"Resistance" to left ventricular outflow studied in anesthetized dogs.

A simple method is proposed to determine resistance to left ventricular output from the evaluation of a parameter Z, given by the ratio of peak systolic pressure and the corresponding instantaneous blood flow. The method, derived from the basic equation of the Windkessel model, is applied to analyze pressure and flow data measured in the ascending aorta of anesthetized dogs. The Z values obtained in this way are found to be closely related to resistance calculated from the ratio of mean aortic pressure and mean flow over the cardiac cycle. Effects of abrupt changes of resistance obtained by removing an aortic constriction are also analyzed according to the same basic Windkessel equation and the results show the same close relationship to resistance.

[A graphic study of levers in the human body]. / Studio grafico delle leve nel corpo umano.

A graphical method is proposed which permits one to determine by a simple drawing procedure for any lever in the human body the intensity of the muscular force and of the force acting on the fulcrum (i.e. on the joint) and the direction of this latter. The method is compared with the conventional one, in which muscular force is first determined by a calculation in which the geometrical lever arms are measured, and then fulcrum force is obtained by means of a vector construction. The new graphic method permits one to simultaneously obtain the intensity and the direction of the forces acting on the lever, without measuring or computing torque values.

Simple physical model of coronary vasodilation.

The coronary diastolic pressure-flow relationship was studied in two groups of dogs; in one group coronary circulation was characterized by normal tone of vascular smooth muscle, while in the other group, complete relaxation of smooth muscle fibres was produced by intravenous infusion of dipyridamole. The coronary flow (CF) was measured in both groups for several values of mean aortic blood pressure (ABP), the variations being obtained by means of a 10 s arterial haemorrhage. The measured CF versus ABP data were found to be well represented by best fit curves calculated by power regression methods. These curves were quite different in the presence and absence of dipyridamole infusion. A simple physical model is proposed for analysis of these curves; the model is based on the Poiseuille equation, modified to take into proper account the variations of the vessel radii under different ABP values during ventricular diastole. These variations are expressed by means of Laplace and Hooke's laws by equating wall tension due to APB, to the sum of tensions due to elastic and smooth muscle forces. Analysis of CF versus APB curves, performed on the basis of this model, shows that to account for the large change of coronary bed conductance observed under dipyridamole infusion one must assume not only that the smooth muscle tone is absent but also that some relevant variations occur in the whole coronary bed, thus pointing to possible recruitment of new blood vessel paths.

The effect of the coronary vascular wall on the changes in the coronary vascular resistance during a sudden reduction and recovery of the aortic blood pressure.

To demonstrate the effect of elastic and myogenic properties of the vessel wall on the mean coronary vascular resistance during ventricular diastole (m.d.c.r.), this resistance was assessed in twenty dogs anaesthetized with sodium pentobarbitone during a 10 s period of arterial haemorrhage and 20 s of recovery from this haemorrhage. In a first stage of the study ten dogs were investigated following sectioning of the vagal nerves in the neck, beta-blockade, and ligation of the carotid arteries. During haemorrhage and reduction in aortic blood pressure (A.B.P.) in these dogs, there was an initial rise (at 2 s after the start of the haemorrhage) in m.d.c.r. which quickly decreased to a value similar to that obtained during the control period. After the abrupt cessation of haemorrhage, and a rise in A.B.P., there was an initial decrease in m.d.c.r. which eventually increased to the control levels. In the second stage of study identical patterns of changes were obtained in a group of five dogs in which the cervical vagi were not sectioned and beta-blocking agents not given. In another group of five dogs, during infusion of dipyridamole to force maximal coronary vasodilation, the increase in m.d.c.r. was maintained throughout the period of haemorrhage and reduction in A.B.P., and m.d.c.r. did not decrease below the control values following the abrupt cessation of haemorrhage and a rise in A.B.P. The findings in the dogs without dipyridamole infusion suggested that the pattern of changes in m.d.c.r. was due to changes in the perfusion pressure and their effect on the elasticity and myogenic properties of the coronary vascular wall. This suggestion was supported in the second stage of the study, when the reductions in m.d.c.r. were abolished by forcing coronary vasodilation using the infusion of dipyridamole.