Evaluation of a combined reflectance photoplethysmography and laser Doppler flowmetry surface probe

Photoplethysmographic (PPG) signals were recorded from the fingers of 16 healthy volunteers with periods of timed and forced respiration. The aim of this pilot study was to compare estimations of arterial oxygen saturation (SpO2) recorded using a dedicated pulse oximetry system while subjects were breathing regularly with and without a mouthpiece containing a flow resistor. The experiments were designed to mimic the effects of mechanical ventilation in manaesthetized patients. The effect of estimated airway pressures of ±15 cmH2O caused observable modulation in the recorded red and PPG signals. SpO2 values were calculated from the pre-recorded PPG signals. Mean SpO2 values were 95.4% with the flow resistor compared with 97.3% with no artificial resistance, with statistical significance demonstrated using a Student’s t-test (P = 0.006)

Intra-operative optical monitoring of bowel tissue viability based on photoplethysmography and laser doppler flowmetry

Determination of bowel viability in patients undergoing bowel resection is essential in gastrointestinal surgery. One of the most common operations in gastrointestinal surgery is bowel resection for patients who have different kinds of bowel cancer or any other occlusion in which anastomosis has to be carried out following the removal of an unhealthy segment of the bowel. Monitoring blood flow in abdominal surgery especially intraoperatively would be a valuable tool for prevention of a postoperative anastomosis complication (e.g. anastomotic leak, which is the main complication after colorectal resection). The development of a continuous method for monitoring perfusion of bowel tissue would assist in early detection of inadequate blood supply which then help to reduce the occurrence of an anastomosis complication. Although various monitoring techniques have been proposed to assess intestinal viability intraoperatively, none of these techniques have proved to be reliable enough to replace visual observation. Therefore, to date there is no widely accepted and readily available intraoperative technique to reliably assess the viability of bowel tissue. The aim of this study was to combine the established techniques, laser Doppler flowmetry (LDF) and Photoplethysmography (PPG), into one probe intended for assessment of perfusion in abdominal tissue during bowel resection intraoperatively. In PPG, changes in transmission of light through tissue due to pulsation of small arteries can be monitored whereas in LDF microcirculatory blood cell velocity and flux can be studied. Such a probe could alert the surgeon immediately of any compromise in blood flow so further investigation and, if necessary, therapeutic steps can be applied immediately to prevent severe consequences. Therefore, custom reflectance PPG along with LDF sensor was designed and built in the form of a probe to investigate the changes in blood volume, blood flow and arterial oxygen saturation in patients undergoing bowel resection. The instrumentation was designed successfully and the data was saved for the further analysis. Twenty-four patients undergoing bowel resection were recruited for monitoring of perfusion and blood flowintraoperatively; twenty had undergone laparoscopy and the remainder had a laparotomy operation. Eight different measurements were performed during each trial. The results revealed that the probe could be an indicator of evaluating perfusion and blood flow changes at different stages of the surgery. The results also suggest that laser Doppler is more sensitive to artefact compared to PPG. Differences in amplitude of PPG between different measurements reveal that the sensor does detect changes in blood volume and flow confirming that it has the ability to verify that pulsatile flow is being preferentially preserved at the last step of the resection procedure (at the edges of the anastomosis sites after anastomosis is been constructed

The development of biomedical instrumentation using backscattered laser light

This thesis is concerned with the measurement of blood flow and oxygen saturation in the microcirculation using the techniques of laser Doppler flowmetry and pulse oximetry. An investigation of the responses of Doppler flowmeters using different signal processing bandwidths and laser sources revealed two major findings. Firstly, that careful choice of processing bandwidth is required in order to sample the whole range of possible Doppler frequencies present in the backscattered light. Secondly, that the choice of laser source is important in governing the output stability of a flowmeter. Another investigation focused on the evaluation of a dual channel laser Doppler flowmeter using both in vitro and in vivo models. It was demonstrated that the instrument permitted a useful method of obtaining flow information by comparing simultaneous responses at experimental and control sites. The choice of laser wavelength was investigated in a study to determine whether blood flow measurements are obtained from different depths within the skin tissue. The results indicate that some depth discrimination is obtainable using instruments operating at different wavelengths, however it is difficult to demonstrate the effect in vivo. In a separate study it was shown that pressure applied to the skin surface greatly affects the underlying blood flow. It is recommended that care has to be taken when positioning Doppler probes on the skin. A reflection pulse oximeter was developed using laser light backscattered from the skin. The instrument was evaluated in vitro and in vivo by comparing desaturation responses with a commercial transmission pulse oximeter. The reflection oximeter was demonstrated to reliably follow trends in oxygen saturation but several problems prevented instrument calibration. Finally, a device combining laser Doppler flowmetry with reflection pulse oximetry was developed and used in vivo to follow trends in blood flow and oxygen saturation from the same tissue sample

Intraoperative monitoring of intestinal viability: Evaluation of a new combined sensor

