1. Cardiac haemodynamics were determined in 196 male untreated subjects, comprising 98 normotensive individuals and 98 hypertensive patients of the same age.
2. In order to study the haemodynamic change in hypertension, a new methodology was proposed. The whole population was classified by increasing diastolic pressures and divided into overlapping subgroups. From subgroup to subgroup, mean values of different haemodynamic variables were plotted against mean values of diastolic pressures. Three phases could be observed in the changes of these variables. In the first phase (for mean diastolic pressures under about 80 mmHg), the rise in pressure was associated with a decrease in blood volume and with an increase both in cardiac output and heart rate. In the second phase (for mean diastolic pressures between 80 mmHg and 95 mmHg approximately), heart rate, cardiac output and blood volume remained at the same values. In the third phase (for mean diastolic pressure above approximately 95 mmHg), heart rate remained elevated while the rise in pressure was associated with a decrease both in cardiac output and blood volume.
2. In the same subgroups, the correlations between cardiac output and heart rate and between cardiac output and blood volume were studied. The correlation between cardiac output and heart rate was significant only in normotensive ranges while the correlation between cardiac output and blood volume was significant only in hypertensive ranges.
3. Classical statistical tests, performed in distinct subgroups, confirmed the above observations on the changes of mean values and correlations.
5. The study provided evidence that cardiac output was predominantly controlled by neural mechanisms within the normotensive range and by volume mechanisms within the hypertensive range.