Biomedical subjects

R S Reneman

Publications and source records attributed to R S Reneman.

At least 55 records · Page 3Linked to original sources

PubMed1998

Estimates of regional work in the canine left ventricle.

Assessment of the magnitude of regional myocardial work requires knowledge of regional fiber stress and fiber shortening. The theoretical development and experimental validation of a method is presented which used values of estimated active and passive fiber stress according to a fluid-fiber model, and measured fiber strain values. This enables the construction of regional stress-strain diagrams, a regional analog of the pressure-volume area model by Suga and co-investigators, which can be linked to regional oxygen consumption. In the left ventricle, either normally or asynchronously activated, the method yields reliable data on strain and active and passive fiber stress. The relation between estimated regional work and myocardial oxygen demand is in quantitative agreement with previously reported relations between global oxygen demand and measured pressure-volume area. During coronary artery occlusion, however, these values were less reliable, which might be due to inaqdequate knowledge of the (passive) material properties of the myocardium.

Animals↗

PubMed1998

Bladder filling reduces femoral artery wall distension and strain: beware of a full bladder!

During a previous study, we noted that the distension and strain of the femoral artery were relatively low when the bladder was full, a situation normally necessary for transabdominal echography. Therefore, in the present study we investigated the influence of bladder filling, if any, on wall properties of the common femoral artery. The results obtained were compared with those obtained in the common carotid artery. The study was performed on the right common carotid and right common femoral arteries of normotensive young (18-35 y) female volunteers (n = 24). Using a specially designed ultrasonic wall-tracking device and automatic brachial artery cuff blood pressure measurements, arterial distension (absolute change in diameter during the cardiac cycle; deltaD), strain (deltaD/D), and cross-sectional distensibility (DC) and compliance (CC) were determined before and after voiding. Distension and strain of the common femoral artery were significantly lower for a full than for an empty bladder. DC and CC were lower when the bladder was filled, but these differences did not reach the level of significance. Blood pressure as measured at the level of the brachial artery and heart rate were not statistically significantly different during a full or an empty bladder. It is concluded that bladder filling affects femoral artery wall properties, an observation that should be kept in mind when performing studies on artery wall properties at this level of the circulation.

Adolescent↗

PubMed1998

A noninvasive method to estimate pulse wave velocity in arteries locally by means of ultrasound.

Noninvasive evaluation of vessel wall properties in humans is hampered by the absence of methods to assess directly local distensibility, compliance, and Young's modulus. Contemporary ultrasound methods are capable of assessing end-diastolic artery diameter, the local change in artery diameter as a function of time, and local wall thickness. However, to assess vessel wall properties of the carotid artery, for example, the pulse pressure in the brachial artery still must be used as a substitute for local pulse pressure. The assessment of local pulse wave velocity as described in the present article provides a direct estimate of local vessel wall properties (distensibility, compliance, and Young's modulus) and, in combination with the relative change in artery cross-sectional area, an estimate of the local pulse pressure. The local pulse wave velocity is obtained by processing radio frequency ultrasound signals acquired simultaneously along two M-lines spaced at a known distance along the artery. A full derivation and mathematical description of the method to assess local pulse wave velocity, using the temporal and longitudinal gradients of the change in diameter, are presented. A performance evaluation of the method was carried out by means of experiments in an elastic tube under pulsatile pressure conditions. It is concluded that, in a phantom set-up, the assessed local pulse wave velocity provides reliable estimates for local distensibility.

Arteries↗

PubMed1998

Optimization of ventricular function by improving the activation sequence during ventricular pacing.

