A finite element model of the human left ventricular systole.

Local wall stress is the pivotal determinant of the heart muscle's systolic function. Under in vivo conditions, however, such stresses cannot be measured systematically and quantitatively. In contrast, imaging techniques based on magnetic resonance (MR) allow the determination of the deformation pattern of the left ventricle (LV) in vivo with high accuracy. The question arises to what extent deformation measurements are significant and might provide a possibility for future diagnostic purposes. The contractile forces cause deformation of LV myocardial tissue in terms of wall thickening, longitudinal shortening, twisting rotation and radial constriction. The myocardium is thereby understood to act as a densely interlaced mesh. Yet, whole cycle image sequences display a distribution of wall strains as function of space and time heralding a significant amount of inhomogeneity even under healthy conditions. We made similar observations previously by direct measurement of local contractile activity. The major reasons for these inhomogeneities derive from regional deviations of the ventricular walls from an ideal spheroidal shape along with marked disparities in focal fibre orientation. In response to a lack of diagnostic tools able to measure wall stress in clinical routine, this communication is aimed at an analysis and functional interpretation of the deformation pattern of an exemplary human heart at end-systole. To this end, the finite element (FE) method was used to simulate the three-dimensional deformations of the left ventricular myocardium due to contractile fibre forces at end-systole. The anisotropy associated with the fibre structure of the myocardial tissue was included in the form of a fibre orientation vector field which was reconstructed from the measured fibre trajectories in a post mortem human heart. Contraction was modelled by an additive second Piola-Kirchhoff active stress tensor. As a first conclusion, it became evident that longitudinal fibre forces, cross-fibre forces and shear along with systolic fibre rearrangement have to be taken into account for a useful modelling of systolic deformation. Second, a realistic geometry and fibre architecture lead to typical and substantially inhomogeneous deformation patterns as they are recorded in real hearts. We therefore, expect that the measurement of systolic deformation might provide useful diagnostic information.

On the significance of fiber branching in the human myocardium.

Myocardial tissue exhibits a high degree of organization in that the cardiac muscle fibers are both systematically aligned and highly branched. In this study, the influence and significance of fiber branching is analyzed mathematically. In order to allow for analytic solutions, a regular geometry and simplified constitutive relations are considered. It is found that branching is necessary to stabilize the ventricular wall.

Excimer laser spectroscopy: influence of tissue ablation on vessel wall fluorescence.

Limited steerability and injury to the normal vessel wall are major drawbacks of laser coronary angioplasty. To overcome these limitations a new generation of laser systems has been developed which allows not only to eliminate the atherosclerotic plaque but to guide the laser beam by analyzing the laser induced tissue fluorescence (= spectroscopy) for the treatment of the atherosclerotic vessel. An excimer laser (MAX 10 LP, 308 nm, Technolas, Munich, Germany) was used with an emitting (phi 1070 microns) and a detecting (phi 130 microns) optical fiber to induce tissue fluorescence which was analyzed quantitatively by a computerized system. Specimens from the descending (thoracic) aorta were obtained from 24 patients (mean age 68.1 years, range 44-92). Tissue fluorescence was induced with ablating (26-30 mJ/mm2) and nonablating (3 mJ/cm2) laser activations. The emitted fluorescence (range 380-575 nm) was normalized to a wavelength of 380 nm; as a measure of tissue fluorescence the intensity ratio at 500 nm divided by 400 nm was calculated in normal (n = 78), mildly atherosclerotic (n = 40), and severely atherosclerotic (n = 48) tissue samples. Repeated laser activations were carried out and tissue fluorescence was checked until the fluorescence spectrum was normalized. All tissue samples were analyzed histologically by a semiquantitative score. Normal tissue samples showed the highest intensity ratios (5.9 +/- 3.4), whereas mildly (2.9 +/- 1.3) and severely atherosclerotic (2.1 +/- 1.0) samples elicited a significantly reduced fluorescence. Repeated tissue ablations were associated with a normalization of fluorescence intensity ratios in the mildly (7.0) as well as in the severely diseased (4.9) vessels. A curvilinear relationship between intensity ratio and the semiquantitative score was observed (r = 0.66) as well as between intensity ratio and intimal wall thickness (r = 0.62). No gender related differences were found but there was an inverse relationship between fluorescence intensity ratio and age (r = 0.56) as well as between intimal thickness and age (r = 0.41). Excimer laser spectroscopy allows reliable detection of atherosclerotic vessel alterations. Fluorescence intensity ratio is inversely proportional to the intimal wall thickness and the severity of the histologic alterations. There is an age dependency of fluorescence intensity ratio which can be explained by an increase in intimal wall thickness. Successful tissue ablation can be obtained by laser angioplasty and allows determination of the optimal point where complete tissue ablation is achieved by laser activation. Thus, excimer laser spectroscopy is an effective method for selective tissue ablation by laser angioplasty.

