Adaptation of cardiac structure by mechanical feedback in the environment of the cell: a model study

In the cardiac left ventricle during systole mechanical load of the myocardial fibers is distributed uniformly. A mechanism is proposed by which control of mechanical load is distributed over many individual control units acting in the environment of the cell. The mechanics of the equatorial region of the left ventricle was modeled by a thick-walled cylinder composed of 6-1500 shells of myocardial fiber material. In each shell a separate control unit was simulated. The direction of the cells was varied so that systolic fiber shortening approached a given optimum of 15%. End-diastolic sarcomere length was maintained at 2.1 microns. Regional early-systolic stretch and global contractility stimulated growth of cellular mass. If systolic shortening was more than normal the passive extracellular matrix stretched. The design of the load-controlling mechanism was derived from biological experiments showing that cellular processes are sensitive to mechanical deformation. After simulating a few hundred adaptation cycles, the macroscopic anatomical arrangement of helical pathways of the myocardial fibers formed automatically. If pump load of the ventricle was changed, wall thickness and cavity volume adapted physiologically. We propose that the cardiac anatomy may be defined and maintained by a multitude of control units for mechanical load, each acting in the cellular environment. Interestingly, feedback through fiber stress is not a compelling condition for such control. [Journal Article; In English; United States

Acute acalculous cholecystitis in children

Acute acalculous cholecystitis (AAC) is a rare disease in African children and usually occurs as a complication of some other diseases, such as systemic infections. AAC can, however, be the primary pathology but, because of its low incidence, it is difficult to establish definite mechanisms and causes. The case histories of six children (mean age seven years) with AAC are presented. They were treated in Queen Elizabeth Central Hospital, Blantyre, Malawi and in Gweru Provincial Hospital in Zimbabwe between 1985 and 1996. In four children, pre-operative ultrasonography showed the typical signs of AAC. Cholecsytectomy was performed on all seven children. One child died postoperatively from generalized sepsis and in two children a wound infection occurred. In the tropics, where many children are seen with gastro-enteritis and other infections diseases, the possibility of AAC must be borne in mind when a child is admitted with right upper abdominal tenderness and fever. The role of ultrasonography is emphasized. It is a reliable, non-invasive and quick investigation, which helps to establish the diagnosis before surgery. Cholecystectomy is the treatment of choice because of the high incidence of necrosis of the gallbladder wall

Cue validity effects in response preparation: A pupillometric study

This study examined the effects of cue validity and cue difficulty on response preparation to provide a test of the Grouping Model [Adam, J.J., Hommel, B. and Umilta, C., 2003. Preparing for perception and action (1): the role of grouping in the response-cuing paradigm. Cognit. Psychol. 46(3), 302-58, Adam, J.J., Hommel, B. and Umiltia, C., 2005. Preparing for perception and action (II) automatic and effortful processes in response cuing. Vis. Cogn. 12(8), 1444 1473.]. We used the pupillary response to index the cognitive processing load during and after the preparatory interval (2 s). Twenty-two participants performed the finger-cuing tasks with valid (75%) and invalid (25%) cues. Results showed longer reaction times, more errors, and larger pupil dilations for invalid than valid cues. During the preparation interval, pupil dilation varied systematically with cue difficulty, with easy cues (specifying 2 fingers on 1 hand) showing less pupil dilation than difficult cues (specifying 2 fingers on 2 hands). After the preparation interval, this pattern of differential pupil dilation as a function of cue difficulty reversed for invalid cues, suggesting that cues which incorrectly specified fingers on one hand required more effortful reprogramming operations than cues which incorrectly specified fingers on two hands. These outcomes were consistent with predictions derived from the Grouping Model. Finally, all participants exhibited two distinct pupil dilation strategies: an "early" strategy in which the onset of the main pupil dilation was tied to onset of the cue, and a "late" strategy in which the onset of the main pupil dilation was tied to the onset of the target. Thus, whereas the early pupil dilation strategy showed a strong dilation during the preparation interval, the late pupil strategy showed a strong constriction. Interestingly, only the late onset pupil dilation strategy revealed the above reported sensitivity to cue difficulty, showing for the first time that the well-known pupil's sensitivity to task difficulty can also emerge when the pupil is constricting instead of dilating