Abnormal electrical activation occurring during ventricular pacing reduces left ventricular (LV) pump function. Two strategies were compared to optimize LV function using ventricular pacing, minimal asynchrony and optimal sequence of electrical activation. ECG and hemodynamics aortic flowprobe, thermodilution cardiac output, LV pressure and its maximal rates of rise (LVdP/dtpos) and fall (LVdP/dtneg) were measured in anesthetized open-chest dogs (n = 7) with healthy hearts. The QRS duration (a measure of asynchrony of activation) was 47 +/- 5 ms during sinus rhythm and increased to 110 +/- 12 ms during DDD pacing at the right ventricular (RV) apex with a short AV interval. During pacing at the LV apex and LV base, the QRS duration was 8% +/- 7% and 15% +/- 7% (P < 0.05) longer than during RV apex pacing, respectively. Stroke volumes, LVdP/dtpos and LVdP/dtneg, however, were higher during LV apex (15% +/- 16%, 10% +/- 12% [P < 0.05], and 15% +/- 10%, respectively) and LV base pacing (11% +/- 12% [P < 0.05], 3% +/- 12%, and 3% +/- 11%, respectively) than during RV apex pacing. Systolic LV pressure was not influenced significantly by the site of pacing. Biventricular pacing (RV apex together with one or two LV sites) decreased the QRS duration by approximately 20% as compared with RV apex pacing, however, it did not improve stroke volumes, LVdP/dtpos and LVdP/dtneg beyond those during pacing at the LV apex alone. In conclusion, the sequence of electrical activation is a stronger determinant of ventricular function than the synchrony of activation. For optimal LV function the selection of an optimal single pacing site, like the LV apex, is more important than pacing from multiple sites.

Animals↗

PubMed1998

Quantitative assessment of [Ca2+]i levels in rat skeletal muscle in vivo.

Intracellular free Ca2+ concentration ([Ca2+]i) plays an essential role in physiological regulatory processes and common pathological conditions. Better understanding of these phenomena is still hampered by problems encountered in the quantitative assessment of [Ca2+]i changes, especially in blood-perfused organs. This study demonstrates that the ratiometric fluorescence technique can be adapted for quantitative in vivo [Ca2+]i determinations. The rat spinotrapezius muscle was topically loaded with indo 1-AM and imaged by a cooled digital camera. Ratio images were calculated in small regions (100 micrometers x 100 micrometers) practically devoid of large vessels in the resting state, after 30 min of ischemia, 20 min of reperfusion, or ionomycin or manganate treatments. When we assumed an average [Ca2+]i of 100 nM in the resting blood-perfused muscle, ischemia increased [Ca2+]i to approximately 200 nM. During reperfusion [Ca2+]i decreased to approximately 140 nM. Ionomycin induced an increase in [Ca2+]i to well above 750 nM. Manganate reduced Ca2+-dependent fluorescence to virtually zero. Our main conclusion is that changes in [Ca2+]i can be monitored and quantitatively determined in vivo.

Animals↗

PubMed1998

Endogenous nitric oxide protects against thromboembolism in venules but not in arterioles.

Because nitric oxide (NO) inhibits aggregation and adhesion of blood platelets, NO may play a role in platelet-vessel wall interactions. Therefore, the purpose of this study was to investigate the involvement of endogenous NO in thromboembolic processes, as induced by wall puncture, in rabbit mesenteric arterioles and venules (diameters 20 to 43 microm). In venules, inhibition of NO synthase by superfusion of the mesentery with N omega-nitro-L-arginine (L-NA; 0.1 mmol/L) significantly increased the duration of embolization (from 50 seconds to 511 seconds) and the number of emboli produced (from 2 to 11 emboli per vessel), while the median period of time needed to produce an embolus was not influenced. On the contrary, in arterioles, L-NA had no significant effect on embolization (duration of embolization: 426 seconds in the control and 382 seconds in the L-NA group, with 20 and 12 emboli per vessel, respectively). Addition to the L-NA superfusate of L-arginine (L-ARG; 1 mmol/L), the active precursor for endogenous NO synthesis, resulted in a complete reversal of the L-NA effects in venules, while addition of the inactive D-arginine (D-ARG; 1 mmol/L) had no effect. Addition of L-ARG and D-ARG had no significant effect in arterioles. Addition to the L-NA superfusate of the exogenous NO donor sodium nitroprusside (0.1 micromol/L) also resulted in reversal of the L-NA effects in venules, while in arterioles, it slightly but significantly decreased embolization duration. The differences in effect of L-NA on embolization between arterioles and venules were not caused by differences in fluid dynamic conditions. It is concluded that the role of endogenous NO in inhibiting thromboembolic processes is more important in venules than in arterioles.

