N.M. van Dijk

Cost optimal allocation of rail passenger lines

We consider the problem of cost optimal railway line allocation for passenger trains for the Dutch railway system. At present, the allocation of passenger lines by Dutch Railways is based on maximizing the number of direct travelers. This paper develops an alternative approach that takes operating costs into account. A mathematical programming model is developed which minimizes the operating costs subject to service constraints and capacity requirements. The model optimizes on lines, line types, routes, frequencies and train lengths. First, the line allocation model is formulated as an integer nonlinear programming model. This model is transformed into an integer linear programming model with binary decision variables. An algorithm is presented which solves the problem to optimality. The algorithm is based upon constraint satisfaction and a Branch and Bound procedure. The algorithm is applied to a subnetwork of the Dutch railway system for which it shows a substantial cost reduction. Further application and extension seem promising.

Closed queueing networks with batch services

In this paper we study queueing networks which allow multiple changes at a given time. The model has a natural application to discrete-time queueing networks but describes also queueing networks in continuous time.It is shown that product-form results which are known to hold when there are single changes at a given instant remain valid when multiple changes are allowed.

Bound performance models of heterogeneous parallel processing systems

Systems of heterogeneous parallel processing are studied such as arising in parallel programs executed on distributed systems. A lower and an upper bound model are suggested to obtain secure lower and upper bounds on the performance of these systems. The bounding models are solved by using a matrix-geometric algorithmic approach. Formal proofs of the bounds are provided along with error bounds on the accuracy of the bounds. These error bounds in turn are reduced to simple computational expressions. Numerical results are included. The results are of interest for application to arbitrary fork-join models with parallel heterogeneous processors and synchronization.

Attitudes towards obesity treatment in GP training practices: a focus group study

BACKGROUND Both patients and government expect the GP to treat obesity. Previous studies reported a negative attitude of GPs towards this task. Little is known about the attitude of GP trainees. OBJECTIVES To assess the attitude and other factors that influence the willingness and ability of GP trainees to provide lifestyle interventions for overweight patients. METHODS A qualitative study was performed using focus groups, consisting of first- and third-year trainees, GP trainers and teachers. Two researchers analysed the data independently. RESULTS First-year trainees lack knowledge and a positive attitude. Third-year trainees, although trained in motivational interviewing techniques, lack specific knowledge and feel cheated when discussing eating habits. Trainers are despondent as they rarely observe long-lasting results. Teachers warn the trainees not to have high hopes. The trainers and trainees fear ruining the relationship with their patient, and all make a request for evidence-based multidisciplinary treatment programmes, joint responsibility and an image change in society to stop the epidemic. CONCLUSIONS Trainees do not feel more competent in treating overweight patients successfully over the course of their GP specialty training and GP trainers are not convinced of the success of the treatment of overweight patients. Therefore, it could be equally important to reflect on the GP trainer as a role model as to concentrate on the education of the trainee. Both need a revived attitude and evidence-based treatment programmes, help from policy makers and an attitude change in society are desired.

Physical countermeasures to increase orthostatic tolerance

Keywords: blood pressure, cardiac output, muscle pump, orthostatic hypotension, syncope, venous return.IntroductionStanding upright challenges the cardiovascularsystem as the pull of gravity displaces about 70%of the circulating blood volume to below heart level,much of it to the compliant veins of the dependentlimbs and the pelvic organs. In patients withautonomic failure due to neurodegenerative dis-eases, the normal cardiovascular adjustments tothis challenge are impaired, and symptomaticorthostatic hypotension becomes a common riskon standing or even sitting quietly. These patientslearn to sway and shift, so that the pumping actionof the muscles can be utilized to counter gravita-tional displacement of blood by squeezing venousblood from the legs upward. Augmentation ofvenous return in the upright posture can also beachieved by deliberate tensing of lower limb andabdominal muscles [1, 2], as depicted in Fig. 1.These clinical observations were the basis forphysical countermeasures, which are taught topatients with autonomic failure to combat symp-tomatic orthostatic hypotension [3–5]. Physicalcounterpressure manoeuvres specifically generatea counterpressure to oppose gravitational venouspooling (e.g. a single bout of lower-body musclecontraction to translocate blood centrally andsustained tensing of the same muscles to preventsubsequent peripheral pooling in the legs andabdomen). More recently, it has been shown thatphysical counterpressure manoeuvres are alsoeffective interventions in otherwise healthy sub-jects with episodic orthostatic syncope due toneurally mediated (i.e. vasovagal reactions) [6, 7]or postexercise syncope [8].In this narrative review, we will primarily considerthese physical counterpressure manoeuvres. Sec-ondarily, we will describe the broader category ofphysical countermeasures that include breathingmanoeuvres and other physical methods, tooppose orthostasis. Existing external devices,which operate through some of the same physio-logical principles as these manoeuvres, will only bediscussed for proof of principle.The defining characteristic of the manoeuvresdescribed in this review is the fact that they canbe employed by patients when a faint is imminent.This is in contrast to devices such as bandages andabdominal belts, which require ongoing use to beeffective. We will discuss both early studies inpatient with primary autonomic failure due toneurodegenerative diseases, as well as more recentexperience obtained in patients with neurally med-iated syncope. The physiology and pathophysiologyof orthostatic blood pressure control and perfusionof the brain are key factors in understanding howphysical countermeasures work. These topics havebeen reviewed extensively [2, 9–12] and will only bediscussed here briefly.Physical counterpressure manoeuvresMuscle tensingIt has been reported that intramuscular pressure isrelated to orthostatic tolerance [2]. Hendersonet al. demonstrated that intramuscular pressuremeasured in the relaxed biceps muscle wasdecreased after prolonged bed rest (38%), followingsurgery (35%), during voluntary hyperventilation(28%) and in the absence of air movement over theskin (31%) [13, 14]. These conditions are stronglyassociated with decreased orthostatic toleranceand a tendency to faint [2, 15]. In addition,intramuscular calf pressure has been shown tobe 15–24 and 6–9 mmHg, respectively, in thosewithout and with a tendency to faint during thehead-up tilt test using a tilt table with a saddle andsuspended legs (Fig. 1) [16].Although these interesting results from studiesperformed in the 1930s and early 1940s have

