Physical dependence, definition

Since operational definition of physical dependence is the tendency to display a withdrawal syndrome after drug termination, the severity of the withdrawal syndrome is the most commonly used indicator of the degree of physical dependence. Rodent models differ in at least three dimensions the means used to induce physical dependence, the means subsequently used to induce a withdrawal syndrome and the variables nsed to estimate the severity of the withdrawal syndrome. Two models may be identical in one of these respects, yet totally different in another. 

In view of the potential pitfalls in giving benzodiazepines to polydrug users, the subject will be examined carefully, starting with probably the only situation in which it is definitely advisable to prescribe, which is in short-term withdrawal from benzodiazepine misuse where there is demonstrable physical dependence and within a closely monitored arrangement... 

A common vocabulary has not been established in the field of prescription drug abuse. Because much of the survey data collected in this area refer to nonmedical use of prescription drugs, this definition of abuse, rather than that of the Diagnostic and Statistical Manual of Mental Disorders (DSM), is used. Also, because physical dependence to prescription medications can develop during medically supervised appropriate use, the term addiction is used to reflect dependence as defined by the DSM. 

Many myths surround the stronger opioids and their use can become restricted when the definitions of addiction, tolerance and physical dependence are confused. Some opioids, e.g. codeine, are less potent and are readily available in OTC products. 

The American Psychiatric Association (APA) and the World Health Organization (WHO) use the word dependence for the same concept. A more common definition for physical dependence, however, is this ... 

Development of physical dependence during the longterm administration of methadone can be demonstrated by drug withdrawal or by administration of an opioid antagonist. Subcutaneous administration of 10 to 20 mg methadone to former opioid addicts produces definite euphoria equal in duration to that caused by morphine, and its overall abuse potential is comparable with that of morphine. 

Evidence does not exclude the participation of psychogenic factors in the modification of the opiate abstinence syndrome (6). The reaction of the subject to his discomfort and to the meaning of withdrawal of drugs certainly contributes to the total picture of abstinence. Addicts with the same degree of physical habit react to the discomfort of withdrawal with widely different degrees of mental intensity. The hypothesis that withdrawal symptoms are entirely psychic is not tenable in view of the definite abstinence syndromes which have been produced in various lower animals and in view of the work of Wilder with chronic spinal and decorticated dogs. Wikler s results indicate that some components of physical dependence still develop under conditions where they can have no symbolic significance for the animal. These components are definitely nonmental (noncerebral). 

A rigorous, theoretical definition of the boiling range distribution of petroleum fiactions is not possible due to the complexity of the mixture as well as the unquantifiable interactions among the components (for example, azeotropic behavior). Any other means used to define the distribution would require the use of a physical process such as a conventional distillation or gas c omatographic characterization. This would therefore result in a method-dependent definition and would not constitute a true value from which bias can be calculated. 

The definition above is a particularly restrictive description of a nanocrystal, and necessarily limits die focus of diis brief review to studies of nanocrystals which are of relevance to chemical physics. Many nanoparticles, particularly oxides, prepared dirough die sol-gel niediod are not included in diis discussion as dieir internal stmcture is amorjihous and hydrated. Neverdieless, diey are important nanoniaterials several textbooks deal widi dieir syndiesis and properties [4, 5]. The material science community has also contributed to die general area of nanocrystals however, for most of dieir applications it is not necessary to prepare fully isolated nanocrystals widi well defined surface chemistry. A good discussion of die goals and progress can be found in references [6, 7, 8 and 9]. Finally, diere is a rich history in gas-phase chemical physics of die study of clusters and size-dependent evaluations of dieir behaviour. This topic is not addressed here, but covered instead in chapter C1.1, Clusters and nanoscale stmctures, in diis same volume. 

The explicit definition of water molecules seems to be the best way to represent the bulk properties of the solvent correctly. If only a thin layer of explicitly defined solvent molecules is used (due to hmited computational resources), difficulties may rise to reproduce the bulk behavior of water, especially near the border with the vacuum. Even with the definition of a full solvent environment the results depend on the model used for this purpose. In the relative simple case of TIP3P and SPC, which are widely and successfully used, the atoms of the water molecule have fixed charges and fixed relative orientation. Even without internal motions and the charge polarization ability, TIP3P reproduces the bulk properties of water quite well. For a further discussion of other available solvent models, readers are referred to Chapter VII, Section 1.3.2 of the Handbook. Unfortunately, the more sophisticated the water models are (to reproduce the physical properties and thermodynamics of this outstanding solvent correctly), the more impractical they are for being used within molecular dynamics simulations. 

The described method can generate a first-order backward or a first-order forward difference scheme depending whether 0 = 0 or 0 = 1 is used. For 9 = 0.5, the method yields a second order accurate central difference scheme, however, other considerations such as the stability of numerical calculations should be taken into account. Stability analysis for this class of time stepping methods can only be carried out for simple cases where the coefficient matrix in Equation (2.106) is symmetric and positive-definite (i.e. self-adjoint problems Zienkiewicz and Taylor, 1994). Obviously, this will not be the case in most types of engineering flow problems. In practice, therefore, selection of appropriate values of 6 and time increment At is usually based on trial and error. Factors such as the nature of non-linearity of physical parameters and the type of elements used in the spatial discretization usually influence the selection of the values of 0 and At in a problem. 

The elastic and viscoelastic properties of materials are less familiar in chemistry than many other physical properties hence it is necessary to spend a fair amount of time describing the experiments and the observed response of the polymer. There are a large number of possible modes of deformation that might be considered We shall consider only elongation and shear. For each of these we consider the stress associated with a unit strain and the strain associated with a unit stress the former is called the modulus, the latter the compliance. Experiments can be time independent (equilibrium), time dependent (transient), or periodic (dynamic). Just to define and describe these basic combinations takes us into a fair amount of detail and affords some possibilities for confusion. Pay close attention to the definitions of terms and symbols. 

Aerosol technology may be defined as involving the development, preparation, manufacture, and testing of products that depend on the power of a hquefied or compressed gas to expel the contents from a container. This definition can be extended to iaclude the physical, chemical, and toxicological properties of both the finished aerosol system and the propellants. 

The value of tire heat transfer coefficient of die gas is dependent on die rate of flow of the gas, and on whether the gas is in streamline or turbulent flow. This factor depends on the flow rate of tire gas and on physical properties of the gas, namely the density and viscosity. In the application of models of chemical reactors in which gas-solid reactions are caiTied out, it is useful to define a dimensionless number criterion which can be used to determine the state of flow of the gas no matter what the physical dimensions of the reactor and its solid content. Such a criterion which is used is the Reynolds number of the gas. For example, the characteristic length in tire definition of this number when a gas is flowing along a mbe is the diameter of the tube. The value of the Reynolds number when the gas is in streamline, or linear flow, is less than about 2000, and above this number the gas is in mrbulent flow. For the flow... 

Climate is often viewed as the aggregate of all of the elements of weather, with quantitative definitions being purely physical. However, because of couplings of carbon dioxide and many other atmospheric species to both physical climate and to the biosphere, the stability of the climate system depends in principle on the nature of feedbacks involving the biosphere. For example, the notion that sulfate particles originating from the oxidation of dimethylsulfide emitted by marine phytoplankton can affect the albedo (reflectivity) of clouds (Charlson et ai, 1987). At this point these feedbacks are mostly unidentified, and poorly quantified. 

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