Animals↗

PubMed1998

Differences in near-wall shear rate in the carotid artery within subjects are associated with different intima-media thicknesses.

In the common carotid artery, reflections originating from the periphery and the flow divider may affect the shape of the flow velocity profile and, hence, near-wall shear rate (WSR) differently just before the bifurcation (location B) than 20 to 30 mm farther upstream (location A). Recent developments in ultrasound technology allow the assessment of WSR and intima-media thickness (IMT) at the same site in the carotid artery in vivo. We therefore determined WSR at locations A and B and investigated whether the differences between both sites, if any, were associated with different IMTs and different mechanical properties of the arterial wall. The effect of age on the possible differences was assessed as well. The study was performed on presumably healthy volunteers (n=53). In all individuals, IMT was larger at location B than at location A. The relative difference in IMT between both locations was not affected by age. No significant differences in diameter and distension were found between locations. Near peak systolic and near mean WSR at the posterior wall (PWSRp and MWSRp, respectively) were significantly lower at location B than at location A. The relative differences in PWSRp and MWSRp between both locations within subjects were independent of age. The velocity profiles were more blunted at location A than at location B. PWSRp and MWSRp significantly decreased and IMT significantly increased with age at both locations. IMT was negatively correlated with PWSRP and MWSRP at location B, but this correlation was not significant at location A. In summary, in the common carotid artery, the lower WSR near the bifurcation, as compared with 20 to 30 mm upstream, is associated with a larger IMT than at the more proximal site. The relative difference between both locations within subjects is independent of age.

Adolescent↗

PubMed1998

Phospholipase A2-mediated hydrolysis of cardiac phospholipids: the use of molecular and transgenic techniques.

Under pathophysiological conditions, like myocardial ischemia and reperfusion, cardiac phospholipid homeostasis is severely disturbed, resulting in a net degradation of phospholipids and the accumulation of degradation products, such as lysophospholipids and (non-esterified) fatty acids. The derangements in phospholipid metabolism are thought to be involved in the sequence of events leading to irreversible myocardial injury. The net degradation of phospholipids as observed during myocardial ischemia may result from increased hydrolysis and/or reduced resynthesis, while during reperfusion hydrolysis is likely to prevail in this net degradation. Several studies indicate that the activation of phospholipases A2 plays an important role in the hydrolysis of phospholipids. In this review current knowledge regarding the potential role of the different types of phospholipases A2 in ischemia and reperfusion-induced damage is being evaluated. Furthermore, it is indicated how recent advances in molecular biological techniques could be helpful in determining whether disturbances in phospholipid metabolism indeed play a crucial role in the transition from reversible to irreversible myocardial ischemia and reperfusion-induced injury, the knowledge of which could be of great therapeutic relevance.

Animals↗

PubMed1998

[Intrinsic and endogenous mechanisms for protecting the heart against ischemia].

The heart has several possibilities to protect itself against ischemia. In general the protection measures are generated by the ischemia itself (intrinsic protection), but the protection can also be induced by external interventions not related to ischemia (endogenous protection). In this survey we have limited ourselves to cellular protection mechanisms. These mechanisms include: 1. The cessation of myocardial contraction immediately following the onset of ischemia; 2. myocardial hibernation, the phenomenon that in ischemic areas cardiomyocytes may dedifferntiate and switch to a fetal phenotype, probably to be better tolerant to hypoxia; 3. preconditioning, the enhanced tolerance against ischemia following short intermittent periods of ischemia and 4. the expression of protective proteins, like heat-shock or other stress proteins. The clinical importance of these protective mechanisms is briefly discussed.

Animals↗

PubMed1997

Skin blood flow abnormalities in a rat model of neuropathic pain: result of decreased sympathetic vasoconstrictor outflow?