A dual wavelength photoplethysmography (PPG) and laser Doppler flowmetry (LDF) sensor was developed to investigate the suitability of these techniques for monitoring bowel viability intraoperatively. Clinical measurements were obtained from thirty patients undergoing bowel surgery. Three measurements were performed at different stages of the operation. The amplitude of infrared PPG decreased from the baseline measurement to the pre-anastomosis measurement by 36% and LDF flux decreased by 21% for the same measurements. An increase of 33% in amplitude for infrared PPG was observed from the pre-anastomotic to post-anastomosis measurement; the equivalent increase was not seen for LDF flux. The results revealed that the sensor could potentially indicate changes in perfusion and blood flow at critical phases of surgery, thereby assisting in the early detection of inadequate blood supply in bowel tissue. The results also suggest that laser Doppler is more sensitive to movement artefact compared to PPG

Influence of blood pulsation on diagnostic volume in pulse oximetry and photoplethysmography measurements

Recent advances in the development of ultra-compact semiconductor lasers and technology of printed flexible hybrid electronics have opened broad perspectives for the design of new pulse oximetry and photoplethysmography devices. Conceptual design of optical diagnostic devices requires careful selection of various technical parameters, including spectral range; polarization and intensity of incident light; actual size, geometry, and sensitivity of the detector; and mutual position of the source and detector on the surface of skin. In the current study utilizing a unified Monte Carlo computational tool, we explore the variations in diagnostic volume due to arterial blood pulsation for typical transmitted and back-scattered probing configurations in a human finger. The results of computational studies show that the variations in diagnostic volumes due to arterial pulse wave are notably (up to 45%) different in visible and near-infrared spectral ranges in both transmitted and back-scattered probing geometries. While these variations are acceptable for relative measurements in pulse oximetry and/or photoplethysmography, for absolute measurements, an alignment normalization of diagnostic volume is required and can be done by a computational approach utilized in the framework of the current study

Optical sensors for the in vivo assessment of flap perfusion in plastic surgery

Following mastectomy for breast cancer a wide variety of surgical techniques are currently available for post mastectomy breast reconstruction where autologous tissue is used to construct a natural looking breast. One of the most common types of reconstructive surgeries use Deep Inferior Epigastric Perforator (DIEP) free flap where skin and adipose tissue along with their blood supplies are transferred from the lower abdomen to the chest. The success of free flap reconstructive surgery depends strongly on the maintenance of adequate perfusion in the flap. Early diagnosis of ischaemia and surgical exploration to restore blood flow can often salvage the flap and may prevent graft failure. Even though many techniques have been used, there is still a need to develop a non-invasive, easy to use, reproducible and inexpensive monitoring device to assess flap perfusion. In an attempt to overcome the limitations of the current flap perfusion monitoring techniques a prototype reflectance three wavelength photoplethysmographic (PPG) sensor was developed. The PPG sensor consisted of two infrared (940 nm), two green (520 nm) and two red (660 nm) LEDs and a photodiode. A PPG processing system was also constructed in order to drive the optical components on the sensor and to detect and pre-process the PPG signals. A Virtual Instrument (VI) was also implemented in LabVIEW in order to display, analyse and archive the PPG signals with the capability of real-time estimation of arterial oxygen saturation (SpO2) values. The system was evaluated in a pilot study on fifteen patients undergoing breast reconstructive surgery using (DIEP) flaps. Good quality red, infrared and green PPG signals were obtained pre-operatively from the donor site (abdomen), intra-operatively (capturing reperfusion of flap following anastomosis) and post-operatively at regular intervals for up to 12 hours post surgery. SpO2 values were also estimated which were found to be in broad agreement with SpO2 values recorded from the commercial pulse oximeter attached to the patients’ finger. The flap PPGs were compared with PPGs and SpO2s acquired from the finger of a small number of patients using a custom made reflectance finger PPG probe, optically and electrically, identical as the flap probe. The finger PPGs were found to be much larger than the flap PPGs which confirms the hypothesis of inadequate perfusion in the flap during and after the operative period. Furthermore the custom made PPG processing system and flap sensor were used successfully on a series of case studies to evaluate the versatility of the system in monitoring PPG signals and estimating blood oxygen saturation in other flaps. These included monitoring two patients undergoing Latissimus Dorsi (pedicle) flap reconstructive surgery and a head and neck free flap surgery where a Vertical Rectus Abdominis Myocutaneous (VRAM) flap was used following total petrosectomy. Also, two patients undergoing reconstructive surgery of the oesophagus using jejunum free flaps were also recruited into the study. For this study a purpose build oesophageal PPG sensor was developed. These case studies demonstrated the ability to use the developed PPG sensors to acquire PPG signals and estimate SpO2s in a variety of flaps. The results have confirmed that the custom made PPG system and sensor has the potential to be used as an alternative technique for monitoring perfusion in various types of flaps at all operative periods