Compatibility considerations for low-mass rigid-belt vehicles.

A number of staged impacts performed by our group with the aid of a test device representing a low-mass vehicle (LMV) indicates that a rigid-belt body (RBB) is a valid means for providing adequate occupant safety also for LMVs in the strict sense (curb mass less than 600 kg). The RBB concept raises the problem of compatibility, however. Ideally, the deformability of car front structures should increase with increasing vehicle weight in order to ascertain compatibility. Published data on frontal deformation characteristics substantiate in contrast that conventional cars today exhibit an opposite behaviour. To evaluate the compatibility properties of ultrastiff LMVs, two crash experiments were performed along with a theoretical model analysis. An LMV with a mass of 680 kg (including batteries, 50% mass of two dummies, instrumentation) designed according to the RBB concept and a conventional care of 1320 kg--(equivalent loading conditions as LMV)--were crashed at 56 km/h in a frontal direction against a deformable barrier (FMVSS 214). Furthermore, a mathematical model was based on estimated deformation characteristics of conventional vehicles to predict intrusion distances into the FMVSS barrier in hypothetical frontal crashes with 56 km/h. The results indicate that due to its low mass an LMV does not represent an excessive compatibility problem for other car occupants in spite of the stiff RBB characteristics.

High-frequency ventilation: oscillatory dynamics.

OBJECTIVES: To determine the influence of the dynamic properties of the oscillator on the oscillatory volume delivered through the endotracheal tube to the lung or lung surrogate (delivered volume) under conditions of high-frequency ventilation. In particular, the relation between the tidal volume of the pump (oscillator) and the delivered volume was analyzed. PaCO2 was measured further as a function of the delivered volume in a number of experiments performed with healthy dogs. DESIGN: Laboratory study. SETTING: Engineering and animal laboratory. SUBJECTS: Lung surrogates and healthy dogs. INTERVENTIONS: An experimental oscillatory system was connected to various lung surrogates. In addition, six beagle dogs received high-frequency ventilation with different delivered volumes during the study. Control of the mean airway pressure was achieved by a peripheral pressure chamber located at the exhaust port of the bias flow tube. RESULTS: The delivered volume, which is the quantity of interest from a physiologic point of view, can deviate considerably from the tidal volume of the pump due to dynamic (particularly resonance) effects. Because the delivered volume and the mean airway pressure have to be controlled independently, two independent quantities are necessary for control purposes (e.g., the tidal volume of the pump and the mean pressure at the exhaust port). Furthermore, it was found that a minimal condition for adequate gas exchange is a delivered volume that exceeds the machine-related deadspace. For this reason, and in order to maximize the CO2 gradient, the exhaust tube must be as short as possible. CONCLUSIONS: a) The delivered volume has to be monitored under clinical conditions; b) however, because the impedance of the endotracheal tube in general considerably exceeds the impedance of the lung, the influence of the impedance of the lung on the delivered volume is generally small, and thus an in vitro calibration may serve as a useful approximation; c) at least two independent quantities are needed for an adequate oscillatory control; d) a necessary (not necessarily sufficient) condition for adequate CO2 removal is that the delivered volume must exceed the machine-related deadspace; e) in a clinical environment involving extremely pathologic lung conditions, e.g., adult respiratory distress syndrome, mechanical lung characteristics may deviate substantially from those characteristics used in this study (i.e., the results obtained may not necessarily be applicable under all clinical situations).

Gas transport during high-frequency ventilation: the significance of direct wash-out.

Gas transport during high-frequency oscillation was investigated in vitro using CO2 elimination from the lung surrogate as a measure of gas transport efficiency. The length of the connecting tube between the piston pump and the three-port connector did not affect gas transport efficiency if the oscillatory volume (VDEL) was constant; inserting an additional tube between the three-port connector and the endotracheal tube decreased gas transport efficiency dramatically. In contradistinction, increasing VDEL caused a steep rise in gas transport efficiency as soon as VDEL surpassed the volume of the tubes connecting the lung surrogate with its surroundings. As gas transport efficiency was found to be very sensitive to the net oscillatory volume, i.e. VDEL minus the volume of the tubes connecting the lung and the surroundings, direct wash-out was considered to be an effective gas transport mechanism during high frequency oscillation. Two preliminary experiments on dogs allowed us to substantiate this hypothesis in vivo.

