Second pass effect

In the concentration time curve the first pass of the bolus appears after the pre-contrast baseline (Fig. 6.4a). After the first pass, concentration does not return to zero, but remains increased (Fig. 6.4b). This effect is due to contrast agent molecules remaining within the capillary network and to second pass effects. One way to avoid these effects is to fit a gamma-variate function to the measured values of contrast agent concentration (Belliveau et al. 1991 Thompson et al. 1964) (Fig. 6.4c) ... 

Alternatively, PP represent a huge compartment within the flowing blood where the drug-wannabe will have free access and binding (PPB) that can sometimes be useful to avoid the first- and second-pass effect. In order to hide out in this or any other type of drug depot, however, binding has to be extremely high. This is because the equilibration rates for PPB and localization versus residence in the blood s aqueous milieu are typically very rapid. Thus, PPB usually is not considered to be a major influence upon a substance s distribution unless it occurs at the level of 90% or more (see Chapter 12 for a more detailed discussion of this topic). 

As discussed in Ref. [38], the compression effect suggests PA with a second PA pulse red-detuned relative to the first one, and populating deeply bound levels in the excited state. In favorable cases, the latter levels could contribute efficiently to the stabilization step toward deeply bound levels of the lower electronic state, and hence to the formation of vibrationally cold stable molecules. Indeed, the bottleneck in the PA reaction being the small probability density for pairs of cold atoms at short relative distance, the existence of a compression effect seems an interesting procedure to improve PA. 

Stroke is the leading cause of major long-term disability in adults and the third leading cause of death in the United States. On average, a new stroke occurs every 45 seconds. Thrombolytic therapy with intravenous recombinant tissue-plasminogen activator (IV rt-PA) is the most effective treatment for acute ischemic stroke. In this chapter, we review the rationale for thrombolysis in acute ischemic stroke, clinical evidence supporting the use of thrombolytics, and the application of thrombolysis in practice. 

Oae found that for both base- and acid-catalyzed hydrolysis of phenyl benzenesul-fonate, there was no incorporation of 0 from solvent into the sulfonate ester after partial hydrolysis. This was interpreted as ruling out a stepwise mechanism, but in fact it could be stepwise with slow pseudorotation. In fact this nonexchange can be explained by Westheimer s rules for pseudorotation, assuming the same rules apply to pentacoordinate sulfur. For the acid-catalyzed reaction, the likely intermediate would be 8 for which pseudorotation would be disfavored because it would put a carbon at an apical position. Further protonation to the cationic intermediate is unlikely even in lOM HCl (the medium for Oae s experiments) because of the high acidity of this species a Branch and Calvin calculation (See Appendix), supplemented by allowance for the effect of the phenyl groups (taken as the difference in between sulfuric acid and benzenesulfonic acid ), leads to a pA, of -7 for the first pisTa of this cation about -2 for the second p/sTa. and about 3 for the third Thus, protonation by aqueous HCl to give the neutral intermediate is likely but further protonation to give cation 9 would be very unlikely. 

The proton affinities (PA) of two restricted subsets of amines were correlated directly with inductive and polarizability effect parameters, respectively (Figs. 19 and 22). These can be combined with data on other hetero-substituted amines to give a set of 80 amines of different skeletal and substitution types (e.g. Fig. 24). In this and all other systems (below), a residual electronegativity value, %l2, (Eq. 5) derived from those of the atoms of the first, 1, and second, 2, sphere neighbors of the nitrogen atom is preferred as a measure of the inductive effect49). 

The standard ruthenium arene and CATHy catalysts are insoluble in water, but are nevertheless stable in the presence of water. Reactions in the I PA system can be carried out in mixtures of isopropanol and water the net effect is a lower rate due to dilution of the hydrogen donor. The use of formate salts in water, with CATHy or other transfer hydrogenation catalysts dissolved in a second immiscible phase was shown to work well with a number of substrates and in some cases to improved reaction rates [34]. The use of water as reaction solvent will be discussed in more detail in Section 35.5. 

Chen, J. Y. and W. Kollmann (1988). PDF modeling of chemical nonequilibrium effects in turbulent nonpremixed hydrocarbon flames. In Twenty-second Symposium (International) on Combustion, pp. 645-653. Pittsburgh, PA The Combustion Institute. 

A second issue relating to long-term medication is the effect of withdrawing medication at the end of a period of treatment. Benzodiazepines are associated with discontinuation symptoms, and their repeated use may foster the development of true physiological dependence. In a study of discontinuation of treatment for panic disorder [Rickels et al. 1993) with either alprazolam [n = 27), imipramine [n = 11) or placebo [n = 10), a withdrawal syndrome was observed in almost all patients treated with alprazolam but in few pa-... 

Data reported in the present work demonstrate that the degree of crystallinity and the acid properties are related the amount of present at the surface of VPP. When the VPP is not fully equilibrated, and hence may contain discrete amounts of it is more selective to MA and less to PA. The reason is that in oxidized catalysts, the olefmic intermediate is preferentially oxidized to MA, rather then being subjected to the acid-catalyzed condensation with a second unsaturated molecule, to yield the precursor of PA. When instead the catalyst is more crystalline, and hence it does contain less oxidized V sites, its surface acid properties predominate over O-insertion properties, and the catalyst becomes more effective in PA formation. In this case, the selectivity to PA at 50% n-pentane conversion becomes comparable to that one of MA. 

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