A.M. de Roos

Pulmonary regurgitation in the late postoperative follow-up of tetralogy of Fallot. Volumetric quantitation by nuclear magnetic resonance velocity mapping.

BackgroundPulmonary regurgitation frequently occurs after surgical correction of tetralogy of Fallot. To date, reliable quantitation of pulmonary regurgitation has not been possible, and therefore the clinical significance of pulmonary regurgitation is controversial. Nuclear magnetic resonance (NMR) velocity mapping allows accurate measurement of volumetric flow. The feasibility and accuracy of NMR velocity mapping to quantify pulmonary regurgitation volumes are studied in patients after Fallot repair. Methods and ResultsIn 18 patients (mean age, 16.5±6.5 years), late (12.6±5.2 years) after Fallot surgery, forward and regurgitant volume flow was measured in the main pulmonary artery with NMR velocity mapping. To validate the measurements of pulmonary forward flow, right ventricular stroke volume was used as an internal reference standard. Pulmonary regurgitation volumes were compared with the differences between the corresponding right and left ventricular stroke volumes. Ventricular volumes were measured with a multisection gradient echo NMR method. In addition, the relation between pulmonary regurgitation and right ventricular volumes was studied. Measurements of pulmonary regurgitation volume with NMR velocity mapping closely corresponded with the tomographically determined volumes (r=.93). Forward pulmonary volume flow was neariy identical to right ventricular stroke volume (r=.98). Pulmonary regurgitation volume was significantly correlated with end-diastolic volume (r=.82, P<.0005), end-systolic volume (r=.63, P<.01), and stroke volume (r=.89, P<.0005) of the right ventricle but not with right ventricular ejection fraction (r= −.41, P=NS). ConclusionsNMR velocity mapping is an accurate method for the noninvasive, volumetric quantification of pulmonary regurgitation after surgical correction of tetralogy of Fallot.

Magnetic resonance imaging during dobutamine stress for detection and localization of coronary artery disease. Quantitative wall motion analysis using a modification of the centerline method.

Quantitative measurement of wall motion is essential to assess objectively the functional significance of coronary artery disease. We developed a quantitative wall thickening analysis on stress magnetic resonance images. This study was designed to assess the clinical value of magnetic resonance imaging (MRI) during dobutamine stress for detection and localization of myocardial ischemia in patients with suspected coronary artery disease. Methods and ResultsThirty-nine consecutive patients with clinically suspected coronary artery disease referred for coronary arteriography and 10 normal volunteers underwent gradient- echo MRI at rest and during peak dobutamine stress (infusion rate, 20 μg· kg−1· min−2). MRI was performed in the short-axis plane at four adjacent levels. Display in a cine loop provided a qualitative impression of regional wall motion (cine MRI). A modification of the centerline method was applied for quantitative wall motion analysis by means of calculation of percent systolic wall thickening. Short-axis cine MRI images were analyzed at 100 equally spaced chords constructed perpendicular to a centerline drawn midway between the end-diastolic and end-systolic contours. Dobutamine MRI was considered positive for coronary artery disease if the percent systolic wall thickening of more than four adjacent chords was < 2 SD below the mean values obtained from the normal volunteers. The overall sensitivity of dobutamine MRI for the detection of significant coronary artery disease (diameter stenosis ≥ 50%) was 91% (30 of 33), specificity was 80% (5 of 6), and accuracy was 90% (35 of 39). The sensitivity for identifying one-vessel disease was 88% (15 of 17), for twovessel disease 91% (10 of 11), and for three-vessel disease 100% (5 of 5). The sensitivity for detection of individual coronary artery lesions was 75% for the left anterior descending coronary artery, 87% for the right coronary artery, and 63% for the left circumflex coronary artery. ConclusionsDobutamine MRI clearly identifies wall motion abnormalities by quantitative analysis using a modification of the centerline method. Dobutamine MRI is an accurate method for detection and localization of myocardial ischemia and may emerge as a new noninvasive approach for evaluation of patients with known or suspected coronary artery disease.

