Spatial modeling using mixed models: an ecologic study of visceral leishmaniasis in Teresina, PiauÝ State, Brazil

Most ecologic studies use geographical areas as units of observation. Because data from areas close to one another tend to be more alike than those from distant areas, estimation of effect size and confidence intervals should consider spatial autocorrelation of measurements. In this report we demonstrate a method for modeling spatial autocorrelation within a mixed model framework, using data on environmental and socioeconomic determinants of the incidence of visceral leishmaniasis (VL) in the city of Teresina, PiauÝ, Brazil. A model with a spherical covariance structure indicated significant spatial autocorrelation in the data and yielded a better fit than one assuming independent observations. While both models showed a positive association between VL incidence and residence in a favela (slum) or in areas with green vegetation, values for the fixed effects and standard errors differed substantially between the models. Exploration of the data's spatial correlation structure through the semivariogram should precede the use of these models. Our findings support the hypothesis of spatial dependence of VL rates and indicate that it might be useful to model spatial correlation in order to obtain more accurate point and standard error estimates.

Diagnostic electrocardiography in epidemiological studies of Chagas' disease: Multicenter evaluation of a standardized method

An electrocardiographic recording method with an associated reading guide, designed for epidemiological studies on Chagas' disease, was tested to assess its diagnostic reproducibility. Six cardiologists from five countries each read 100 electrocardiographic (ECG) tracings, icluding 30 from chronic chagasic patients, then reread them after an interval of 6 months. The readings were blind, with the tracings numbered randomly for the first reading and renumbered randomly for the second reading. The physicians, all experienced in interpreting ECGs from chagasic patients, followed printed instructions for reading the tracings. Reproducibility of the readins was evaluated using the Kappa (k) index for concordance. The results showed a high degree of interobserver concordance with respect to the diagnosis of normal vs. abnormal tracings (k= 0.66; SE 0.02). While the interpretations of some categories of ECG abnormalities were highly reproducible, others, especially those having a low prevalence, showed lower levels of concordance. Intraobserver concordance was uniformly higher than interobserver concordance. The findings of this study justify the use by specialists of the recording of readings method proposed for epidemiological studios on Chagas' disease, but warrant caution in the interpretation of some categories of electrocardiographic alterations

Clasificación de electrocardiogramas y sistema abreviado de derivaciones para encuestas de poblaciones en relacion con la enfermedad de Chagas / Electrocardiographic classification and abbreviated lead system for population-based studies of Chagas' disease

Se presenta una modificacion de la Clave de Minnesota para uniformar el registro de datos electrocardiograficos en encuestas sobre la cardiopatia chagasica y se evaluan los resultados obtenidos al aplicar dicha clave modificada. Tambien se describe un sistema electrocardiografico para el examen rapido de poblaciones grandes