Pulmonary vascular resistance in the fluorocarbon-filled lung

Pulmonary vascular resistance was investigated in the fluorocarbon-filled lung in an in situ isolated lung preparation. Lungs were perfused at constant flow (100 ml X min-1 X kg-1) with whole blood from a donor cat. left atrial pressure was held constant at zero pressure. Measurements of pulmonary arterial pressure enabled calculation of pulmonary vascular resistance. Regional changes in pulmonary blood flow were determined by the microsphere technique. During quasi-static deflation over a range of 0-30 mmHg, dependent alveolar pressure was consistently greater for a volume of fluorocarbon than for gas, with each pressure-volume curve for the fluorocarbon-filled lung shifted to the right of the curve for the gas-filled lung. In turn, pulmonary vascular resistance was found to increase linearly as a function of increasing alveolar pressure, independent of the medium in the lung. Thus, for a given volume, pulmonary vascular resistance was consistently greater in the fluorocarbon-filled lung compared with the gas-filled lung. This increase in pulmonary vascular resistance was accompanied by a redistribution of pulmonary blood flow in which blood flow to the dependent region was decreased in the fluorocarbon-filled lung compared with the gas-filled lung. Conversely, the less-dependent regions of the lung received a relatively greater percentage of blood flow when filled with fluorocarbon compared with gas. These findings suggest that pulmonary vascular resistance is increased during liquid ventilation, largely as the result of mechanical interaction at the alveolar-vascular interface.     

Cardiovascular alterations in Macaca monkeys exposed to stationary magnetic fields: Experimental observations and theoretical analysis

Simultaneous measurements were made of the electrocardiogram (ECG) and the intraarterial blood pressure of adult male Macaca monkeys during acute exposures to homogeneous stationary magnetic fields ranging in strength up to 1.5 tesla. An instantaneous, field strength-dependent increase in the ECG signal amplitude at the locus of the T wave was observed in fields greater than 0.1 tesla. The temporal sequence of this signal in the ECG record and its reversibility following termination of the magnetic field exposure are consistent with an earlier suggestion that it arises from a magnetically induced aortic blood flow potential superimposed on the native T-wave signal. No measurable alterations in blood pressure resulted from exposure to fields up to 1.5 tesla. This experimental finding is in agreement with theoretical calculations of the magnetohydrodynamic effect on blood flow in the major arteries of the cardiovascular system.     

Assessment of dynamic changes in cerebral autoregulation.

Cerebral autoregulation (CA) is a control mechanism that adjusts cerebral vasomotor tone in response to changes in arterial blood pressure (ABP) to ensure a nearly constant cerebral blood flow. Patient treatment could be optimized if CA monitoring were possible. Whereas the concept of static CA assessment is simply based on comparison of mean values obtained from two stationary states (e.g., before and after a pressure change), the evaluation of dynamic CA is more complex. Among other methods, moving cross-correlation analysis of slow waves in ABP and cerebral blood flow velocity (CBFV) seems to be appropriate to monitor CA quasi-continuously. The calculation of an "instantaneous transfer function" between ABP and CBFV oscillations in the low-frequency band using the Wigner-Ville distribution may represent an acceptable compromise in time-frequency resolution for continuous CA monitoring.     

Skin blood flow in the Wistar-Kyoto rat and the spontaneously hypertensive rat

• 1.1. Using laser Doppler techniques in man, we have previously demonstrated differences in skin blood flow properties at sites with primarily nutritive (NUTR) perfusion, such as the elbow or knee, as compared to sites such as the finger pulp, with predominantly arteriovenous anastomotic (AVA) perfusion.
• 2.2. Basal and heat stimulated flow is greater at AVA sites. In man, blood pressure changes are reflected primarily by changes at AVA rather than NUTR sites.
• 3.3. These blood pressure induced changes affect the red blood cell velocity (VEL) component at AVA sites more than microvascular volume (VOL).
• 4.4. Given these findings in man, we decided to compare skin blood flow properties in a suitable animal model.
• 5.5. We chose the Wistar-Kyoto (WKY) and Spontaneously Hypertensive Rat (SHR) strains, in view of the marked difference in systemic blood pressure in these two related strains.
• 6.6. Skin blood flow varied considerably at different skin sites in the rats. Skin sites with hair covering, on the back and at the base of the tail, showed low basal and heat stimulated blood flow.
• 7.7. In contrast, the plantar surface of the paw behaved similarly to the finger or toe pulps in man, with 3–4-fold higher basal flow than the hair covered areas and a 7–8-fold rise with local heating to 44°C.
• 8.8. Furthermore, there was a 25% greater blood flow at the plantar paw surface in the SHR rats as compared to the WKY rats, corresponding to the 25% higher systemic blood pressure in these animals.
• 9.9. The heat induced increase in flow at the plantar surface of the paw was primarily a result of a marked increase in VEL rather than VOL.
• 10.10. The higher flow at this site in SHR as compared to WKY rats was likewise ascribable to an increase in VEL, VOL being equivalent in the two strains.

