Skin color independent robust assessment of capillary refill time

Capillary refill time (CRT) is a method of measuring the peripheral perfusion status through a visual assessment. We have developed a robust system to measure CRT using a webcam (RGB) that monitors polarized light reflected on the skin. The average image intensity changes exponentially in time after the application of compression. The characteristic time constant we designated polarized capillary refill time (pCRT). We tested the pCRT performance in twenty-two healthy adult volunteers with different ages 37 $\pm$ 16, by momentarily applying a low compression (7 kPa) to the forearm of the volunteers. Results show positive behavior in conditions including; poor perfusion, high and low concentration of melanin, anemic conditions, and damaged skin. In 80% of unique measurements, the result is within $\pm$ 20% of the expected pCRT.Comment: 17 pages, 7 figure

Non-invasive optical measurements of blood perfusion response times

O interesse no uso de métodos ópticos não invasivos para a avaliação da perfusão microvascular da pele, cresceu nos últimos anos. Neste trabalho, estudamos duas técnicas ópticas para avaliação da perfusão. Para isso, utilizamos um oxímetro de dedo para medir as curvas do sinal de fotopletismográficos (sigla PPG do inglês Photoplethysmograph) e simultaneamente avaliamos a perfusão periférica, através do teste do tempo de recarga capilar (sigla CRT do inglês Capillary Refill Time). Trabalhos anteriores relatam limitações quanto a execução do teste de CRT, pois tradicionalmente é realizado de forma visual e suscetível à subjetividade de julgamento, gerando alta variabilidade entre observadores. Para contornar essas e outras limitações, estudos anteriores, realizaram o processamento das imagens durante o teste de CRT, porém não alcançaram resultados satisfatórios. Sendo assim, propomos uma metodologia baseada no uso de imagens de uma câmera digital com luz polarizada. Para isso, utilizamos uma câmera com polarizador, luz polarizada e um dispositivo para aplicar compressão, desenvolvido especificamente para este projeto. Foram recrutados 55 voluntários de 20 a 70 anos, nos quais aplicamos compressão por cerca de 5 s no antebraço esquerdo e adquirimos vídeos para determinar o CRT (sigla rCRT Reflection Capillary Refill Time). Os vídeos, foram processados em MATLAB. Os resultados indicam que o procedimento desenvolvido requer compressão cerca de 5 vezes menor que o especificado na literatura, além da possibilidade de medição do CRT em todo o espectro de fototipos de pele (escalas Fitzpatrick I a VI), e baixa variabilidade entre medidas repetidas. Em voluntários de pele negra (Tipo V e VI), as medidas de CRT apresentaram incertezas semelhantes às dos participantes de outros fototipos de pele (Tipo I, II, III e IV), diferentes das relatadas em outras pesquisas. Também, essa pesquisa mostrou que os valores de CRT se devem principalmente ao tempo de preenchimento de sangue total, em especial, sangue oxigenado. Observamos que o CRT é proporcional com a ativação do sistema simpático do sistema nervoso através de correlações com a variabilidade da frequência cardíaca (HRV) medida pelo oxímetro digital (PPG). Finalmente, analisamos os dados do PPG a fim de relacionar com a perfusão periférica no dedo com o fluxo sanguíneo na aorta abdominal, usando um modelo de análise de decomposição de pulso (PDA). Determinamos que, no modelo PDA, os pulsos cardíacos são melhor modelados por um conjunto de ondas de função secante hiperbólica do que por funções de onda gaussiana, como proposto na literatura. Em suma, este trabalho permite o desenvolvimento de métricas confiáveis para determinação da perfusão periférica, abrindo caminho para novas possibilidades de desenvolvimentos de equipamentos, visando diagnósticos em pacientes com problemas de perfusão periférica, como por exemplo, em pacientes diabéticos.The interest in the use of non-invasive methods for the hemodynamic evaluation of microvascular perfusion of the skin has increased in recent years. A clinically well-established technique is the measurement of Capillary Refill Time (CRT). The limitations of CRT, which were performed only visually, include being susceptible to the subjectivity of visual judgment, which was highly variable among different evaluators, incorrect results for dark-skinned people, irritation, or sensitive skin. Previous studies to overcome these limitations with the use of cameras haven\'t achieved the desired results. Seeking to overcome such limitations, we propose the used methodology based on the use of images from a digital camera and, as an innovation in CRT measurements, the use of polarized light. We use a webcam with polarizers, polarized light, and a device to apply compression, developed specifically for this project. 55 volunteers from 20 to 70 years old were recruited, on whose left forearms we applied compression for about 5 s and acquired footage of the process, which was then processed through MATLAB scripts. The results have shown that the procedure requires compression about 5 times lower than that specified in the literature, measurement of CRT in the entire spectrum of skin colors (Fitzpatrick scales 1 to 6), and low variability in repeated measurements. In dark-skinned volunteers, CRT measurements have shown uncertainties similar to those of participants of other skin colors, unlike what is presented by other studies. We have also found that CRT is primarily determined by the time necessary to fill whole blood, mainly oxygenated blood. Also, it was observed that CRT proportional to the activation of the sympathetic system of the nervous system through correlations with heart rate variability (HRV) measured by digital plethysmograph (PPG). Finally, we analyzed the PPG data to relate peripheral perfusion in the finger to blood flow in the abdominal aorta, using a pulse decomposition analysis (PDA) model. We show that, in the PDA model, cardiac pulses are better modeled by a wave function Hyperbolic Secant than by Gaussian pulses, as proposed in the literature. In short, this work allows the development of solid metrics to determine peripheral perfusion, paving the way for new possibilities for developments aimed at diagnosing patients with peripheral diseases, such as sedentary people and people with diabetics

Blood Pressure Estimation by Photoplethysmogram Decomposition into Hyperbolic Secant Waves

Photoplethysmographic (PPG) pulses contain information about cardiovascular parameters. In particular, blood pressure can be estimated using PPG pulse decomposition analysis, which assumes that a PPG pulse is composed of the original heart ejection blood wave and its reflections in arterial branchings. Among pulse decomposition wave functions that have been studied in the literature, Gaussian waves are the most successful ones. However, a more adequate pulse decomposition function could be found to improve blood pressure estimates. In this paper, we propose pulse decomposition analysis using hyperbolic secant (sech) waves and compare results with corresponding Gaussian wave decomposition. We analyze how the parameters of each of the two types of decomposition waves correlate with blood pressure. For this analysis, continuous blood pressure data and PPG data were acquired from ten healthy volunteers. The blood pressure of volunteers was varied by asking them to hold their breath for up to 60 s. The results suggested sech wave decomposition had higher accuracy in estimating blood pressure than the Gaussian function. Thus, sech wave decomposition should be considered as a more robust alternative to Gaussian wave pulse decomposition for blood pressure estimation models