When to measure flow and when to hold it constant

There is a time to measure blood flow and a time to hold it constant. Natural pressure/natural flow (pressure and flow measured), controlled pressure (flow measured), and controlled flow (pressure measured) are the possibilities, and there are definite indications for the use of each. For example, when the aim is to determine the natural hemodynamics of vascular beds, we operate in the natural pressure/natural flow mode. When the objective is to study autoregulation of blood flow, the controlled pressure mode is used. When the aim is to determine the activity of a substance on an in situ vascular bed, it is convenient to set flow constant and measure perfusion pressure, because in this mode blood concentrations can be predetermined and repeated as often as desired. In this review, the virtues and indications for each of these three possible modes of study are considered.