Significance of bulk convection during high-frequency oscillation.

In 7 anesthetized supine dogs with an anatomic dead space of 115-162 ml, gas transport during high-frequency oscillation (HFO) was investigated at an oscillatory frequency of 15 Hz. Starting with an oscillatory volume effectively delivered to the lungs (VDEL) of 60 ml, measured on line with an ultrasonic airflow meter, VDEL was reduced in steps of 10 ml, down to a VDEL of 30 ml, whereby fresh gas flow rate, airway occlusion pressure and lung volume above functional residual capacity were kept constant. An HFO-circuit without bias tube was used. The volume of endotracheal tube and three port connector, designated as HFO-circuit related rebreathing volume, was 35 ml. PaCO2 continuously increased, when VDEL was reduced from 60 ml to 40 ml and the data fit perfectly to a reciprocal regression (1/PaCO2 = a + b.VDEL), r2 ranging from 0.95 to 1.00. Measured PaCO2 values at a VDEL of 30 ml (8.26 +/- 1.77 kPa), however, were significantly (P less than 0.025) higher than PaCO2 values predicted by the individual reciprocal regression equations (6.25 +/- 1.46 kPa). This overproportionate increase in PaCO2 due to a reduction of VDEL from 40 ml to 30 ml may be explained by the sudden drop out of bulk convection as a gas transport mechanism between central airways and the surrounding because bulk convection is only possible as long as VDEL exceeds the HFO-circuit related rebreathing volume. Bulk convection therefore is considered an essential gas transport mechanism during HFO and the efficiency of CO2 elimination during HFO is critically dependent on the net oscillatory volume, i.e. VDEL minus the HFO-circuit related rebreathing volume and not on the relationship between VDEL and anatomic dead space.

Gas transport enhancement in high-frequency oscillation.

Gas transport during high-frequency oscillation (HFO) with (HFO+BT) and without bias tube (HFO-BT) was investigated in 10 anesthetized supine dogs. The oscillatory volume effectively delivered to the lungs, airway occlusion pressure and lung volume above functional residual capacity (FRC), regulated by a newly deviced pressure control system, as well as the oscillatory frequency (20 Hz) were adjusted to equal levels in HFO+BT and HFO-BT. At a fresh gas flow rate (fgf) of 3 L/min (room air), arterial CO2 partial pressures (PaCO2) decreased from 49.9 +/- 6.5 mm Hg (mean +/- SD) to 40.2 +/- 6.3 mm Hg (P less than 0.01) i.e. by 19.2 +/- 8.7%, and arterial O2 partial pressures (PaO2) increased from 71.5 +/- 13.1 mm Hg to 85.6 +/- 14.6 mm Hg (P less than 0.01) or by 20.5 +/- 12.0% in HFO-BT as compared to HFO+BT. At a fgf of 6 L/min, PaCO2 decreased less but still significantly (P less than 0.025) from 42.1 +/- 6.5 mm Hg to 37.8 +/- 6.8 mm Hg (10.4 +/- 5.6%) and PaO2 increased from 78.1 +/- 12.9 mm Hg to 84.6 +/- 16.4 mm Hg i.e. by 8.1 +/- 6.4% (P less than 0.05) in HFO-BT. The higher gas transport efficiency after removing the bias tube can be explained by two mechanisms: (1) By removing the bias tube, the volume of the bias system decreased from 54 ml in HFO+BT to 1 ml in HFO-BT and rebreathing of exhaust gas from the bias system is therefore eliminated in HFO-BT.(ABSTRACT TRUNCATED AT 250 WORDS)

Augmentation of CO2 elimination during high frequency oscillation by removing the bias tube--an in vitro study.

