The determination of the optimal time of the patients in weaning trial process from Mechanical Ventilation (MV), between patients capable of maintaining spontaneous breathing and patients that fail to maintain spontaneous breathing, is a very important task in intensive care unit. Symbolic Dynamic (SD) and Neural Networks (NN) techniques were applied in order to develop a classifier for the study of patients on weaning trial process. The respiratory pattern of each patient was characterized through different time series. In order to reduce the dimensionality of the system Forward Selection is implemented, obtaining a classification performance result of 85,96 ±6,26% with 64 variables differentiating between 3 classes analyzed at same time.