Measurement of volumetric cardiac output is technically difficult and poorly reproducible, and the technique is unsuitable for use in the majority of patients in whom a knowledge of overall cardiovascular function would be desirable. Interpretation of a single measurement in a patient is difficult because little is known about volumetric cardiac output in normal subjects and the available techniques of measurement are very complex or invasive. Linear cardiac output, in contrast, can be simply, non-invasively, and reproducibly measured at the bedside by doppler ultrasound. The systolic-velocity integral of a single heart beat (stroke distance) multiplied by the heart rate gives minute distance; this may be envisaged as the distance travelled by mid-stream blood in the aorta in a minute. Providing there is no aortic disease, minute distance may be used as an absolute indication of cardiac output and for following serial changes.