Abstract: The invention estimates a cardiac performance value such as cardiac output (CO) and cardiac ejection fraction (EF) by sensing a downstream indicator concentration signal y(t) that corresponds to an indicator (preferably heat) signal x(t) injected upstream in a patient's blood channel. The signal x(t) is preferably generated as a series of alternating transitions between a high state and a low state, such as a PRBS signal. The signal y(t) is then divided into at least one sub-signal that is synchronous with x(t). A decay parameter &tgr; that minimizes a cost function that includes a time-domain channel relaxation model is then calculated. The cardiac performance value is then calculated based on this value for &tgr;. For each transition of the injected indicator signal, a corresponding segment of the indicator concentration signal is preferably isolated, and a segment relaxation parameter is then calculated.