ABSTRACT
One of the conditions for the application of Poiseuille’s equation (eqn 2.7) is that the flow is ‘steady’, i.e. that it does not vary with time (a condition also known as ‘stationary flow’). It has long been realized that the equation could not be applied to arterial flow, which is highly pulsatile. While flow measurements were confined to the determination of mean flow only with the use of such instruments as Ludwig’s stromuhr, the problem aroused little interest. This probably accounts for the complete neglect of pioneering mathematical analyses such as that of Witzig (1914). A somewhat empirical approach to the problem was made by Shipley et al. (1943) as a result of their detailed studies of pulsatile flow patterns in a wide range of arteries using an orifice meter. They could find no clear relation between the pressure and flow waveforms and, with considerable insight, suggested that flow was more closely related to the derivative of the pressure rather than to the pressure itself. They did not pursue the quantitative relationship and their comment seems to have escaped notice as it was generally assumed that an increase in pulsatile flow would cause an increase in pulsatile pressure. This is indeed true when there are no reflections in the system.
