Dynamic dead space

Improved alveolar ventilation may be partly compromised by an increase in the dynamic dead space (VDdyn), derived from the physiologic dead space (VDphys) plus the dead space of the apparatus (VDap). Whereas the physiologic dead space is influenced by the tidal volume, the dead space of the apparatus is a fixed consequence of the internal volume of the interface. Differences in flow pattern and pressure waveform associated with the machine and mode of ventilation, also affect the dead space of the apparatus. Saatci et al. (36) noted that during spontaneous breathing, a face mask increased VDdyn from 32% to 42% of tidal volume (VT) above VDp ys. Positive pressure during the expiratory phase reduced VDdyn close to VDphys, while inspiratory pressure support without positive end-expiratory pressure decreased VDdyn from 42% to 39% of VT, i.e., VDdyn remained higher than VDphys. When the exhalation port was placed close to the nasal bridge, VDdyn was lower than VDp ys as a consequence of a beneficial flow path that decreased VDdyn (from 42% to 28% of VT), in the presence of an expiratory positive pressure. 

Saatci E, Miller DM, Stell IM, et al. Dynamic dead space in face masks used with noninvasive ventilators a lung model study. Eur Respir J 2004 23 129-135. 

The effectiveness of a fixed-bed operation depends mainly on its hydraulic performance. Even if the physicochemical phenomena are well understood and their application in practice is simple, the operation will probably fail if the hydraulic behavior of the reactor is not adequate. One must be able to recognize the competitive effects of kinetics and fluid dynamics mixing, dead spaces, and bypasses that can completely alter the performance of the reactor when compared to the ideal presentation (Donati and Paludetto, 1997). The main factor of failure in liquid-phase operations is liquid maldistribution, which could be related to low liquid holdup in downflow operation, or other design problems. These effects could be critical not only in full-scale but also in pilot- or even in laboratory-scale reactors. 

Again in the case of flowing liquids, the supply of the cathodic reagents or of the inhibitors to the surface depends on the fluid dynamic conditions, and it may have contrasting effects. The general increase in the thickness of the boundary layer can reduce the supply of the cathodic reactant and that of the inhibitor, and, within the dead spaces, a cathodic process can be substituted for another, such as in the case of aerated and sulfatic waters, where anaerobic conditions that are established in the dead spaces permit the development of sulfate-reducing bacteria. 

In respiratory failure from airflow obstruction, the increased pressures required for airflow may overload ventilatory muscles and the narrowed airways predispose to intrinsic PEEP (9). The slow expiratory phase and the increased dead space promote dynamic... 

To what extent helium is adsorbed has been of major concern in adsorption studies for both volumetric and gravimetric methods. Until recently, the experimental error was often attributed to the finite adsorption of helium at high pressures, and different remedial methods were suggested [38-40]. The effect of helium adsorption on the gravimetric technique is clearly shown in Eq. (8). The volume difference, AF, will be overestimated if the adsorption of heUum is not negligible. Its effect on the volumetric technique ean be explained in terms of Fig. 1. The volume of the solid phase of adsorbent, F, is experimentally determined by heUum. This volume is sometimes called dead space or hehum volume of the adsorption cell, which is, indeed, the volume of adsorbent inaccessible to the hehum molecules. However, this value is usually taken for the volume of adsorbent inaccessible to the adsorbate molecules. The difference in molecular dynamic size and shape between helium and adsorbate is logically a source of error. The irregular solid surface and/or the complex strueture of micropores inevitably render uncertainty in the determination of P. As a eonsequenee of helium adsorption, the dead volume is underestimated. 

CEMA dead times, typically lns (i.e. of the same order as the width of a TOFMS mass peak but usually less than the spacing between mass peaks 1 Th apart), provide a limit to the upper end of the dynamic range for any ion counting strategy. However, since ion arrival times are described by Poisson statistics (see the text box in this... 

The interconnected pore space is often denoted as the effective pore space, while unconnected pores may be considered from the hydro-dynamic point of view as part of the solid matrix, since those pores are ineffective as far as flow through the porous medium is concerned. They are dead-end pores or blind pores, that contain stagnant fluid and no flow occurs through them. 

It is necessary to assume that the medium is isotropic, uniform, randomly constructed and represented by a nore-throat geometry between its boundaries. Connectivity of 1 implies a poorly connected or dead-ended pore connectivity of zero defines an isolated pore. Parallel, non-intersecting pores would define a one dimension void space of connectivity 2. Time effects due to intrapatticle back-mixing would create an extra dimension and lead to dynamic sensitivity of apparent transport resistance. 

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