Determination of the efficacy of the mechanism changing the relative rate of blood flow through cutaneous and deep lying veins with temperature.

An analytical model of the circulation in the upper extremities of man is used; it consists of one artery, a deep vein lying close to the artery and returning a fraction p of the arterial blood flow, and a cutaneous vein carrying the fraction (1 - p). Using the model, a method is given for finding, for any subject, the efficiency of the mechanism which changes the circulation pattern with temperature. A patient's extremity is immersed in water at 25 and 10 degrees c and the rate of blood flow and rate of heat loss from that flow are measured. Computation of p for these two temperatures is reduced to the solution of two linear equations. The method is applied to 11 subjects and gives good agreement with results obtained by a more elaborate method.

Mathematical circulation model for the blood-flow-heat-loss relationship in the rat tail.

A mathematical model for the heat-loss-blood-flow relationship is developed for the rat tail. When supplied with experimental values of heat loss and blood flow, the model allows one to compute the distribution of flow in deep and cutaneous vessels as a function of body core and tail temperature and to determine the savings in heat loss that result from alterations in the pattern of circulation and from counter-current heat transfer. Blood flow in the cutaneous and deep lying veins of the tail is controlled by both central and local temperatures and increases fairly linearly with deep body temperature. However, the distribution of blood flow in the tail is controlled only by local tail temperature and is independent of deep body temperature. The change in venous distribution of flow has a great impact on the conservation of heat and can reduce the heat loss from the circulating blood by more than 50% when venous return is directed to deep lying veins. On the other hand, counter-current heat transfer is of only minor importance in the control of heat loss from the tail, resulting at most in a 10% saving of heat loss, and that only at the smallest rate of blood flow.

Elastic properties of human aortas in relation to age and atherosclerosis: a structural model.

A new structural model is described for the tension-radius relationship of blood vessels, taking into account their mechanically important constituents: collagen, elastin and smooth muscle. The model has four characteristic parameters: EC, the Young's modulus of the collagen fibres; ESE, the Young's modulus of the combined smooth-muscle/elastin network; epsilon mu, the amount of strain at which the high stiffness region on the tension-radius curve is reached, and eta an indicator for the degree of collagen fibre stretching. The structural stiffness of the wall constituents is reflected by EC and ESE whereas the global stiffness of the entire blood vessel is described by epsilon mu and eta. All these elasticity parameters are pressure independent, in contrast to generally quoted values for the incremental modulus or vascular compliance which are strongly pressure dependent. Hence, an objective comparison of the mechanical properties for various types of blood vessel, based on the present model parameters, has been made possible. The model was successfully fitted to tension-radius data of 65 human aortas, age range 30-88 years, with moderate or severe atherosclerosis. The structural as well as the global stiffness changes with age, e.g. collagen stiffness shows a ninefold increase over 60 years. Global stiffness depends on atherosclerosis.

A thermodynamically consistent constitutive equation for the elastic force-length relation of soft biological materials.

Starting from the laws of thermodynamics of reversible processes, a temperature-dependent constitutive equation is derived for the elastic force-length relation of soft biological tissues. These tissues are composed of a network of fibres (mainly collagen). The equation is based on a model which uses a simplified two-dimensional representation of the alpha-helix of collagen.

In vivo noninvasive determination of abnormal water diffusion in the rat brain studied in an animal model for multiple sclerosis by diffusion-weighted NMR imaging.

In vivo NMR images of the rat brain were obtained using a NMR microscope (7 T) from SMIS (England). Four animals were imaged every 3-4 days during a pathological cycle (starting after induction and up to 37 days) of experimental allergic encephalomyelitis (EAE), an animal model for multiple sclerosis. The EAE rats were weighted and clinically scored daily. We aimed at measuring the apparent diffusion coefficient (ADC) or the mean diffusivity (D) with a high accuracy, and within a reasonable experimental time frame, because of the clinical situation of the animals. Therefore, we fitted the ADC value from five diffusion-weighted images--with an experimental time of 17 min/image--and chose to apply diffusion-sensitizing gradients in a direction intersecting all fiber directions of the external capsule. With this, we also obtained high b-values. For the control rats, we obtained a statistical mean value of ADC = (388 +/- 16) 10(-12) m2/s for gray matter and a statistical mean value of (D) of (750 +/- 30) 10(-12) m2/s for white matter, measured in the external capsule. For the EAE rats, no alterations in ADC values of gray matter with increasing clinical scores were observed. Concerning white matter, as determined in the external capsule, there were no significant differences in (D) values between controls and EAE rats before clinical signs occurred. However, when clinical signs were observed, we could demonstrate a significant positive correlation between the clinical score and the (D) values in the external capsule. As the clinical signs became more severe, we measured a rise in water diffusion (increase in (D)) in the external capsule, which was accompanied by the occurrence of interstitial edema as revealed by a complementary histological study.

Venous occlusion plethysmography with mercury-in-rubber strain gauges.