Loose ligation of a sciatic nerve in rats provokes signs and symptoms like those observed in human conditions of neuropathic pain. Some of these have been associated with sympathetic dysfunction. Since the skin microcirculation in the rat is strongly influenced by sympathetic tone, abnormalities in skin blood flow may be used as an indirect measure of sympathetic dysfunction. We measured, by means of laser Doppler flowmetry, skin blood flow at the plantar surface of the rat hind paw before and after ipsilateral loose sciatic nerve ligation. We assessed basal skin blood flow as well as the vasoconstrictor response which follows cooling of the rat abdomen. The effectiveness of this response may be used as a measure of sympathetic vasoconstrictor outflow. As compared to the values obtained before ligation (= 100%): (1) the vasoconstrictor response was impaired (65%, P < 0.01) from day 1 onwards, whereas (2) basal skin blood flow was increased (171%; P < 0.01) from day 3 until day 5, and decreased (51%, P < 0.0001) from day 7 until day 28. At day 28, blockade of impulse propagation in the loosely ligated sciatic nerve (by means of lidocaine) did not increase the lowered level of skin blood flow. These findings suggest that in the chronic construction injury model loose ligation of a sciatic nerve reduces sympathetic vasoconstrictor outflow, which, in turn may induce supersensitivity of skin microvessels to catecholamines.

Animals↗

PubMed1997

Cloning and cellular distribution of a group II phospholipase A2 expressed in the heart.

Phospholipase A2 has been considered to play a role in physiological membrane turnover in cardiac tissue and in the degradation of membrane lipids under pathophysiological conditions, such as ischemia and reperfusion. We report the cloning of a cDNA encoding a member of the Ca2+-dependent, low molecular mass phospholipase A2 (PLA2) present in rat heart. The cDNA predicts a mature protein of 146 amino acid residues including a 21 amino acid sequence at the N-terminal end, which has the features characteristic of eukaryotic secretory signal peptides. The deduced amino acid sequence constitutes an enzyme of the group II class of PLA2s, and resembles PLA2s from other mammalian sources. A Northern blot analysis performed to determine the tissue distribution showed that rat ileum contains the largest amount of the PLA2 transcript among the tissues examined, a weaker signal was present in heart, spleen and soleus muscle, and no signal could be detected in EDL muscle, stomach, liver, kidney, brain and lung. Northern blot analysis and reverse transcriptase-polymerase chain reaction (RT-PCR) techniques indicate the presence of this enzyme in neonatal and adult rat cardiomyocytes and in a cultured rat cardiac fibroblast-like cell line, but not in rat cardiac-derived endothelial cell lines. Transcription levels of rat heart group II PLA2 in isolated neonatal rat cardiomyocytes were found to increase after stimulating the cells with tumor necrosis factor-alpha (TNF-alpha) or the alpha1-adrenergic agonist phenylephrine.

Age Factors↗

PubMed1997

Tracking markers with missing data by lower rank approximation.

Motion and deformation of an object may be quantified by following attached markers in video or cine frame sequences. When recording cardiac motion by video (256 x 256 pixels, 50 Hz), generally no more than approximately 20 markers can be followed due to difficulties in proper identification of marker images. In the present study we developed the lower rank (LR) tracking method which can automatically follow considerably more than 20 markers. The performance of the method was evaluated in computer simulations of naturally moving myocardial markers observed in a sequence of 60 video frames. White noise was added to the marker coordinates. Realistic loss of data due to detection failure was simulated by deleting a generated marker image when the distance to another marker image was below a given minimum value. In a test, realistic values were substituted for the noise level sigma (0.5 pixel) and the minimum marker distance dm (4 pixels). For numbers of markers ranging from 50 to 100, 95-90% of the detected marker images was correctly tracked. Less than 0.7% was part of a false track, i.e. a track containing images of different markers. Under less favourable conditions (sigma = 1 pixel; dm = 8 pixels) the method was robust: for 75 markers with 40% of the marker images missing, still 70% of the detected images was correctly tracked, while the fraction in false tracks did not increase. The LR tracking method appears reliable for automatic tracking of large amounts of moving markers in a sequence of video or cine frames.

Algorithms↗

PubMed1997

Relation between torsion and cross-sectional area change in the human left ventricle.

