1. A method of measuring cardiac output of a patient, the method comprising the steps of: a) measuring electrical impedance between two locations on the patient, for a predetermined first time interval, thereby obtaining an impedance signal over said first time interval, b) differentiating said impedance signal to obtain a signal representative of a first derivative of said impedance signal over said first time interval, c) computing a frequency transform of said first derivative of said impedance signal, for each of a plurality of discrete second time intervals, each second time interval being at least an order of magnitude shorter than said first time interval, each of said frequency transforms comprising a function of power versus frequency at one of said second time intervals, d) computing a plurality of,definite integrals of each of said frequency transforms, the definite integrals being taken over a predetermined range of frequencies, e) assembling a graph having an abscissa which comprises time, and an ordinate comprising one of said definite integrals which corresponds to a time represented by the abscissa, f) identifying points in time at which extrema appear in the graph produced in step (e), and determining corresponding points in time in said first derivative of said impedance signal, g) using said points in the first derivative of said impedance signal to determine a maximum excursion of said first derivative of said impedance signal and to determine a time interval corresponding to ventricular ejection time of the patient, and h) calculating cardiac output according to the maximum first derivative of said impedance signal and the ventricular ejection time determined in step (g).