Mobility versus density-limited predator-prey dynamics on different spatial scales

We investigate the dynamics of a predator-prey model that explicitly accounts for the spatial position and the movement behaviour of individual prey and predators, and does not assume the law of mass action. We show that limited individual mobility greatly reduces fluctuations in total density, although average densities and vital rates are virtually unaffected. We analyse the dynamics of patterns in the spatial distribution of prey and predator, which are generated by the model, and show that population dynamic observations at different spatial scales depend on the characteristic scale imposed by the individual biology.

Impact of Intraguild Predation and Stage Structure on Simple Communities along a Productivity Gradient

We analyze the consequences of intraguild predation and stage structure for the possible composition of a three‐species community consisting of resource, consumer, and predator. Intraguild predation, a special case of omnivory, induces two major differences with traditional linear food chain models: the potential for the occurrence of two alternative stable equilibria at intermediate levels of resource productivity and the extinction of the consumer at high productivities. At low productivities, the consumer dominates, while at intermediate productivities, the predator and the consumer can coexist. The qualitative behavior of the model is robust against addition of an invulnerable size class for the consumer population and against addition of an initial, nonpredatory stage for the predator population, which means that the addition of stage structure does not change the pattern. Unless the top predator is substantially less efficient on the bottom resource, it tends to drive the intermediate species extinct over a surprisingly large range of productivities, thus making coexistence generally impossible. These theoretical results indicate that the conditions for stable food chains involving intraguild predation cannot involve strong competition for the bottommost resource.

Large-amplitude cycles of Daphnia and its algal prey in enriched environments

Ecological theory predicts that stable populations should yield to large-amplitude cycles in richer environments. This does not occur in nature. The zooplankton Daphnia and its algal prey in lakes throughout the world illustrate the problem. Experiments show that this system fits the theorys assumptions, yet it is not destabilized by enrichment. We have tested and rejected four of five proposed explanations. Here, we investigate the fifth mechanism: inedible algae in nutrient-rich lakes suppress cycles by reducing nutrients available to edible algae. We found three novel results in nutrient-rich microcosms from which inedible algae were excluded. First, as predicted by theory, some Daphnia-edible algal systems now display large-amplitude predator-prey cycles. Second, in the same environment, other populations are stable, showing only small-amplitude demographic cycles. Stability is induced when Daphnia diverts energy from the immediate production of young. Third, the system exhibits coexisting attractors—a stable equilibrium and large-amplitude cycle. We describe a mechanism that flips the system between these two states.

Interference and generation cycles

In this paper we re-examine the derivation of an interference limited functional response due to Beddington (1975 J. Anim. Ecol. 44, 331–340) and extend his treatment to more realistic models of the interference process. We study the dynamic effects of interference in the context of a structured population model and show that the stabilising effect of interference against paradox of enrichment cycles is unaffected by age-structure. We also demonstrate that single generation cycles are much more weakly affected by interference than prey-escape cycles. Thus the net effect of weak interference is to prevent single generation cycles from being masked by the prey-escape cycles which would otherwise dominate the population dynamics.

Progression of brain atrophy and cognitive decline in diabetes mellitus A 3-year follow-up

Objective: To investigate progression of MRI-assessed manifestations of cerebral degeneration related to cognitive changes in a population of elderly patients with diabetes mellitus (DM) compared to age-matched control subjects. Methods: From a randomized controlled trial (PROSPER study), a study sample of 89 patients with DM and 438 control subjects without DM aged 70–82 years were included for brain MRI scanning and cognitive function testing at baseline and reexamination after 3 years. Changes in brain atrophy, white matter hyperintensities (WMHs), number of infarctions, and cognitive function test results were determined in patients with DM and subjects without DM. Linear regression analysis was performed with correction for age, gender, hypertension, pravastatin treatment, educational level, and baseline test results. In patients with DM, baseline MRI parameters were correlated with change in cognitive function test result using linear regression analysis with covariates age and gender. Results: Patients with DM showed increased progression of brain atrophy (p < 0.01) after follow-up compared to control subjects. No difference in progression of WMH volume or infarctions was found. Patients with DM showed increased decline in cognitive performance on Stroop Test (p = 0.04) and Picture Learning Test (p = 0.03). Furthermore, in patients with DM, change in Picture Learning Test was associated with baseline brain atrophy (p < 0.02). Conclusion: Our data show that elderly patients with DM without dementia have accelerated progression of brain atrophy with significant consequences in cognition compared to subjects without DM. Our findings add further evidence to the hypothesis that diabetes exerts deleterious effects on neuronal integrity.