During the ejection phase, motion of the left ventricular (LV) wall is such that all myocardial fibers shorten to the same extent. In a mathematical model of LV mechanisms it was found that this condition could be satisfied only if torsion around the long axis followed a unique function of the ratio of cavity volume to wall volume. When fiber shortening becomes non-uniform due to cardiac pathology, this pathology may be reflected in aberration of the torsional motion pattern. In the present study we investigated whether the predicted regular motion pattern could be found in nine healthy volunteers, using Magnetic Resonance Tagging. In two parallel short-axis cross-sections, displacement, rotation, and area ejection were derived from the motion of tags, attached non-invasively to the myocardium. Information from both sections was combined to determine area ejection, quantified as the change in the logarithm of the ratio of cavity area to wall area, and torsion, represented by the shear angle on the epicardium. Linear regression was applied to torsion as a function of area ejection. The slope thus found (-0.173 +/- 0.024 rad, mean +/- S.D.) was similar to the slope as predicted by the model of LV mechanics (-0.194 +/- 0.026 rad). In conclusion, the relation between area ejection and torsion could be assessed noninvasively in humans. In healthy volunteers, the relation was close to what was predicted by a mathematical model of LV mechanics, and also close to what was found earlier in experiments on animals.

Algorithms↗

PubMed1997

A radio frequency domain complex cross-correlation model to estimate blood flow velocity and tissue motion by means of ultrasound.

This article introduces a mean frequency estimator based on a radio frequency (RF) domain complex cross-correlation model (C3M). The C3M estimator differs from the real cross-correlation model (CCM) estimator in two respects; it is an unbiased estimator of blood flow velocity and/or tissue motion independent of the bandwidth of the RF ultrasound signals, and it provides an estimate of the spatial bandwidth of the RF-signal. The estimators derived from the complex cross-correlation model (mean spatial frequency, mean temporal frequency, spatial bandwidth and signal-to-noise ratio) are based on three complex cross-correlation coefficients. A full derivation and mathematical description of both estimators (C3M and CCM), starting from a Gaussian model of the complex power spectral density distribution of sampled RF signals, are presented. In addition, a thorough performance evaluation of the C3M estimator in comparison with the CCM estimator is carried out by means of simulations to document the effect of signal-to-noise ratio, bandwidth and sample frequency. In the context of the specific simulation conditions considered, the quality of the C3M estimator is shown to offer the best performance (no bias, low standard deviation of the estimate). Taking into account the computational load and the robustness of the C3M estimator, it may be concluded that the C3M estimator combines high quality and modest complexity.

Blood Flow Velocity↗

PubMed1997

Reproducibility of shear rate and shear stress assessment by means of ultrasound in the common carotid artery of young human males and females.

In the present study, the reliability of an ultrasonic shear rate estimating system, in terms of intrasubject intrasession, intersubject intrasession and intersubject intersession variability coefficients for the assessment of wall shear rate (WSR) in the common carotid artery (CCA) was determined in eight presumed healthy volunteers. Measurements were performed on consecutive days (day 1, day 2 and day 7). To investigate whether there were differences in WSR due to gender, dynamic WSR in the CCA was assessed in 11 presumed healthy males (mean age 24 y) and 11 presumed healthy females (mean age 25 y). Wall shear stress (WSS) was estimated from WSR and calculated whole blood viscosity. The average intrasubject intrasession variability was about 15% for peak WSR and about 12% for mean WSR. The intersubject intrasession variability for peak WSR decreased from 19% on day 1 to 16% on day 7 and for mean WSR from 17% on day 1 to 11% on day 7. The intersubject intersession variability is on the order of 5% for peak WSR and about 4% for mean WSR. No significant differences could be detected between peak and mean WSR values on day 1, day 2 and day 7, indicating good short- and medium-term intersubject intersession reproducibilities. No differences in peak and mean WSR were found between the left and the right CCA in the male group as well as in the female group. Mean WSS was similar in males (1.3 +/- 0.3 Pa) and in females (1.2 +/- 0.2 Pa), but peak WSS was slightly, but significantly, higher in males (4.3 +/- 1.3 Pa) than in females (3.3 +/- 0.7 Pa). It can be concluded that peak and mean WSR can be reliably determined noninvasively using ultrasound.

