Theoretical studies on the human arterial pressure and flow pulse by means of non-uniform tube model

[Principles of the genesis of pressure and flow pulse contours of the human carotid artery (author's transl)]

In order to clarify the genesis of the characteristic pressure and flow pulse contours of the common carotid artery of man, at first transcutaneous pulse recordings were carried out on healthy resting subjects during breathing of room air and of a mixture of 95% O2 and 5% CO2 (Carbogen). The latter was used to cause vasodilation of the cerebral resistance vessels and thus reducing the peripheral reflection coefficient. Secondly pulse constructions were performed by means of a theoretical inhomogeneous tube model using a digital computer. A side branch of this model represented the carotid artery. The pressure pulse of the aortic arch of the model was used as generating pulse for the carotid system. A satisfactory simulation of the carotid pressure and flow pulse contours under normal conditions and during peripheral vasodilation requires an intermediate reflection (positive for antegrade waves) at a site near the middle of the total transmission time of the carotid model system. The adequacy of the model is demonstrated by comparison of the recorded and constructed pulses.

The genesis of the pulse contours of the distal leg arteries in man

In order to clarify the genesis of the human pressure and flow pulse contours of the distal leg arteries, in particular the posterior tibial artery, pulse recordings were performed with transcutaneous techniques under normal conditions and in the state of strong vasodilatation (reactive hyperaemia) in the distal parts of the lower legs. From the experimental results it is concluded that the contour of the incident pressure wave arriving in the leg arteries is very similar to the pressure pulse contour of the abdominal aorta, while the resulting contour in the leg arteries is determined by this incident wave and superimposed reflected waves. The latter arise from positive reflection in the periphery of the lower legs. The travel in retrograde direction, are reflected negatively in proximal regions, particularly in the abdominal aorta, and appear again, with opposite sign, in the leg arteries. In addition, retrograde waves reflected positively at the aortic valve and then traveling in antegrade direction also influence the pulse contours. By considering this wave travel, the genesis of the characteristic contours of the pressure and flow pulses of the lower leg arteries is explained in a satisfactory way. This is demonstrated by a simplified graphical pulse construction as well as by the calculation of pulse contours on the basis of a theoretical tube model of the arterial system with the aid of a digital computer. The results of these calculations are discussed with respect to the findings of previous investigators who used analog and digital models of the arterial system.

The resolution of arterial pulses into forward and backward waves as an approach to the determination of the characteristic impedance

Pressure and flow recordings from a given site in an artery can be used for the resolution of the pulse wave into its forward and backward components if the characteristic impedance (Z) is known. The principle of this method was reported by von Kries in 1892. In the present work, the procedure is first applied to a theoretical, non-uniform tube model. The characteristic impedance of the model are assumed to be real magnitudes. From the results it is seen that the calculated backward wave provides a criterion which indicates whether the true value of Z actually has been used for the resolving procedure. If, as in the case of natural pulses, Z is not known, its value can be obtained by repeatedly performing the resolving procedure using various values of Z, and choosing the value of Z employed to calculate the backward wave which best fits the criterion. The method is demonstrated on pulses of the abdominal aorta and carotid artery of the dog. The results are compared with the average values of the input impedance in the higher frequency range.

The central arterial pulses. Experiments on a hybrid model of the heart and the arterial system

In order to examine the contours of central aortic and coronary flow pulses as well as those of pressure and flow pulses along the aorta, a hybrid model of the arterial system and the heart was designed. The digitally programmed model of the aortic system is an inhomogeneous transmission line with adjustable reflection factors at the end and at three intermediate locations. For the reflection factor at the entrance different values may be chosen for the ejection time and the diastole. The influence of a stenosis and of frequency-independent damping may be examined. The model of the ventricle is the analog solution of a system consisting of an internal isometric pressure source and an internal resistance and capacitance. The model of the coronary artery is the analog solution of a windkessel model of the system. The digital and analog models are interlocked by AD and DA converters. All programs can be executed in real time. The characteristic contour of the aortic flow is determined by the relation between the internal impedance of the ventricle and the magnitude of the characteristic impedance of the aorta. Furthermore, the influence of reflections within the arterial system is shown to be quite remarkable. Natural pressue pulses can be simulated by the model under normal and pathological conditions including aortic coarctation. The contour of the left coronary artery flow is determined on the one hand by the fraction of the ventricular pressure which acts as a counterpressure at the site of the peripheral resistance, and by the time constant of the coronary windkessel on the other hand.