Contingency management (CM) has recently shown efficacy in promoting abstinence and retention in treatment among crack cocaine users in Brazil. However, partially because of unawareness and resistance among health care providers, CM has not been widely employed. The objective of this study was to conduct a secondary analysis in order to evaluate how CM participants perceive their treatment experience.
Twenty-seven crack cocaine users, previously assigned to 12 weeks of CM treatment, were assessed with a structured questionnaire designed to assess their personal opinion of, difficulty in understanding, and acceptance of the CM intervention, as well as their opinion regarding its impact on their treatment responses.
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In the last 20 years, the demand for crack cocaine dependence treatment has increased drastically in Brazil, and crack cocaine use has become a severe health concern in the country [1, 2]. In Brazil, crack cocaine use is associated with high rates of psychiatric comorbidities, cognitive impairment, unemployment, homelessness, sexually transmitted infections, involvement in illegal activities, incarceration, and death [3,4,5,6,7,8,9,10]. When compared to snorted cocaine users, crack cocaine users present poor treatment outcomes [11], with high dropout rates [6] and low post-treatment abstinence rates [12].
A recent randomized clinical trial conducted in the city of São Paulo, Brazil, presented strong evidence that CM is an effective treatment for crack cocaine dependence [25]. In that trial, CM was significantly more efficacious than was usual care in increasing rates of treatment session attendance and retention in treatment, as well as in reducing crack cocaine use and promoting continuous abstinence from crack cocaine. The authors also observed significant secondary effects of CM, including reductions in alcohol and marijuana use, as well as in post-treatment anxiety and depressive symptomatology [25, 26].
Although CM shows promise as an effective intervention to address the high morbidity and mortality associated with crack cocaine use, health providers at substance abuse treatment facilities are unaware of the positive effects of CM on treatment responses. Given the evidence of strong resistance to the use of CM among regular treatment staff, as shown in studies conducted in the United States [27, 28], it is important to provide health professionals with additional information regarding the benefits of incorporating CM into routine substance abuse treatment protocols [29]. One strategy to accomplish this is to provide crack cocaine users with the perspectives of patients regarding their experience with the CM intervention. Therefore, the aim of this study was to access how crack cocaine users exposed to a CM intervention in Brazil evaluated their experience with this novel treatment intervention.
For this study, we conducted secondary analyses of data collected in a randomized clinical trial developed between August 2012 through to July 2015, which was designed to evaluate the efficacy of CM in improving attendance and retention in treatment, as well as in reducing crack cocaine use and promoting continuous crack cocaine abstinence, in a sample of crack cocaine-dependent individuals seeking treatment at the Vila Maria Specialized Medical Outpatient Clinic for Alcohol and Drug Treatment, located in the northern region of the city of São Paulo [25]. Treatment at Vila Maria Outpatient clinic consisted of 90 min group therapy sessions a week that were focused on coping skills, training and relapse prevention; 90 min group occupational therapy weekly sessions; at least one individual session per month with a psychiatrist; and one individual psychotherapy session per week.
Individuals between 18 and 60 years of age, seeking treatment for crack cocaine dependence, as defined by the current criteria of the Diagnostic and Statistical Manual of Mental Disorders, Fourth edition (DSM-IV), assessed with the Structured Clinical Interview for DSM-IV Axis I Disorders [30], were screened for eligibility. Polydrug users were included if crack cocaine was the drug of choice. The exclusion criteria were being abstinent from crack cocaine for 4 or more weeks, being diagnosed with schizophrenia (confirmed using the structured interview), and not being able to attend treatment sessions three times per week. A total of 65 individuals met the study criteria and were enrolled in the study. Of those 65 participants, 33 were allocated to receive the CM intervention and 32 were allocated to receive the usual care intervention. All participants provided written informed consent. The study was approved by the Research Ethics Committee of the Federal University of São Paulo and by the Ethics Committee from the Brazilian National Ministry of Health (CAAE No: 00745912.4.0000.5505).
