Haemodynamics in essential hypertension

The arterial blood pressure is mainly determined by the cardiac output and the total peripheral resistance. The blood pressure and the cardiac output are measured and the total peripheral resistance is calculated by dividing mean arterial blood pressure by cardiac output. This calculation is valid only for laminar flow in rigid straight tubes, but still this calculation is currently used to estimate the complex total peripheral resistance in man, although this means a rough approximation (Peterson, 196 1). In an extensive review of the haemodynamics of hypertension, Freis (1960) stated that according to the prevailing concept the primary and cardinal haemodynamic disturbance in practically all forms of hypertension was an increase in total peripheral resistance, probably caused by neurogenic or humoral mechanisms. The cardiac output was normal as long as heart failure was not present and, from a pathogenetic point of view, the heart was not of much interest. However, in the late 1950s and the early 1960s a few observations in experimental and human hypertension seemed to indicate that an increased cardiac output could play an important role in the starting phase of hypertension. The total peripheral resistance could become increased later as a sort of autoregulatory phenomenon. Ledingham & Cohen (1963) demonstrated that in some rats with experimental renal hypertension the cardiac output was increased when the pressure started to rise, although the total peripheral resistance was still normal. After some time the cardiac output fell and the total peripheral resistance increased, maintaining the