[Measurement of cardiac output by invasive methods: methods and variability]

Invasive techniques for the measurement of cardiac output include the Fick method, the marker dilution method and radiological ventriculography. The principles, methods and accuracy of these techniques are described. The reference method remains that of Fick, despite its being less widely used now because of its complicated nature and its technical limitations (need for strict baseline conditions, precision of sampling). The thermodilution method is often performed. It is easy and provides reliable results, except in conditions of low cardiac output or when there is severe tricuspid insufficiency. Radiological ventriculography is used less for the measurement of cardiac output, since it is heavily dependent upon heart rate and theoretical geometrical models. It remains valuable for the quantification of mitral and aortic regurgitation.