Adult↗

PubMed1997

Automated detection of local artery wall thickness based on M-line signal processing.

The Young's modulus of an arterial segment, a measure of the elastic properties of the arterial wall, requires the simultaneous and local assessment of pulse pressure, wall thickness, diameter, and distensibility (relative increase in cross-sectional area per change in blood pressure). The diameter and relative increase in cross-sectional area can be obtained with a wall track system, processing the radiofrequency (r.f.) ultrasound signals received along a single line of observation (M-line processing). It will be demonstrated that it is feasible to combine, in a single measurement, the assessment of wall thickness and the (relative change in) diameter involving a minimum of user interaction. Phantom tests show a standard error of the estimate for intima-media thickness measurements of less than 20 microns; in vivo registrations exhibit a variation on the order of 45 microns. It is concluded that processing of the radiofrequency ultrasound signal, acquired along an M-line, provides an accurate and time-efficient alternative for videoprocessing of 2-dimensional B-mode ultrasound images to estimate artery wall thickness.

Adult↗

PubMed1997

Accumulation of arachidonic acid in ischemic/reperfused cardiac tissue: possible causes and consequences.

Under physiological conditions, the content of unesterified arachidonic acid in cardiac tissue is very low. The bulk of arachidonic acid is present in the membrane phospholipid pool. Incorporation of arachidonic acid into phospholipids (reacylation) and liberation of this fatty acid from the phospholipid pool (deacylation) are controlled by a set of finely tuned enzymes, including lysophospholipid acyltransferase and phospholipase A2. At present, at least three subtypes of phospholipase A2 have been identified in cardiac structures, i.e., a low molecular mass group II phospholipase A2, a cytoplasmic high molecular mass phospholipase A2 and a plasmalogen-specific phospholipase A2. Cessation of flow to the heart (ischemia) gives rise to net degradation of membrane phospholipids accompanied by accumulation of fatty acids, including (unesterified) arachidonic acid. Restoration of flow to the previously ischemic cells results in a continued accumulation of fatty acids. The mechanism(s) underlying net phospholipid degradation in ischemic/reperfused myocardial tissue is (are) incompletely understood. Impaired reacylation, enhanced hydrolysis of phospholipids, or a combination of both may be responsible for the phenomena observed. Elevated tissue levels of arachidonic acid may exert both direct and indirect effects on the affected myocardium and healthy cardiac cells adjacent to the injured cardiomyocytes. Indirect effects might be evoked by arachidonic acid metabolites, i.e., eicosanoids. Arachidonic acid may directly influence ion channel activity, substrate metabolism and signal transduction, thereby affecting the functional characteristics of the ischemic/reperfused myocardium.

Acylation↗

PubMed1997

Female sex hormones do not influence arterial wall properties during the normal menstrual cycle.

1. In previous studies, the elastic properties of the common carotid artery were found to differ between men and women. In these studies, however, the phase of the menstrual cycle was not taken into consideration. It was the aim of the present study to investigate the effect of changing ovarian hormone levels during the normal menstrual cycle on the arterial wall properties of female large arteries. 2. We investigated the elastic right common carotid artery and the muscular right common femoral artery of normotensive young (18-35 years) female subjects (n = 12). The arterial distensibility and cross-sectional compliance coefficients were determined by the use of a specially designed ultrasonic wall-tracking device and measurements of automatic brachial artery cuff blood pressure. The phase of the menstrual cycle was assessed by ultrasonographic evaluation and measurement of 17 beta-oestradiol and progesterone blood plasma levels. 3. The distensibility coefficient and the cross-sectional compliance coefficient of both the common carotid and the common femoral artery did not change significantly during the normal menstrual cycle despite evidently changing ovarian hormone levels. 4. We conclude that the menstrual cycle does not influence the arterial wall properties of either the elastic common carotid artery or the muscular common femoral artery.

Adolescent↗