Introduction: Measurement of total lung capacity (TLC) is important in the assessment of lung disease. Of the two methods routinely used - helium dilution and whole body plethysmography (body box) - the body box method is quicker and less subject to limitations resulting from the underlying disease processes. Unfortunately until recently, the capital costs of the body box were high and the technique has been limited in general to major centres with research facilities. To take advantage of this technology the Medical Physics Department supported a project to build, instrument and interface a body box for clinical and research use by the Department of Respiratory Medicine in South Tees NHS Trust. This abstract describes the principles behind the development and refinement of the instrumentation.
Methods: The body-box uses the principles of Boyle's Law (V1.P1 = V2.P2) to measure the volume of thorax gas (Vtg). The subject sits in a large (1000 litre) box with sealed door and pants gently in and out against a shuttered mouthpiece for a few seconds. Alternating compression and rarefaction of Vtg cause small pressure changes inside the body-box (ΔPbox) which occur simultaneously with the small changes in Vtg (ΔV). Measurements made of the pressure changes in the body-box and at the mouth (ΔPmo) during the panting manoeuvre can be used to calculate Vtg and from this TLC. The body-box requires sensitive pressure transducers, accurate calibration and interfacing with standard laboratory PCs to make the technique practicable in day-to-day use.
Data acquisition and Display: The data from the mouth and box pressure transducers was acquired at a frequency of 200Hz using Labview software and a data acquisition card (DAQ 1200) purchased from National Instruments. All data was displayed in real time for analysis from a circular first in first out buffer (FIFO) which stored the last 2000 data points allowing analysis of the preceding 10 seconds or most recent breathing manoeuvres. An automated gradient was then fitted to the traces to allow for the ratio of Δ Pbox / Δ Pmo to be determined.
Results: Between 2002 and 2004 a total of 44 patients had their total lung capacity measured using the bodybox. We reliably measured total lung capacity and its sub divisions in the assessment of various lung diseases. When compared to helium dilution, the results from the bodybox, as anticipated slightly over estimated thoracic gas volume.
Conclusion: Since the start of the project, falling prices of electronic components and lab PCs have allowed major manufacturers to develop body boxes which are relatively cheap and available to smaller lung function laboratories. Data acquisition and handling have been refined to make calibration and daily use of the body-box more routine and less time consuming for technicians. The locally developed body-box has been superseded but was a successful instrument at a time of limited funding and an imaginative collaboration for Respiratory Medicine and Medical Physics.
Daya A, Tapper B, Chapman-Brown D, Gribbin H. The development and instrumentation of a body plethysmograph for the determination of lung volume. Annual Research and Development Day. The James Cook University Hospital, Middlesbrough. March 2008.
Abdul Daya, Cleveland Unit, Middlesbrough. Tel: +44 (0)1642-854-778.
Regional Medical Physics Department, Freeman Hospital, Newcastle upon Tyne, NE7 7DN, UK.
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