Strong Stochastic Bounds for the Stationary Distribution of a Class of Multicomponent Performability Models

We consider a class of models for multicomponent systems in which components can break down and be repaired in a dependent manner and where breakdown and repair times can be arbitrarily distributed. The problem of calculating the equilibrium distribution and, from this, the expected performability for these models is intractable unless certain assumptions are made about breakdowns and repairs. In this paper we show that the performability of multicomponent systems that do not satisfy these rules can be bounded by tractable modifications. Our results are proved by stochastic comparability arguments and a Markov reward technique, which is of interest in itself as it enables one to prove that the equilibrium distribution of one process can be bounded by that of another even when the sample paths of the process are not. This is illustrated by an example.

Factors influencing the EBM behaviour of GP trainers: A mixed method study

Background: General practitioner (GP) trainees state that their trainers are not consistent in using evidence-based medicine (EBM) or are even dismissive of it. As trainers are important role models in the Dutch GP training system this could have a large influence on the EBM training of GP trainees. Aim: To establish the motivations and barriers of Dutch GP trainers in using EBM. Methods: A questionnaire on personal characteristics, knowledge, skills (Berlin, score 0–15) and attitude (McColl, VAS score 0–100), and statements about EBM barriers were presented to 106 GP trainers. Additionally, three focus group sessions with trainers (n = 30) were held. Results: Knowledge and skills were less than half correct (mean 6.1, standard deviation (SD) 2.9); the overall score on attitude was 58.8 (SD 9.4). Factor analysis showed four categories of barriers: EBM competence (mean 3.5 (SD 0.8)), search activities (mean 3.5 (SD 0.8)), motivation (mean 3.8 (SD 0.7)) and time (mean 2.5 (SD 0.9)). After analysis of the focus group sessions, five categories of motivations and barriers predominated: EBM competence, attitude and behaviour, sources, time and logistics. Conclusion: GP trainers experience motivations in EBM; however, these motivations can also be barriers, depending on the trainers level of knowledge and attitude.

At the heart of the arterial baroreflex: a physiological basis for a new classification of carotid sinus hypersensitivity

The aim of this review is to provide an update of the current knowledge of the physiological mechanisms underlying reflex syncope. Carotid sinus syncope will be used as the classical example of an autonomic reflex with relatively well‐established afferent, central and efferent pathways. These pathways, as well as the pathophysiology of carotid sinus hypersensitivity (CSH) and the haemodynamic effects of cardiac standstill and vasodilatation will be discussed. We will demonstrate that continuous recordings of arterial pressure provide a better understanding of the cardiovascular mechanisms mediating arterial hypotension and cerebral hypoperfusion in patients with reflex syncope. Finally we will demonstrate that the current criteria to diagnose CSH are too lenient and that the conventional classification of carotid sinus syncope as cardioinhibitory, mixed and vasodepressor subtypes should be revised because isolated cardioinhibitory CSH (asystole without a fall in arterial pressure) does not occur. Instead, we suggest that all patients with CSH should be thought of as being ‘mixed’, between cardioinhibition and vasodepression. The proposed stricter set of criteria for CSH should be evaluated in future studies.

Product forms for stochastic interference systems

Systems are studied that consist of interfering components which are alternatively active (busy) and passive (idle) for random periods. Such systems naturally arise from the performance evaluation of computer models. A concrete invariance condition is imposed on the interferences allowed. Under this condition, the steady-state vector of active components is shown to be of product form as well as to be robust to distributional forms of active and passive periods. The proof is simple and self-contained. A number of concrete and generic examples is provided. These include resource sharing mechanisms, hierarchical circuit switchings, parallel processing, priorities, and breakdowns.

Blended learning in CME: the perception of GP trainers

INTRODUCTION Blended learning (the combination of electronic methods with traditional teaching methods) has the potential to combine the best of traditional education with the best of computer-mediated training. We chose to develop such an intervention for GP trainers who were undertaking a Continuing Medical Education (CME) course in evidence-based medicine (EBM). This study reports on our experience and investigated the factors influencing the perception on usefulness and logistics of blended learning for learners in CME. METHODS In total, 170 GP trainers participated in the intervention. We used questionnaires, observations during the four face-to-face meetings and evaluations in the e-course over one year. Additionally we organised focus groups to gain insight in some of the outcomes of the questionnaires and interpretations of the observations. RESULTS The GP trainers found the design and the educational method (e-course in combination with meetings) attractive, instructive and complementary. Factors influencing their learning were (1) educational design, (2) educational method, (3) topic of the intervention, (4) time (planning), (5) time (intervention), (6) learning style, (7) technical issues, (8) preconditions and (9) level of difficulty. A close link between daily practice and the educational intervention was considered an important precondition for the success of the intervention in this group of learners. CONCLUSION GP trainers were positive about blended learning: they found e-learning a useful way to gain knowledge and the meetings a pleasant way of transferring the knowledge into practice. Although some preconditions should be taken into consideration during its development and implementation, they would participate in similarly designed learning in the future.