Respiratory inductive plethysmography (RIP) : calibration, breathing pattern analysis and external CO2 dead space measurement

• Respiratory inductive plethysmography (RIP) is an important tool for ventilatory monitoring in research studies because it has minimal influence on the spontaneous breathing pattern and because the rib cage and abdominal contributions to the breaths are measured. RIP measures changes in rib cage and abdominal cross-sectional areas which are translated into lung volume estimates via RIP calibration against e.g., a pneumotachometer (PTM). The commercially available RIP (Respitrace ®) was used to evaluate models and methods for RIP calibration, to study changes in breathing pattern during induced bronchial obstruction in asthmatics and to improve a unique method for measuring the external CO2 dead space volume in facial visors and respiratory protective devices.Several models of the respiratory system and various methods and volume references can be used during RIP calibration. We found that conventional use of the PTM can result in large volumetric PTM errors, but with proper compensations the PTM is a useful volume reference. A linear model of the respiratory system, relating lung volume changes from the start of inspiration or expiration to rib cage and abdominal excursion from the start of respiratory motion, was the most accurate. The voluntarily preferential rib cage and abdominal breathing method for RIP calibration proved to be appropriate and robust with similar accuracy as that obtained with other methods.The spontaneous breathing pattern was recorded in asthmatics at rest and during bronchial histamine and methacholine challenges (HiCh/MeCh). The relationship between the airways' obstruction and the breathing pattern was explored. In one study, only four of eight patients showed a significant increase in minute ventilation (V'I: mean 72%) and mean inspiratory flow (VTI/TI; mean 80%) during HiCh. In another study a groupwise increase of 20% in V'I and V'TI/TI was found during repeated HiCh and repeated MeCh. However, the individual repeatability was poor. The ventilatory response did not correlate to the central or the peripheral airways' obstruction, nor to hypoxaemia or sensations of dyspnoea. Breathing pattern analysis is consequently not an adequate method for airways' obstruction monitoring during bronchial challenge. The ventilatory response appears to be the result of a complex interaction between several afferent stimuli and central ventilatory control mechanisms.Analysis of the variability of the breathing pattern components confirmed that the timing component (phase switching; inspiration/expiration) of the respiratory control system is more constant than the drive and volume function. Furthermore, the variability did not change during induced obstruction.A unique method for assessment of the external CO2 dead space volume in facial visors and respiratory protective devices was evaluated and further improved. The method was found to have sufficient accuracy (dead space measurement error ≤ 20%), provided that great concern was taken to measure the initial CO2 containing part of inspiration correctly with RIP.

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MEDICIN

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