For 12 weeks, participants allocated to the CM intervention were encouraged to come to treatment sessions three times per week (Mondays, Wednesdays, and Fridays), at which time they would submit urine samples in order to identify crack cocaine use. Participants could earn vouchers with monetary value when they submitted samples testing negative for crack cocaine. Vouchers values increased when consecutive negative samples for crack cocaine were submitted but reset to the original value if participants missed a screening appointment or tested positive for crack cocaine. Vouchers could be used to obtain any goods available in the surrounding community, with the exception of tobacco and alcohol. If abstinent for all 12 weeks of the CM intervention, participants would receive a total of US$235.50 in vouchers. For a full description of the methods employed in the clinical trial, see Miguel et al. (2016). At the end of treatment, a total of 27 participants (81.8%) underwent post-treatment evaluation with a 6-item structured questionnaire designed to assess participant opinions regarding the ease or difficulty of understanding the CM protocol, acceptance of the CM intervention, and the impact of CM on treatment responses (Fig. 1).
Despite its limitations, our study provides important information on how crack users perceive the CM intervention. In a country were public treatment services for crack cocaine dependent individuals lack evidence of efficacy, the propagation of evidence based interventions is paramount. CM has recently shown efficacy in promoting retention in treatment, reducing crack cocaine use, promoting abstinence and also, reducing psychiatric symptomatology for treatment seeking crack cocaine dependent individuals in Sao Paulo, Brazil. This study has shown that, according to this population, CM was clearly and easily understood, well accepted and had a positive effect on their treatment response. Our results offer additional support for the applicability of CM in outpatient public treatment settings and advocates for the distribution of CM to other substance abuse treatment centers in Brazil.
Needs to impart appropriate elasticity and high toughness to viscoelastic polymer materials are ubiquitous in industries such as concerning automobiles and medical devices. One of the major problems to overcome for toughening is catastrophic failure linked to a velocity jump, i.e., a sharp transition in the velocity of crack propagation occurred in a narrow range of the applied load. However, its physical origin has remained an enigma despite previous studies over 60 years. Here, we propose an exactly solvable model that exhibits the velocity jump incorporating linear viscoelasticity with a cutoff length for a continuum description. With the exact solution, we elucidate the physical origin of the velocity jump: it emerges from a dynamic glass transition in the vicinity of the propagating crack tip. We further quantify the velocity jump together with slow- and fast-velocity regimes of crack propagation, which would stimulate the development of tough polymer materials.
Theoretical understanding of the velocity jump has been very limited, although it is important for toughening polymer materials. Previous theories based on linear fracture mechanics5 and linear viscoelasticity1 are unable to reproduce the velocity jump12, 18, 19. Although there is a theory that reproduces the jump20, the theory predicts an extremely high-temperature region near the crack tip whereas only a slight temperature-increase was experimentally observed21.
To construct the minimal model, we start from the two-dimensional square-lattice model (Fig. 2a), often used to simulate the structure and dynamics of sheet materials, with the lattice spacing l and the sheet height L under zero strain. Then, we derive a simplified model illustrated in Fig. 2b by decimating most of the lattice points. As shown in Fig. 2c, the survivors (lattice points) represent the minimum number of variables essential to describe crack propagation. To realize a crack propagation in the x-direction (i.e., horizontal direction), we assume that each bond is broken if the local strain at the crack tip is larger than the critical strain ε c . For simplicity, we assume that the sheet is symmetric about the x-axis, and thus we consider only the lattice points on the upper side.
To elucidate the physical origin of the velocity jump, we focus on a crossover among the three types of dynamic responses of Zener elements, corresponding to soft-elastic, viscoelastic, and hard-elastic regimes (Fig. 3b), depending on the time scale of the propagation dynamics. Since we are interested in a crack propagation closely related to relaxation responses (rather than oscillatory responses in Fig. 3b) of Zener elements, we introduce the two parameters
Physical pictures on the crack propagation revealed by the present exact solution. We draw the three illustrations based on Fig. 5. (a) The slow-velocity propagation is characterized by viscous dissipation in the vicinity of the crack tip. (b) The velocity jump induced by emergence of a hard-elastic regime (as a result of a dynamic glass transition) in the close vicinity of the crack tip. (c) The fast-velocity propagation is characterized by viscous dissipation on the rear side (with the hard-elastic regime in the close vicinity of the crack tip). Note that away from the crack tip viscous dissipation in the viscoelastic regime decays with the distance from the tip (see the text for details). 2ff7e9595c
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