The reliability of automatic speech recognition systems depends mainly on the local perplexity of the language to recognise. In the framework of vocal command dialogue systems, we propose an approach based on pragmatic, mainly through a precise treatment of referential expressions, which we use in order to reduce dynamically the local perplexity that the recognition process is confronted with. Therefore, we take into account not only the left context of the current hypothesis but also the state of the application. The article justifies the architecture we propose, describes the treatments and shows the resulting reduction of perplexity when using contextual information as compared to that obtained when using only semantic ones. Keywords: vocal command system - natural language - pragmatics - language perplexity - reference calculus