Photonic Biosensors: Detection, Analysis and Medical Diagnostics

The role of nanotechnologies in personalized medicine is rising remarkably in the last decade because of the ability of these new sensing systems to diagnose diseases from early stages and the availability of continuous screenings to characterize the efficiency of drugs and therapies for each single patient. Recent technological advancements are allowing the development of biosensors in low-cost and user-friendly platforms, thereby overcoming the last obstacle for these systems, represented by limiting costs and low yield, until now. In this context, photonic biosensors represent one of the main emerging sensing modalities because of their ability to combine high sensitivity and selectivity together with real-time operation, integrability, and compatibility with microfluidics and electric circuitry for the readout, which is fundamental for the realization of lab-on-chip systems. This book, “Photonic Biosensors: Detection, Analysis and Medical Diagnostics”, has been published thanks to the contributions of the authors and collects research articles, the content of which is expected to assume an important role in the outbreak of biosensors in the biomedical field, considering the variety of the topics that it covers, from the improvement of sensors’ performance to new, emerging applications and strategies for on-chip integrability, aiming at providing a general overview for readers on the current advancements in the biosensing field

Reflectance photoplethysmography for non-invasive monitoring of tissue perfusion

Monitoring blood perfusion and oxygenation changes is of vital importance and for this reason many different techniques have been developed over the decades. Photoplethysmography (PPG) is an optical technique that measures blood volume variations in vascular tissue and it is well known for its utilisation in pulse oximetry for the estimation of arterial blood oxygen saturation (SpO2). In pulse oximetry, mainly the pulsatile component of the signal (AC PPG) is used while the continuous DC component is mostly excluded. Near Infrared Spectroscopy (NIRS) is another optical technique that measures changes in the concentration of oxygenated (ΔHbO2), deoxygenated (ΔHHb), and total haemoglobin (ΔtHb) from the variations in light attenuations at different wavelengths. The main motivation of this research is to explore the capability of Photoplethysmography in assessing tissue perfusion and oxygenation similarly as NIRS. The hypothesis underlining this research is that the DC component of the PPG signal contains information on the overall absorbed light and this part of the PPG signal, acquired at least two wavelengths, may be used to obtain ΔHbO2, ΔHHb, and ΔtHb as performed in NIRS. Therefore, DC PPG attenuations may be related to haemoglobin concentrations by the modified Beer-Lambert law (MBLL). In order to investigate this, novel reflectance, custom-made PPG sensors and measurement systems, including advanced signal processing algorithms, have been developed for the acquisition and analysis of raw PPG signals (AC + DC) from different anatomical locations. Three in vivo studies on healthy volunteers were carried out in order to investigate if ΔHbO2, ΔHHb, and ΔtHb estimated from PPG could indicate changes in blood perfusion and oxygenation. The studies consisted of vascular occlusions on the forearm, negative bed tilting, and whole body cold exposure. Raw PPG signals were acquired from different locations such as the forearm, fingers, and forehead, whereas simultaneous NIRS signals were used as a reference. The results showed that ΔHbO2, ΔHHb, and ΔtHb could be effectively estimated from PPG signals. These parameters indicated the changes in blood volumes and/or oxygenation, whereas comparison with NIRS signals showed good levels of correlation and trending. These promising results showed that DC PPG signals could be used to monitor changes in blood perfusion and oxygenation, extending the range of applications of Photoplethysmography

Laser doppler perfusion imaging of the normal and diseased vulva.

Vulval lichen sclerosus (LS) and high-grade intraepithelial neoplasia (VIN 3) are two common and distressing diseases. Significant morbidity is caused by symptoms of persistent pruritus and surgical treatment of skin areas suspicious of malignancy. The risk of developing cancer in a background of LS and VIN 3 is poorly defined. The methods currently available for clinical assessment of the vulva are limited. There is abundant research on the application of the LASER Doppler technique - laser Doppler Flowmetry (LDF) - showing changes in perfusion within the small blood vessels of the skin as a useful parameter for more accurate disease classification. There is also research on immunohistochemical microvessel density (MVD) studies showing increases in blood supply in tissues prone to develop cancer or as a prognostic marker of cancer outcome. The Laser Doppler perfusion imager (LDPI) provides a rapid, real time, non-invasive and non-contact method to measure skin blood flow in an area as opposed to a single point by the LDF, making the LDPI more suitable for application to the vulva. This thesis reports for the first time, the application of the LDPI to the vulva. Initially the LDPI was applied to the clinically normal vulva to study perfusion variance related to menstrual cycle, age and local skin temperature provocation. The application was then extended to vulval disease, LS and VIN 3, and validated against morphological differences in MVD. The LDPI and MVD studies suggest that in VIN 3 there is an actual increase in skin perfusion. In LS the situation is more complex and suggests that the LDPI measured perfusion at a greater depth than the MVD. Studies on base line perfusion variance of vulval LS to topical therapy show that there is no overall difference in baseline perfusion in spite of symptom improvement. Temperature provocation studies suggest differences in skin blood flow response in diseased compared to the normal vulva