Study Of Aerodynamics On Pressure Changes Inside Whole-body Plethysmograph

BackgroundPlethysmography is widely used in small animal experiments, which contain Whole-body plethysmography and Double-chamber plethysmography. Based on different measuring principles, there are two types of whole-body plethysmography including pressure plethysmography and flow plethysmography.When using pressure whole-body plethysmography to test lung function of rodents, pressure changes inside are proportional to tidal volume, which follows Boyle's law. The precondition of Boyle's law is that perfect air is static. But air inside pressure plethysmograph is flowing all the time when a rodent breathes in it. So Boyle's law, a principle of air statics, may be not convenient for measuring pressure changes of flowing air.Incurrent or excurrent volume of air inside flow plethysmograph is proportional to the integral of pressure changes, which is according to Poiseuille's law. The precondition of Poiseuille's Law is that air flowes through a regular passage and the air flow is laminar. When a rodent is in flow chamber, however, flowing air is turbulent. So compression and turbulent flow of air inside chamber may influence measurement. Objective1. To testify that Boyle's law is not appropriated to calculate tidal volume when using pressure plethysmography to measure lung function.2. The objective of this part is to prove that Poiseuille's law may be not convenient for measuring tidal volume.MethodsIn prat 1, we firstly duduce equations about pressure changes inside pressure plethysmograph. Then we design 3 experiments to testify our theoretic deduction.Experiment 1: After injecting air into pressure plethysmograph, peak pressure and horizontal pressure are measured.Experiment 2 : Air of different volumes and frequencies are introduced into plethysmograph to observe if air with different frequencies would play an extra role on pressure changes inside chamber.Experiment 3: Different air fluxs are made by small animal ventilator and pipette. This experiment is designed to prove the effects of different air fluxs on pressure changes inside chamber.In prat 2, we also duduce equations about pressure changes inside pressure plethysmograph and design experiments to testify our theory.Experiment 4: Air of different volumes and frequencies are pumped into flow plethysmograph. Integrals of pressure changes inside chamber are measured to testify the extra effect of flowing air.Experiment 5: Different air fluxs are made by small animal ventilator and pipette. This experiment is designed to prove if different air fluxs would exert the different effects on integrals of pressure changes.Flow chart of our research Results:In part 1, theoretic deduction expressed that increased pressure was generated from two sources: one was according to Boyle's law, another was based on law of conservation of momentum. When airflow was different function of time, pressure changes was different.Experiment 1: After air injection, pressure inside pressure plethysmograph increasedsharply to peak pressure, then decreased to horizontal pressure and kept no change. Peak pressure was significantly higher than horizontal pressure (P<0.001).Experiment 2: With increasing frequencies, pressure changes inside chamber was becoming higher and there were significant differences when ANOVA was used (P<0.001).Experiment 3: Though air was pumped into pressure plethysmograph with the same frequency and volume, pressure changes from ventilator were very significantly different from that produced by pipette (P<0.001).In part 2, theoretic equations informed that integral of pressure change was not proportional to volume of injected air, but is influenced by effect of volume and time together. When the air flux was different function of time, integral of pressure change was different.Experiment 4: With increasing frequencies, the integrals of pressure changes inside chamber were becoming lower.Significant differences were found in main effects of both volume (F=39885.639, P<0.001) and frequency (F=1083.922, P<0.001).Experiment 5: Though air was pumped into flow plethysmograph with the same frequency and volume, integrals of pressure changes from ventilator were significantly different from that derived from pipette (P<0.001). Conclusion1. When using pressure plethysmography to measure tidal volume, Boyle's law is not appropriate to express pressure changes inside chamber.2. Both volume and speed of air changes play roles on pressure changes, and type of air flux also influence the measurement.3. Poiseuille's law is not convenient for expressing pressure changes inside chamber.4. Both volume and speed of air changes play roles on integral of pressure changes, and type of air flux also influence the measurement.5. When using plethysmography to precisely calculate tidal volume of rodents, more principles of aerodynamics would be used.