Nowadays the “gold clinical standard” of hemodynamics diagnostic and cardiac output measurements is pulmonary artery catheterization by means of the Swan-Ganz catheter and thermodilution. The method itself is sensitive to numerous disturbances which cause inaccurate results. One of the well-known disadvantages of thermodilution is the overestimation of results at low values of cardiac output. This effect may concern the limited slew rate of the thermoelement mounted at the tip of the catheter. In this paper the relationship between the dynamic response of the thermoelement and the uncertainty of cardiac output measurements by means of thermodilution has been investigated theoretically and experimentally.