such as the rate of oxygen consumption. Additionally, this study is the first of its kind that shows in-vivo results for different types of airflow sensor method contribution to technical error in measurements within EGAIC.