The Whitney plethysmograph consists of a mercury-in-rubber strain gauge together with a Wheatstone bridge. It can be used to measure, in a non-invasive way, the blood flow in extremities. A simple theoretical and practical study is lined out, showing that the apparatus is fully acceptable for clinical use. (Acta anaesth. belg., 1980, 31, 5-14).

Circulatory changes during a hypotensive technique of anesthesia.

The blood circulation and the oxygen consumption of the tissues during general hypotensive anesthesia for stapes surgery is quantitatively studied on 40 patients. The rate of oxygen consumption of the whole body and of the tissue of an upper extremity is determined together with the rate of blood flow in that extremity. A series of other parameters such as the PaCO2 and the acid-base balance of arterial and venous blood, the blood pressure, the pulse frequency and the temperature are also measured. Most measurements are performed before premedication, after premedication and after surgery. The state of the patient in the immediate postoperative period is completely comparable with the peroperative anesthetic state. Some of the parameters mentioned above are measured also on a series of 8 normal persons at awake and at asleep state. The numerical results show clearly that the used type of general anesthesia "Protected Sleep" obeys the demands necessary for performing microsurgery of the ear in a successful way. (Acta anaesth. belg., 1980, 31, 15-27).

An electrical analogue for a pressure-controlled, fluid flow generator for arterial blood-flow simulation.

To study the human arterial system, we constructed a pressure-controlled, fluid flow generator [1] that allows simulation of pressure and flow waves in a hydraulic system in which the arteries are simulated by collapsible tubes. In this paper we elaborate an electrical analogue of this flow generator. The analogue circuit contains only five electrical components: a source resistance and inductance, and a downstream resistance, inductance and capacitance. The values of the components are obtained from a function fit to the pressure and flow waves generated during steady and pulsatile flow measurements. The pulsation and relaxation times calculated from the analogue model are compared with experimental values. This electrical analogue allows computer simulation of the arterial pressure and flow waves.

Temperature dependence of the circulation pattern in the upper extremities.

1. The rate of blood flow D through the hand and the rate of heat H transferred by that blood flow into the hand were measured on eleven subjects between 18 and 22 years old. The hand was immersed in water, at temperatures ranging from 10 to 40 degrees C. 2. The rate of blood flow was measured with a water-air displacement plethysmograph, which was adapted also to give heat loss measurements. 3. Comparison of the relative values of D and H at different temperatures gives information about the change in distribution of blood flow with temperature. On decreasing water temperature, the change in distribution starts at about 25 degrees C and continues gradually to about 15 degrees C, suggesting that more and more blood is shunted to deep veins. For one of the subjects, no change in distrubiton pattern was found.

Analytical model for the temperature dependence of the circulation pattern in upper extremities.

An analytical model for the circulation in the upper extremities of man is studied under the condition that the hand is immersed in water at different temperatures. The arterial and venous system is represented by one artery and two veins. One of the veins lies close to the deep lying artery; the other one is a cutaneous vein. The model permits a qualitative description of the dependence on temperature of the rate of heat loss of the hand as a function of the rate of blood flow and the water temperature, by means of parameters which determine the behaviour of the considered system. One parameter accounts for the relative amount of the rate of blood flow along deep and cutaneous veins. Comparison with experiment leeds to the following results. At water temperatures 0Hw higher than 25 degrees C about 10% of the blood is returning through the deep vein; for water temperatures lower than 13 degrees C this percentage becomes about 80%. In between the circulation pattern changes gradually. The influence of the counter-current effect, between the artery and the deep vein, on the heat loss of the hand is studied. The arterial blood temperature at the entrance in the hand is compared with the available experimental results.

Body temperature control of rat tail blood flow.

Tail blood flow (BF) and heat flow (HF) were measured in five albino rats during transients in rectal temperature (Tre) caused by body heating at rest. During heating, tail temperature (Tt) was kept at 15, 20, 25, 30, 35, or 42 degrees C by enclosing the tail in a water-perfused tube. Thermal conductance (K) was computed as HF/(Tre-Tt). BF was measured by venous occlusion plethysmography. Heating caused a rise in Tre that was accompanied by proportional increases in both K and BF. The ratio R = K/BF represents conductance per unit BF and reflects the amount of heat exchange for a given BF. R can thus be used to estimate the distribution of BF within the tail. R was independent of Tre at all Tt, indicating that BF distribution is controlled by the tail. R was low at low Tt and rose at higher Tt. This suggests that at low Tt, blood flows primarily in central veins of the tail and at higher Tt blood flows in peripheral tail veins.

Two-component versus admittance tympanometry.

The probe tone frequency of electro-acoustic bridges has a direct effect on the shape of the tympanograms obtained. At higher frequencies (800 Hz) typical patterns are generated for eardrum abnormalities and ossicular discontinuities. The 625- and 660-Hz frequencies sometimes prove to be not quite high enough to obtain the expected 'W' or undulating admittance tympanograms. Our investigation proves that much more certainty is given even at 660-Hz probe tone when components B and G are both recorded. The same information for ossicular disruption is found in an admittance (Y) together with a new kind of tympanogram, namely the phase tympanogram.