Pulmonary medicine research using mass flow control

University of Oxford: Department of Engineering Science, and Nuffield Department of Clinical Neurosciences

Assessment of lung function using a non-invasive oscillating gas-forcing technique

Monitoring lung function typically requires an array of specialised, complex gas analysers as well as patient cooperation. Making the estimation of lung variables to deduce the appropriate therapeutic measures in an intensive care unit or operating theatre a difficult task.

In this study a compact and non-invasive system was created to obtain a tidal ventilation model using a non-invasive oscillating gas-forcing technique. Using O2 and N2O as on-demand indicator gases to estimate cardiopulmonary variables. The compact system comprises of just a flow rate sensor, a gas concentration analyser, and two mass flow controllers.

Two Alicat mass flow controllers were used to deliver the O2 and N2O indicator gases simultaneously into the patient’s airway flow during inspiration (breath-by-breath). The gas delivery rates were maintained proportional to the subject’s inspiratory flow rate at any instant, such that the indicator concentration remained constant within the breath, but could be forced to vary between breaths. In order to avoid errors in the estimation of the differential mass of gases, the sub-30ms response time of the mass flow controllers is key.

 

Concentration of indicator gas O2 in the airway flow of a healthy male volunteer. The forcing sinusoidal period T = 2 min. The green and red sinusoids are fitted to the green and red circles, respectively. The sinusoids show that the concentration of the indicator gas varies sinusoidally, at the chosen value of T. (Original figure source)