     

Mouth pressure curve on abrupt interruption of airflow during forced expiration

An attempt was made to investigate how the mouth pressure curve represents the process of air flowing into the collapsed segment downstream to the choke point when the airflow is abruptly interrupted at the mouth during forced expiration. Immediately after the interruption of airflow, the mouth pressure suddenly increased (phase 1), followed by a slower rise in pressure (phase 2) within approximately 100 ms until the pressure reached the alveolar pressure. The pleural and alveolar pressures remained constant during this process. The first phase of the abrupt rise represented the pressure induced by the instantaneous interruption of the airflow itself. Analysis of the supramaximal flow (Vsupramax) observed after resumption of the airflow suggested that the choke point remained constant during the second phase of the mouth pressure after interruption of maximal flow (Vmax). From these results, examination of the second phase of the mouth pressure curve may provide useful information about the downstream segment of the airway.     

Instantaneous blood flow, impedance and elastic properties computed in man from aortic pulse waves

A mathematical model of the pressure-flow relationship in the arterial circulation and its possible use in routine hemodynamics in man are described. The instantaneous blood flow velocity in the ascending aorta can be calculated from two pressure curves simultaneously recorded 5 cm apart. The mechanical aortic input impedance is computed from the recorded pressure and the calculated blood flow velocity curves. Projection of the pulse waves on a time-length plane leads to the determination of the pulse wave velocity and then an estimation of the elastic modulus of the aortic wall.     

Systems engineering analysis of aortic root blood pressure

This paper describes the aortic blood pressure as a function of aortic blood flow and the parameters of the blood and circulatory system. The method of performance involves the analogue of a multi-branched electrical to hydraulic transmission line applying graphical convolution to the blood flow-transform impedance relationship resulting in a theoretical pressure curve for the infinite aorta. The difference between the single pressure pulse and the computed adjusted infinite aorta pressure curve is described as the reflected wave. This reflected wave is then shown to be of reasonable configuration in time and velocity. The blood pressure is thus finally described completely by the physical parameters of the blood and the circulatory system and the blood flow.     

The inverse Womersley problem for pulsatile flow in straight rigid tubes

In this study a numerical solution for the problem of pulsating flow in rigid tubes is described. The method applies to the case of known flow rate waveform, as opposed to Womersley solution where the pressure gradient was the known quantity. The solution provides the pressure gradient and wall shear stress waveforms as well as the instantaneous velocity profiles. Results show that the method can be used to study the blood flow characteristics in large arteries.     

Some factors affecting oxygen uptake by red blood cells in the pulmonary capillaries

In this study we investigate the equations governing the transport of oxygen in pulmonary capillaries. We use a mathematical model consisting of a red blood cell completely surrounded by plasma within a cylindrical pulmonary capillary. This model takes account of convection and diffusion of oxygen through plasma, diffusion of oxygen through the red blood cell, and the reaction between oxygen and haemoglobin molecules. The velocity field within the plasma is calculated by solving the slow flow equations. We investigate the effect on the solution of the governing equations of: (i) mixed-venous blood oxygen partial pressure (the initial conditions); (ii) alveolar gas oxygen partial pressure (the boundary conditions); (iii) neglecting the convection term; and (iv) assuming an instantaneous reaction between the oxygen and haemoglobin molecules. It is found that: (a) equilibrium is reached much more rapidly for high values of mixed-venous blood and alveolar gas oxygen partial pressure; (b) the convection term has a negligible effect on the time taken to reach a prescribed degree of equilibrium; and (c) an instantaneous reaction may be assumed. Explanations are given for each of these results.     