Adaptation of cardiac structure by mechanical feedback in the environment of the cell: a model study

In the cardiac left ventricle during systole mechanical load of the myocardial fibers is distributed uniformly. A mechanism is proposed by which control of mechanical load is distributed over many individual control units acting in the environment of the cell. The mechanics of the equatorial region of the left ventricle was modeled by a thick-walled cylinder composed of 6-1500 shells of myocardial fiber material. In each shell a separate control unit was simulated. The direction of the cells was varied so that systolic fiber shortening approached a given optimum of 15%. End-diastolic sarcomere length was maintained at 2.1 microns. Regional early-systolic stretch and global contractility stimulated growth of cellular mass. If systolic shortening was more than normal the passive extracellular matrix stretched. The design of the load-controlling mechanism was derived from biological experiments showing that cellular processes are sensitive to mechanical deformation. After simulating a few hundred adaptation cycles, the macroscopic anatomical arrangement of helical pathways of the myocardial fibers formed automatically. If pump load of the ventricle was changed, wall thickness and cavity volume adapted physiologically. We propose that the cardiac anatomy may be defined and maintained by a multitude of control units for mechanical load, each acting in the cellular environment. Interestingly, feedback through fiber stress is not a compelling condition for such control. [Journal Article; In English; United States

Optimization of cardiac fiber orientation for homogeneous fiber strain during ejection

The strain of muscle fibers in the heart is likely to be distributed uniformly over the cardiac walls during the ejection period of the cardiac cycle. Mathematical models of left ventricular (LV) wall mechanics have shown that the distribution of fiber strain during ejection is sensitive to the orientation of muscle fibers in the wall. In the present study, we tested the hypothesis that fiber orientation in the LV wall is such that fiber strain during ejection is as homogeneous as possible. A finite-element model of LV wall mechanics was set up to compute the distribution of fiber strain at the beginning (BE) and end (EE) of the ejection period of the cardiac cycle, with respect to a middiastolic reference state. The distribution of fiber orientation over the LV wall, quantified by three parameters, was systematically varied to minimize regional differences in fiber shortening during ejection and in the average of fiber strain at BE and EE. A well-defined optimum in the distribution of fiber orientation was found which was not significantly different from anatomical measurements. After optimization, the average of fiber strain at BE and EE was 0.025 ± 0.011 (mean ± standard deviation) and the difference in fiber strain during ejection was 0.214 ± 0.018. The results indicate that the LV structure is designed for maximum homogeneity of fiber strain during ejection

Optimization of cardiac fiber orientation for homogeneous fiber strain at beginning of ejection

Mathematical models of left ventricular (LV) wall mechanics show that fiber stress depends heavily on the choice of musele fiber orientation in the wall. This finding brought us to the hypothesis that fiber orientation may be such that mechanical load in the wall is homogeneous. Aim of this study was to use the hypothesis to compute a distribution of fiber orientation within the wall. In a finite element model of LV wall mechanics, fiber stresses and strains were calculated at beginning of ejection (BE). Local fiber orientation was quantified by helix (HA) and transverse (TA) fiber angles using a coordinate system with local r-, c-, and l-directions perpendicular to the wall, along the circumference and along the meridian, respectively. The angle between the c-direction and the projection of the fiber direction on the cl-plane (HA) varied linearly with transmural position in the wall. The angle between the c-direction and the projection of the fiber direction on the cr-plane (TA) was zero at the epicardial and endocardial surfaces. Midwall TA increased with distance from the equator. Fiber orientation was optimized so that fiber strains at BE were as homogeneous as possible. By optimization with TA = 0°, HA was found to vary from 81,0° at the endocardium to - 35.8 at the epicardium. Inclusion of TA in the optimization changed these angles to respectively 90.1° and - 48.2° while maximum TA was 15.3°. Then the standard deviation of fiber strain (ef) at BE decreased from ± 12.5% of mean ef to ± 9.5%. The root mean square (RMS) difference between computed HA and experimental data reported in literature was 15.0° compared to an RMS difference of 11.6° for a linear regression line through the latter data