In the framework of the general problem of diagnosing pacemakers, the problem of estimating the propagation velocity of a transmembrane potential from its measurements is considered. Based on the mathematical model of a pacemaker, which is represented by a two-dimensional non-linear dynamic Van der Pol-Duffing system, methods for the synthesis of state observers of low and full order with non-linear and linear corrective actions are developed. The results of a comparative analysis of the quality of transmembrane potential velocity estimation using various observers and differentiating filter are presented. The comparison results are based on numerical modeling in MATLAB SIMULINK.