a portable apparatus for measuring the thoracic lung volume of a patient without enclosing the patient in a sealed chamber , comprising first and second impedance belts , a flow meter , shutter and a microprocessor - based controller . the flow meter includes pressure transducers for measuring the change in volume and pressure as the patient respires therethrough . the change in thoracic cage volume of the patient &# 39 ; s lungs is directly correlated with the change of impedance in the belts . the thoracic lung volume is then determined from a measured barometric pressure , the measured change in pressure and the measured volume change in the thoracic cage volume utilizing a correction factor to determine the thoracic cage volume .