Theory and design of disposable clinical blood viscometer

A disposable clinical whole blood viscometer which can produce viscosity measures over a wide range of shear rates in a single rapid determination has been developed and is currently under test. The design is based upon the time varying flow of blood through a capillary. The flow is driven by the pressure in a fixed volume air chamber and transmitted to the sample through a compliant membrane. The time varying pressure in the air chamber is measured by a suitable transducer. The instantaneous shear stress of the blood in the capillary is proportional to the air pressure, while the instantaneous shear rate is proportional to the pressure-time derivative. Proper design ensures that the system operates as a first order dynamic system with flow resistance entirely determined by the nonlinear sample viscosity. By constructing the air chamber in two parts coupled by a quick disconnect fitting the design can allow for the blood-containing part of the instrument to be discarded, eliminating handling and cleaning of blood contacted components. The entire determination is completed in less than a minute, so that anticoagulants are not necessary. Tests on a prototype show that the instrument gives results in excellent agreement with those obtained on a cone-plate rheogoniometer.     

Source parameters of the left ventricle related to the physiological characteristics of the cardiac muscle.

An attempt is made here to correlate the physiological muscle parameters with the dynamic source parameters of the left ventricle (LV), i.e. the source (isovolumic) pressure Po and the source (internal) resistance, Rs. The internal resistance is described here as a time-dependent parameter, corresponding to the pressure drop (from the theoretical instantaneous isovolumic pressure) associated with the instantaneous ejection flow rate. The source pressure, which relates to the muscle stress and the ventricular volume, is represented by the time-varying elastance concept and a spheroidal model relating the average wall stress to LV pressure. Linear and exponential force-velocity relationships (FVR), expressed in stress-strain rate terms, are compared. Two possible characteristics of the dynamic FVR in the partially active state, based on either a parallel or a fanlike shift of the stress-strain rate curve, are studied by utilizing simple analytical models as well as a computer simulation model. Comparing the calculated results with experimental data indicates that the dynamic FVR shift occurs in a fanlike pattern in which the maximum strain rate remains constant throughout the cycle. This pattern of the FVR shift is consistent with experimental data that show that the internal resistance is linearly related to the instantaneous isovolumic pressure. The analysis also indicates that the difference between the hyperbolic and linear FVR is rather minor, and in spite of some effects on the ejection pattern and the value of Rs, the functional shape has no effect on the global LV characteristics, such as the ejection fraction and stroke volume.     

Multimodal pressure-flow method to assess dynamics of cerebral autoregulation in stroke and hypertension

Background

This study evaluated the effects of stroke on regulation of cerebral blood flow in response to fluctuations in systemic blood pressure (BP). The autoregulatory dynamics are difficult to assess because of the nonstationarity and nonlinearity of the component signals.

Methods

We studied 15 normotensive, 20 hypertensive and 15 minor stroke subjects (48.0 ± 1.3 years). BP and blood flow velocities (BFV) from middle cerebral arteries (MCA) were measured during the Valsalva maneuver (VM) using transcranial Doppler ultrasound.

Results

A new technique, multimodal pressure-flow analysis (MMPF), was implemented to analyze these short, nonstationary signals. MMPF analysis decomposes complex BP and BFV signals into multiple empirical modes, representing their instantaneous frequency-amplitude modulation. The empirical mode corresponding to the VM BP profile was used to construct the continuous phase diagram and to identify the minimum and maximum values from the residual BP (BP_R) and BFV (BFV_R) signals. The BP-BFV phase shift was calculated as the difference between the phase corresponding to the BP_R and BFV_R minimum (maximum) values. BP-BFV phase shifts were significantly different between groups. In the normotensive group, the BFV_R minimum and maximum preceded the BP_R minimum and maximum, respectively, leading to large positive values of BP-BFV shifts.

Conclusion

In the stroke and hypertensive groups, the resulting BP-BFV phase shift was significantly smaller compared to the normotensive group. A standard autoregulation index did not differentiate the groups. The MMPF method enables evaluation of autoregulatory dynamics based on instantaneous BP-BFV phase analysis. Regulation of BP-BFV dynamics is altered with hypertension and after stroke, rendering blood flow dependent on blood pressure.

