Affecting Factors On Pressure Changes Occurring Inside Barometric Plethysmograph

Background:In 1868, Bert first used plethysmography in his research. The pressure changes inside a closed chamber occur while a small animal breathes spontaneously inside a plethysmograph, some parameters of lung function can be induced by the pressure changes. Currently, there are three kinds of plethysmography to measure lung function of small animals 1) invasive plethysmography 2) whole-body plethysmography 3) double-chamber plethysmography. Whole-body plethysmography is classified into barometric plethysmography and flow plethysmography.The pressure changes within a barometric plethysmograph are considered from two components: air compression and air temperature and humidity. The gas compression and expansion within the chamber caused by thoracic movement produce the pressure changes inside the plethysmograph as a small animalbreathes. The gas flows into the lung as the animal inhales. The inspired air mixed with lung gas is heated and humidified because the temperature and humidity is higher in respiratory system than in ambient gas, the gas expands and the thorax inflates, the air inside the chamber is compressed. As shown by Lundblad, the magnitude of pressure excursion within the box reduced by two thirds when the box gas was he ated and humidified to the same of animal body. Recordings of pressure changes within the plethysmograph are basis of all plethysmography, research on pressure changes and affecting factors is essential and crucial to respiratory physiology of small animals. Objectives:First, we analyze the resources and components of box pressure inside whole-body plethysmograph, look into the basis of pressure fluctuation and effects of tidal volume and thoracic inflating volume on the pressure changes occurring inside barometric plethysmograph. Second, we try to know how mice temperature effects on the box pressure changes, and how these pressure changes affect mice respiratory frequency. Material and methods:In the first part, male BALB/C mice weighted 18-22g were provided. Two cube plethysmographs were made of polymethylmethacrylate with containing volume 3×4×12cm~3. Two chambers can be connected by a three-way adapter. We recorded pressure waves within whole-body plethysmograph and double-chamber plethysmograph inside which mice breathed, and calculated the volume changes. Paralyzed mice were ventilated through a fixed tidal volume by a small animal ventilator, thoracic inflating volume inside body chamber was measured and compared with tidal volume.In the second part, animals and plethysmographs were same as before. A 10 liters glass chamber was provided to produce various air pressure. Effects of mice body temperature on chamber temperature and pressure within whole-body plethysmograph and double-chamber plethysmograph were observed. The pressure as before was applied to head chamber and body chamber of double-chamber plethysmograph and whole-body plethysmograph, effects of applied pressure on respiratory frequency and pressure magnitude were observed. How the pressure changes caused by temperature changes influence on mice respiratory frequency and depth inside whole-body plethsmograph? Results: 1. The pressure change within barometric plethysmograph was0.088±0.008mmHg, corresponding to a volume change V_P 0.013 ±0.001ml. The pressure change within head chamber was 1.856 ±0.062mmHg, corresponding to a volume change V_t 0.356 ± 0.012ml. The pressure change within body chamber was 2. 434 ±0.089 mmHg, corresponding to a volume change V_e 0.369 ± 0.014ml. V_e was significantly higher than V_t ( t=5.27, P=0.003) .After paralyzed, mice were ventilated with tidal volume 0.3ml and frequency 1. 5HZ, the calculated thoracic inflating volume was 0.233±0.003ml which was considerably different from tidal volume 0.3ml (t=-21.4, P<0. 001) . 2. There was no difference between pressure or temperature changes caused by anesthetized mice and dead mice. The pressure or temperature changes in body chamber were significantly higher than head chamber.While extra pressure applied on body or head chamber, respiratory frequency showed significance on main effects of pressure, time and cross effect, but the magnitude of pressure changes didn't reveal significance on main effects of pressure, time and cross effect.In the closed chamber, respiratory frequency showed significance at various time points (F=4.751, P<0.001) , themagnitude of pressure changes also showed significance atvarious time points (F=2.535, P=0.009) .Conclusion:1. the volume measured inside body chamber in invasive, head-out, or double-chamber plethysmograph is truly the thoracic inflating volume of small animal, tidal volume can be measured by head chamber of double-chamber plethysmograph. The pressure changes within barometric plethysmograph are affected by thoracic expansion and tidal volume, and tidal volume can be calculated by Fenn method. In addition to air heating and humidifying, one reason for thoracic inflating volume grater than tidal volume is thoracic pressure. Thoracic pressure can affect the pressure change inside the whole-body plethysmograph.2. The mice body temperature considerably influences the pressure change within the whole-body plethysmograph. The temperature and pressure change in head chamber is significantly less than that in body chamber. The extra pressure on head chamber or body chamber can significantly affect respiratory frequency of the mice, as well as the pressure change caused by the temperature variation inside the chamber.