Monitorización del patrón ventilatorio (PV) mediante tomografía por impedancia eléctrica (TIE) en paciente con enfermedad pulmonar obstructiva crónica (EPOC)

Abstract

El objetivo de esta investigación fue obtener un modelo matemático de calibración que permita estimar cuantitativamente el patrón ventilatorio (PV), en términos de volumen, en un grupo de voluntarios sanos y pacientes con enfermedad pulmonar obstructiva crónica (EPOC). Todo ello a partir de la información regional de la variación temporal de impedancia eléctrica obtenida con el equipo TIE4sys (sistema de tomografía por impedancia eléctrica), prescindiendo de la calibración individual con un neumotacómetro. Para lograr este objetivo la presente investigación se dividió en dos etapas.
 La primera etapa consistió en obtener y evaluar un grupo de ecuaciones de calibración utilizando 7 regiones de interés (ROI) en un grupo de voluntarios sanos (hombres y mujeres). La primera ROI incluyó todos los elementos de la imagen TIE. Dos ROI fueron definidas usando solamente los pixeles con variación superior al 70% y 30% de la máxima (método porcentajes). Dos fueron obtenidas definiendo el contorno de forma manual (método mouse) usando como referencia las ROI anteriores. Las dos últimas fueron máscaras elípticas con excentricidad 0,8 y eje mayor de 32 y 25 pixeles, respectivamente. En el caso de hombres, la media del coeficiente de determinación (R2) de las ecuaciones de calibración fue de 93 ± 5%. Los parámetros comunes en todas ellas fueron el pliegue subescapular y peso. Las menores dispersiones en la estimación de volumen se obtuvieron usando todos los elementos de la imagen, la ROI reducida al 70% con el método porcentajes y ambas máscaras elípticas. El valor medio de dichas dispersiones fue del 8%. En el caso de mujeres se analizaron 4 grupos: el grupo total y 3 que resultaron de estratificar la muestra en función al IMC. La media del R2 de todas las ecuaciones de calibración fue del 58 ± 12%. El parámetro más significativo en todas ellas fue la edad. En todos los grupos de mujeres se obtuvieron dispersiones en la estimación de volumen muy elevadas cuyo valor medio fue del 28 ± 5%. The goal of this research is to obtain a mathematical calibration model to estimate, in a quantitative way and in terms of volume, the ventilatory pattern in a group of healthy volunteers and chronic obstructive pulmonary disease (COPD) patients. Several models were built from the changes on the regional electrical impedance obtained by TIE4sys (electrical impedance tomograph), without requiring an individual calibration with a pneumotachometer. In order to achieve this aim, the research was divided into 2 parts. The first part involved obtaining and assessing a set of calibration equations, using 7 regions of interest (ROI), in a group of healthy volunteers (males and females). The first ROI included all EIT image elements. Two ROI were defined by only including the pixels displaying changes larger than 70% and 30% of the maximum value of pixel change (percentage method). Two were determined by defining the region boundaries manually (mouse method), using as a reference the above ROI. The remaining two were elliptical masks with eccentricity of 0,8 and major axis of 32 and 25 pixels, respectively. In the case of healthy males, the mean value of the determination coefficients (R2) of all calibration equations was 93±5%. The parameters appearing in all of them were the subscapular skinfold and the weight. The smallest volume estimation dispersions obtained with TIE4sys were those using all elements of ROI, the reduced ROI by percentage method to 70% and both elliptical masks. The mean value of these volume estimation dispersions was of 8%. The group of female volunteers was further divided in 4 groups, one including all females and 3 groups obtained from the stratification of the sample according to BMI. The mean value of R2 of all calibration equations was of 58±12%. The most significant parameter involved in all of them was the age. The dispersion in the estimation of volume for all groups was of 28±3%.The second part involved the assessment of the previous healthy male calibration equations in a group of COPD patients. Preliminary results showed an underestimation of volume estimations obtained by TIE4sys. Due to this fact, a set of adjustment mathematical models were obtained in order to improve the estimation of volume. The mean value of R2 of all adjustment equations obtained in this group was 87±7%. The most significant parameters involved in these equations were subscapular skinfold and weight. The smallest volume estimation dispersions by TIE4sys were obtained by using the elliptical mask with major axis of 25 pixels. The mean value of these dispersions was 14±1%. Furthermore, a subgroup of patients for which all pulmonary function tests (PFT) were available, was analyzed. The mean value of R2 of all adjustment equations obtained in this sample was 88±6%. From all PFT parameters, only the pressure of carbon dioxide in arterial blood (PCO2) was involved in 4 of 7 adjustment equations obtained in this group. The mean value of volume dispersions obtained by TIE4sys, using this 4 equations, was of 19±6%. This value is higher than the one obtained for the whole group. Conclusion. The mathematical adjustment of the calibration equations for healthy males was better than the one obtained for females. This fact highlighted that the anthropometric parameters used in the male group were statistically significant, while they were not for females. For this reason, larger volume estimation dispersions were obtained in this latter group. In the case of COPD patients, all healthy male calibration equations were improved by a mathematical adjustment model. In this case, the smallest volume estimation dispersions were obtained by using the elliptical mask with major axis of 25 pixels. The results obtained in the group of healthy males and COPD patients have been encouraging and it leads us to think that EIT technique can become part of PFT. In the case of females additional efforts should be made to find a calibration equation