In clinical applications of high frequency oscillation (HFO), sufficient CO2 elimination (VCO2) may represent a problem mainly at higher oscillation frequencies. With the intention of examining how to increase VCO2 a modified bias flow system was investigated in vitro with wash-out experiments. In bias flow systems, long tubes have been used in order to minimize the loss of oscillatory volume; however, a distinct increase of VCO2 was achieved in the present study by removing the bias tube. This improvement occurred over the whole frequency range of 2-60 Hz, although the oscillatory volume, effectively delivered to the lungs was smaller with the HFO circuit without bias tube (HFO-BT) as compared to the arrangement with bias tube (HFO + BT). A long bias tube flattens the CO2 concentration gradient from the alveoli to the atmosphere. Removing the bias tube results in a steeper CO2 concentration gradient and in a correspondingly enhanced VCO2. Furthermore, the large oscillatory volume at the exit of the bias flow system in HFO-BT supports VCO2 as an additional wash-out mechanism. Based upon longitudinal tracer gas concentration measurements between the alveoli and the atmosphere during HFO16,17, an increase of gas transport up to 20% can be expected for in vivo applications by removing the bias tube.

Flow separation, an important mechanism in the formation of mean pulmonary pressure during high-frequency oscillation.

Mean pressures within the lungs and lung volume, respectively, are clinically important parameters. During ventilation by way of high-frequency oscillation (HFO), these parameters have been shown to be strongly frequency dependent. To identify mechanisms leading to mean pressure formation during HFO, findings of the theory of stationary flow were extended to oscillatory flow by a quasi-stationary approach. To confirm the theoretical findings, in-vitro experiments on HFO-models were performed. Flow separation was found to be an important mechanism in the formation of mean pressure. Flow separation causes a significant flow resistance, which may be distinctly different for in- and outflow. During oscillatory flow, a mean pressure difference thus results. This mechanism is of particular importance in bifurcations, which are present in the HFO-circuit as well as in the airways. With the direction-dependent flow separation, a general mechanism was found, which accounts for differing mean pressure values within the lungs with different HFO-circuits. This mechanism also contributes to interregionally different mean pressure values within the lungs.

Lung surrogates.

In the development and evaluation of mechanical ventilation on the basis of high-frequency oscillation, appropriate surrogates of the lung are important, because they allow the measurement and control of various parameters which are not accessible in animal models. Yet, criteria have to be established according to which results obtained with a surrogate may be assessed with a view to extrapolation to humans. Theoretical considerations and impedance measurements are used for this purpose. It is found that for each given frequency a model can be made which exhibits realistic properties. However, no uniformly valid surrogate in the entire frequency range of 10-50 Hz is available at present.

Motion patterns of pedestrian surrogates in simulated vehicle-pedestrian collisions.

In about 80-85% of all vehicle-pedestrian collisions, a pedestrian is hit by the frontal area of a vehicle. Thereby, an enormous variety of spatial motion patterns of the impacted pedestrian is observed. The analysis of injury mechanisms and injury prevention measures depends largely on a sufficient knowledge of the relevant impact-induced motions. Accordingly, the investigation and classification of these motions is an important task in accident biomechanics. To this end, about 150 collision experiments with the aid of a catapult and several types of pedestrian surrogates were performed. Also, extensive use of a mathematical gross motion analysis model was made. The experimental impacts were analysed with the aid of automated high-speed cinephotogrammetry and acceleration measurements while the mathematical model was validated and calibrated for a number of impact configurations chosen strategically. Results of the experimental and theoretical impact simulations are presented which are related to the observed motion characteristics. In particular, a method is discussed which allows the assessment of the sensitivity of a given motion with respect to the impact parameters. An attempt is made to classify these parameters according to the significance of their influence, whereby pedestrian-related and vehicle-related parameters are discerned. It was found, that such a classification is necessary and possible and results are shown.

On the propagation of a wave front in viscoelastic arteries.

In formulating a mathematical model of the arterial system, the one-dimensional flow approximation yields realistic pressure and flow pulses in the proximal as well as in the distal regions of a simulated arterial conduit, provided that the viscoelastic damping induced by the vessel wall is properly taken into account. Models which are based on a purely elastic formulation of the arterial wall properties are known to produce shocklike transitions in the propagating pulses which are not observed in man under physiological conditions. The viscoelastic damping characteristics are such that they are expected to reduce the tendency of shock formation in the model. In order to analyze this phenomenon, the propagation of first and second-order pressure waves is calculated with the aid of a wave front expansion, and criteria for the formation of shocks are derived. The application of the results to the human arterial system show that shock waves are not to be expected under normal conditions, while in case of a pathologically increased pressure rise at the root of the aorta, shocklike transitions may develop in the periphery. In particular, it is shown that second-order waves never lead to shock formation in finite time for the class of initial conditions and mechanical wave guides which are of interest in the mammalian circulation.