     

Blood pressure and mesenteric blood flow in rats during the infusion of biogenic amines. Effect of supralethal irradiation]

The action of biogenic amines (noradrenaline, dopamine), infused at different concentration into the aorta of the urethane anesthetized control and irradiated rats for 2 min., was followed on the basis of systemic blood pressure and mesenteric blood flow. The mesenteric blood flow was measured by means of an electromagnetic flow meter. The changes observed i.e. after dopamine an increase in pressure and flow, after noradrenaline an increase in pressure and a decrease in flow with an increase after infusion had been stopped, correspond to those obtained in larger animals. In many, but not in all cases, the response is proportional to the log of the concentration of the amine infused. Irradiation with 2 kR, i.e. a dose which causes the animals to die from the gastrointestinal syndrome after 3 days, modified the response to dopamine and noradrenaline. The changes are, for noradrenaline, a greater pressure and a lower flow responses and for dopamine a greater pressure response at low and middle doses.     

Effects of gastric distension on the cardiovascular system in rainbow trout (Oncorhynchus mykiss)

When animals feed, blood flow to the gastrointestinal tract increases to ensure an adequate oxygen supply to the gastrointestinal tissue and an effective absorption of nutrients. In mammals, this increase depends on the chemical properties of the food, as well as, to some extent, on the mechanical distension of the stomach wall. By using an inflatable nitrile balloon positioned in the stomach, we investigated the cardiovascular responses to mechanical stretch of the stomach wall in rainbow trout (Oncorhynchus mykiss). Distension with a volume equivalent to a meal of 2% of the body mass increased dorsal aortic blood pressure by up to 29%, and central venous blood pressure increased transiently nearly fivefold. The increase in arterial pressure was mediated by an increased vascular resistance of both the systemic and the intestinal circulation. Cardiac output, heart rate, and stroke volume (SV) did not change, and only transient changes in gut blood flow were observed. The increase in arterial pressure was abolished by the alpha-adrenergic antagonist prazosin, indicating an active adrenergic vasoconstriction, whereas the venous pressor response could be the consequence of a passive increase in intraperitoneal pressure. Our results show that mechanical distension of the stomach causes an instantaneous increase in general vascular resistance, which may facilitate a redistribution of blood to the gastrointestinal tract when chemical stimuli from a meal induce vasodilation in the gut circulation. The normal postprandial increase in gut blood flow in teleosts is, therefore, most likely partly dependent on mechanical stimuli, as well as on chemical stimuli.     

Intrinsic indices of the left ventricle as a blood pump in normal and infarcted left ventricles

To assess the left ventricle as a blood pump, data are collected from contrast angiograms and analysed by computer, using two-dimensional finite element analysis, to provide instantaneous distributions of intra-LV flow and differential pressure during the diastolic and ejection phases. Characteristic indices are derived for normal and infarcted LVs, and for cases before and after administration of nitroglycerin. These indices may be used to assess the degree and nature of dysfunction in coronary artery disease.     

A new theory of pulmonary blood flow in zone 2 condition

In pulmonary blood flow, if arterial pressure (Pa) and pleural pressure (Ppl) were fixed, the flow increases with decreasing venous pressure (Pv) only when venule pressure (Pven) greater than airway pressure (PA) (i.e., in zone 3). When Pven less than or equal to PA (i.e., in zone 2), with Pa fixed, the flow decreases with decreasing Pv. The pressure-flow relationship has a hysteresis loop. This phenomenon can be explained by conservation of mass and momentum and the morphology and material properties of the lung, including the observation that reseparation of adhered cells requires an extra force. The key mathematical observation is that the solution h = 0 (h being the blood sheet thickness in the interalveolar septa) can coexist with the solution h not equal to 0 in zone 2 condition, resulting in "patchy" filling of the alveolar walls. When h = 0, the sheet is collapsed and endothelial cells adhere. Experimental results show that the adhered endothelial cells do not reseparate by raising Pv in zone 2 but can be accomplished under zone 3 condition.     

Flow measurements in a highly curved atherosclerotic coronary artery cast of man.

Flow visualization and wall pressure measurements were made in a polyurethane cast of a cadaver coronary artery with a significant "s" shaped reverse curvature. A sucrose solution was used to simulate the kinematic viscosity of blood, with flow rates in the physiologic range. Flow visualization demonstrated significant secondary flow patterns in the wall vicinity, which increased with increasing Reynolds number. Random dye dispersion was observed at a Reynolds number of about 400, but not at 200. Dye filament patterns in the transition between the first and second curved region were predominantly influenced by the second curved region at lower Reynolds numbers, and by the first curved region at higher Re. Local wall pressure measurements demonstrated a significant centrifugal effect with large radial pressure differences across the casting. Flow resistances for the casting were considerably greater than reference Poiseuille flow values, and increased further with pulsatile flow.     

