Developments in fibre optic cardiac and respiratory plethysmography

Maletras, Francois-Xavier (2002) Developments in fibre optic cardiac and respiratory plethysmography. (PhD thesis), Kingston University,


This work is the continuation of previous research by A. Raza and other contributors to the field of fibre optic plethysmography. Plethysmography is defined as the volume estimation of an object according to its external dimensions. fu particular, this technique can be used to produce an estimation of the respiratory volume of a human subject according to the dimension of his chest, measured at the thoracic and abdominal levels. A respiratory plethysmograph simply attempts to deliver a signal being the closest possible estimation of the true respiratory volume, as measured by a spirometer or a pneumotachometer. There are essentially two instrumental approaches to respiratory plethysmography: 1) The Respiratory Inductive Plethysmograph (RIP) estimates the cross section area of the chest by monitoring the variation of inductance in an electrical wire encircling the chest. 2) The Fibre Optic Respiratory Plethysmograph (FORP) sees the contribution of fibre optic sensors to measure the chest's circumference variations. The purposes of the present investigation were to improve the performance of previous FORP prototypes, and to extend its capabilities to cardiac monitoring. Both these targets have been reached, and the new prototype is now demonstrating the potential of plethysmography as a sound investigation technique for both cardiac and respiratory monitoring. Overall, the improved sensor and acquisition system permitted the resolution of details of the plethysmographic waveforms that were beyond the reach of the previous prototype. The new FORP prototype is generally more reliable and more precise, if not less compact. From a medical point of view, research carried out with the new FORP prototype has had two major outcomes: 1) The increased temporal resolution-of the new acquisition system has given us the possibility to precisely measure the phase shifts between the plethysmographic signals, and the spirometric signal. Such measurements have contributed. to producing a better estimation of the spirometric signal, therefore increasing the credibility of the FORP as a non-invasive, respiratory volume monitoring device. 2) The increased amplitude resolution of the new acquisition system, coupled with the better linearity, better precision and smaller hysteresis of the new sensor, has enabled the FORP to detect body circumference variations due to cardiac activity around head, neck, thorax and abdomen of a patient. Observations of heart movements at thoracic level had already been reported with the RIP, the direct analogue of the FORP. The signal processing required by the RIP for such monitoring only permitted offline, Electro-Cardio-Gram (ECG) assisted interrogation of cardiac displacements. However, thanks to better signal processing, the FORP has been made capable of real time cardiac position monitoring, without referencing to a simultaneous ECG signal. The combined impact of this research and previous research by A. Raza and A. Augousti on respiratory gating with the FORP, is potentially important in the field of cardiac imaging with Magnetic Resonance and Computed Tomography scanners. The FORP should permit better synchronisation with cardiac movements, while helping the patient to maintain stable chest position, subsequently increasing the image resolution by limiting motion blur.

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