Abstract: Impedance plethysmography is of medical importance because it can Perform cardiac measurements noninvasively and indicate possible diseases.We have developed a computer recognition system for automatic analysis of impedance waveforms and this paper gives a mathematical model used in our system for describing the impedance plethysmogram.The model is based on the attributed grammar which proves to be a quite general and powerful scheme for waveform recognition。A structure directed parsing and backtracking-based semantics computation algorithm is presented for analysing the attributed grammar,which is general enough and believed to be applicable to other waveforms when the structural and semantic information are important.