Pressure drop increase by biofilm accumulation in spiral wound RO and NF membrane systems: role of substrate concentration,flow velocity,substrate load and flow direction

In an earlier study, it was shown that biofouling predominantly is a feed spacer channel problem. In this article, pressure drop development and biofilm accumulation in membrane fouling simulators have been studied without permeate production as a function of the process parameters substrate concentration, linear flow velocity, substrate load and flow direction. At the applied substrate concentration range, 100–400 μg l^?1 as acetate carbon, a higher concentration caused a faster and greater pressure drop increase and a greater accumulation of biomass. Within the range of linear flow velocities as applied in practice, a higher linear flow velocity resulted in a higher initial pressure drop in addition to a more rapid and greater pressure drop increase and biomass accumulation. Reduction of the linear flow velocity resulted in an instantaneous reduction of the pressure drop caused by the accumulated biomass, without changing the biofilm concentration. A higher substrate load (product of substrate concentration and flow velocity) was related to biomass accumulation. The effect of the same amount of accumulated biomass on the pressure drop increase was related to the linear flow velocity. A decrease of substrate load caused a gradual decline in time of both biomass concentration and pressure drop increase. It was concluded that the pressure drop increase over spiral wound reverse osmosis (RO) and nanofiltration (NF) membrane systems can be reduced by lowering both substrate load and linear flow velocity. There is a need for RO and NF systems with a low pressure drop increase irrespective of the biomass formation. Current efforts to control biofouling of spiral wound membranes focus in addition to pretreatment on membrane improvement. According to these authors, adaptation of the hydrodynamics, spacers and pressure vessel configuration offer promising alternatives. Additional approaches may be replacing heavily biofouled elements and flow direction reversal.     

Low frequency flowmotion/(vasomotion) during patho-physiological conditions

The role of low frequency flowmotion in physiological or pathophysiological settings is unclear.We performed various series of experiments in young anesthetized New Zealand white (NZW) rabbits. Many animals exhibited flowmotion during control conditions. However, they very often seemed to be in unstable physiological conditions, and our preset inclusion criteria (as to arterial pressure and blood gases) were frequently not met.Therefore, in a first series, we correlated these systematically with the incidence of flowmotion. Eleven of 35 anesthetized rabbits, subjected to extensive surgery, showed flowmotion with a median frequency of 1.5 cpm and a relative "amplitude" of 32%. Arterial pressure was 10 mmHg lower, bicarbonate, base-excess, and PCO(2) values and relative blood flow were also significantly lower compared to animals not exhibiting flowmotion.In a second series, we tested whether flowmotion could be induced by an isolated metabolic acidosis in animals meeting the inclusion criteria and not showing flowmotion at control. Here, flowmotion was induced in 9/10 cases (p < 0.01) 30 min after the start of an HCl-infusion.In a third study, we related the onset of flowmotion to the pressure/flow autoregulation curve. At locally reduced blood pressure all 23 rabbits exhibited flowmotion (p < 0,00001) in the gastrocnemius and the tenuissimus muscles, with maximum flowmotion at a locally reduced blood pressure of 30 mmHg; the LDF-flux level showing 67% of control flow.These results support the concept that low frequency periodic hemodynamics are a characteristic of pathophysiological conditions like hypoperfusion or acidosis rather than indicating a normal physiological state.     

Changes in mechanics of breathing during experimental cough and sneeze in anaesthetized cats

Pleural pressure, airflow and tidal volume during experimental cough and sneeze elicited by mechanical stimulation of the tracheobronchial and nasal mucous membranes were investigated in fifty anaesthetized cats (pentobarbital, 40 mg/kg i.p.). Pressure-volume, pressure-flow and flow-volume relations were studied during these expulsive processes. In comparison to quiet breathing there was a decrease in dynamic lung compliance in both respiratory tract reflexes (p less than 0.001), especially in their expiratory phases. As compared to quiet breathing, the total work of breathing was significantly increased (p less than 0.001) in cough (20 times) as well as in sneeze (13 times). The total lung resistance increased markedly (p less than 0.001) in both cough and sneeze compared to quiet breathing. In these expulsive processes there was also a high "cough index" (resistance calculated from the peak flow and instantaneous pressure). The flow-volume curve in cough, in contradistinction to sneeze, indicated a significantly reduced airflow of the end of expiration (at 85% of the expired volume), demonstrating a concomitant bronchoconstriction.