Cannibalism as a life boat mechanism

Under certain conditions a cannibalistic population can survive when food for the adults is too scarce to support a non-cannibalistic population. Cannibalism can have this lifeboat effect if (i) the juveniles feed on a resource inaccessible to the adults; and (ii) the adults are cannibalistic and thus incorporate indirectly the inaccessible resource. Using a simple model we conclude that the mechanism works when, at low population densities, the average yield, in terms of new offspring, due to the energy provided by one cannibalized juvenile is larger than one.

Diagnostic significance of gadolinium-DTPA (diethylenetriamine penta-acetic acid) enhanced magnetic resonance imaging in thrombolytic treatment for acute myocardial infarction: its potential in assessing reperfusion.

The diagnostic value of gadolinium-DTPA (diethylenetriamine penta-acetic acid) enhanced magnetic resonance imaging in patients treated by thrombolysis for acute myocardial infarction was assessed in 27 consecutive patients who had a first acute myocardial infarction (14 anterior, 13 inferior) and who underwent thrombolytic treatment and coronary arteriography within 4 hours of the onset of symptoms. Magnetic resonance imaging was performed 93 hours (range 15-241) after the onset of symptoms. A Philips Gyroscan (0.5 T) was used, and spin echo measurements (echo time 30 ms) were made before and 20 minutes after intravenous injection of 0.1 mmol/kg gadolinium-DTPA. In all patients contrast enhancement of the infarcted areas was seen after Gd-DTPA. The signal intensities of the infarcted and normal values were used to calculate the intensity ratios. Mean (SD) intensity ratios after Gd-DTPA were significantly increased (1.15 (0.17) v 1.52 (0.29). Intensity ratios were higher in the 17 patients who underwent magnetic resonance imaging more than 72 hours after the onset of symptoms than in the 10 who underwent magnetic resonance imaging earlier, the difference being significantly greater after administration of Gd-DTPA (1.38 (0.12) v 1.61 (0.34). When patients were classified according to the site and size of the infarcted areas, or to reperfusion (n = 19) versus non-reperfusion (n = 8), the intensity ratios both before and after Gd-DTPA did not show significant differences. Magnetic resonance imaging with Gd-DTPA improved the identification of acutely infarcted areas, but with current techniques did not identify patients in whom thrombolytic treatment was successful.

Safety of ruling out acute pulmonary embolism by normal computed tomography pulmonary angiography in patients with an indication for computed tomography: systematic review and meta-analysis.

Introduction: Several outcome studies have ruled out acute pulmonary embolism (PE) by normal computed tomography pulmonary angiography (CTPA). We performed a meta‐analysis in order to determine the safety of this strategy in a specific group of patients with a strict indication for CTPA, that is, ‘likely’ or ‘high’ clinical probability for PE, an elevated D‐dimer concentration, or both. Methods: Studies that ruled out PE by normal CTPA, with or without subsequent normal bilateral compression ultrasonography (CUS), in patients with a strict indication for CTPA, were searched for in Medline, EMBASE, Web of Science and the Cochrane dataset. The primary endpoint was the occurrence of (fatal) venous thromboembolism (VTE) in a 3‐month follow‐up period. Results: Three studies were identified that excluded PE by CTPA alone (2020 patients), and three studies that performed additional CUS of the legs after normal CTPA (1069 patients). The pooled incidence of VTE at 3 months was 1.2% [95% confidence interval (CI) 0.8–1.8] based on a normal CTPA result as a sole test, and 1.1% (95% CI 0.6–2.0) based on normal CTPA and negative CUS findings, resulting in negative predictive values of 98.8% (95% CI 98.2–99.2) and 98.9% (95% CI 98.0–99.4), respectively. This compares favorably with the VTE failure rate after normal pulmonary angiography (1.7%, 95% CI 1.0–2.7). The risk of fatal PE did not differ between the diagnostic strategies (0.6% vs. 0.5%). Conclusion: A normal CTPA result alone can safely exclude PE in all patients in whom CTPA is required to rule out this disease. There is no need for additional ultrasonography to rule out VTE in these patients.