Vincent P. Crabtree

Non-invasive measurement of peripheral venous oxygen saturation using a new venous oximetry method: evaluation during bypass in heart surgery.

Monitoring of mixed venous oxygen saturation (SvO(2)) is currently performed using invasive fibre-optic catheters. This procedure is not without risk as complications may arise from catheterization. This paper describes an alternative, non-invasive method of monitoring peripheral venous oxygen saturation (SxvO(2)) which, although it cannot replace pulmonary artery catheters, can serve as an adjunct/early warning indicator of when there is an imbalance in oxygen supply and demand. The technique requires the generation of an artificial venous pulse at the finger, thereby causing modulation of the venous blood volume within the digit. The blood volume changes are monitored using an optical sensor. Just as pulse oximetry utilizes the natural arterial pulse to perform a spectrophotometric analysis of the peripheral blood in order to estimate the arterial blood oxygen saturation, the proposed venous oximetry technique uses the artificially generated venous pulse to estimate SxvO(2). A prototype device was tested in a pilot study with patients undergoing heart surgery. Data from this study support the notion that the method is capable of tracking haemodynamic changes and suggests the technique is worthy of further development and evaluation.

Prospective Venox Feasibility Study

Venox is a propriety venous oximetry system, capable of measuring peripheral venous oximetry. In this ongoing study, Venox is being compared against mixed central venous oximetry during human cardiac surgery, with fibre optic reflectance spectrophotometry being used as the gold standard, placed in the pulmonary artery. A background review of the pulse oximetry, current venous oximetry techniques and the potential advantage of the Venox system are discussed. Lessons learnt, preliminary results, and future plans are included in discussion

Poincaré plot analysis for pulse interval extracted from non-contact photoplethysmography

An analysis of pulse-to-pulse interval (PPI) derived from non-contact photoplethysmography (PPG) is performed using the Poincare plot technique. Eleven healthy volunteers participated in a protocol utilising both spontaneous and paced breathing methodologies with a customised non-contact PPG system to obtain adequate quality signals for a distance of 80 mm between source and detector. An adaptive trough-searching algorithm (ATSA) is developed to extract the PPI which is effective in the presence of artefact during non-contact PPG measurement. A Poincare plot for each 5 minute period of the protocol shows significant pattern differences between spontaneous and paced breathing measurements. Geometric parameters extracted from the Poincare plot are shown to be sensitive to breathing interval changes and are a useful indicator for the variation of pulse rate variability measured using non-contact PPG

Effect of postural changes on lower limb blood volume, detected with non-invasive photoplethysmography

This paper describes the effect of passive leg raising on blood volume change in the lower limb, using a dual probe photoplethysmography (PPG) system employing a tissue optics model. The normalized AC/DC ratio and DC value are introduced from the model to evaluate the dynamic pulsation and total blood volume changes due to postural effects. The AC and DC components of PPG signals were collected from a passive leg raising protocol. With the leg raised, the normalized AC/DC ratio significantly decreased when supine, while the normalized DC value increased significantly in both supine and reclining positions. The parameters from the stationary leg showed similar but smaller responses. These results demonstrate a local and systemic physiological phenomenon in the lower limb blood volume change caused by postural changes. The normalized AC/DC ratio and DC value derived from the tissue optics model could be applied to assess the blood volume change.

Dimensional analysis of blood vessels in the pressure myograph

The accuracy of conventional and emerging methods for the dimensional analysis of optically imaged arterial vessels, isolated in a pressure myograph, is investigated. The pressure myograph is a device used to study the structure and function of isolated sections of small resistance arteries, as a function of chemical, mechanical and electrical stimuli. The arterial wall and lumen dimensions are particularly important indicators of anatomy and pathology. The conventional method of dimensional analysis uses edge detection, however the accuracy of this approach is questionable when the vessel is in a contracted state since contrast deteriorates or is lost between lumen and vessel wall. The conventional and emerging methods are examined experimentally with vessel phantoms, to provide known characteristics. A novel algorithm, based on a measurement of the vessel extinction coefficient, is also examined theoretically and experimentally. A discussion centers on the possibility for realistic lumen size measurement when edge detection can not be applied and when the accuracy of edge detection is questionable.