Bi-level devices

Table 1 provides a list of the most commonly used pressure controlled ventilators, of which the bi-level devices are the majority. 

The variable controlled during the expiratory phase is known as the baseline variable, most commonly, pressure, typically expressed as EPAP or PEEP. It is necessary to have a positive baseline pressure in bi-level devices to assure CO2 washout. Bi-level devices have also been shown to be effective in managing upper airway collapse, in patients with obstructive sleep apnea and overlap (14). Einally, a positive baseline pressure has been shown to decrease the work of breathing associated with intrinsic PEEP and improve... 

There are three main types of ventilator circuits used for HMV Single circuits with a leak, single circuits with a valve, and double circuits with a valve. When used with NIV, it is important that the appropriate mask be used. Table 5 summarizes the important points associated with each type of circuit. Circuits with a leak are used for bi-level devices. The intentional leak and CO2 washout for a given pressure vary significantly depending on the manufacturer and the type of mask. There are many choices of masks for the leak circuit as these masks are also used for CPAP. We have already reviewed the issue of CO2 washout and concluded that this is not a major concern when using a nasal mask (20). The only type of mask where this might be of clinical concern is with a full-face mask. A fiill-face mask is... 

This dramatic shift occurred partly because of the advantages of bi-level devices as listed above, but also because of technical advances. Bi-level devices are now available that weigh just a few pounds, are quieter, have built-in humidifiers, and newer, potentially more comfortable modes that lower airway pressure early during expiration (BiFlex , Respir-onics, Inc., Murrysville, Pennsylvania, U.S.). Patients receiving tracheostomy ventilation are still ventilated most often using volume-limited ventilators, partly because of their more sophisticated alarm capabilities than those of bi-level devices. 

Strumpf D, Carlisle C, Millman R, et al. An evaluation of the Respironics BiPap bi-level C-Pap device for delivery of assisted ventilation. Respir Care 1990 35 415-422. 

In an Italian multicenter trial (146), 122 stable hypercapnic COPD patients on LTOT for more than six months were randomized to continue LTOT alone, or LTOT plus NIV using a bi-level positive pressure device, over a two-year period. Although dropout rates were similar in both groups and compliance with NIV was impressive at 9 hours per... 

Flynn WE, Piper AJ, Willson GN, et al. Non-invasive ventilatory support clinical experience with Bi-level pressure preset devices. Proceedings of the Thoracic Society of Australia and New... 

Bi-level positive pressure ventilators are the most commonly used for NIV. Although precise numbers are difficult to ascertain, it has been estimated that more than 90% of NIV patients use these devices (N. Hill, personal observation). Compared with volume-limited positive pressure ventilators, these devices are easier to use, more comfortable and portable, quieter, and less expensive. Their major disadvantages compared with volume-limited ventilators are their lack of sophisticated alarms, lack of internal batteries, and requirement for high capacity external batteries to support their continuously functioning turbines. They are also unable to assist patients who wish to enhance secretion clearance by breath stacking, as can volume-limited ventilators (33). It is likely that the North American experience parallels that of Switzerland, where ventilators for NIV shifted from 100% volume-limited in 1992 to 85-95% bi-level or pressure support in 2000 (20). 

FIGURE 19 Daily rabbit blood plasma levels of levonorgestrel from a crosslinked polymer prepared from a 3,9-bis(ethylidene-2,4,8,10-tetraoxaspiro[5,5Jundecane)/3-methyl-l,5-pentanediol prepolymer crosslinked with 1,2,6-hexane triol. Polymer rods, 2.4 x 20 mm, containing 30 wt% levonorgestrel and 7.1 mol% Mg(OH)2. Devices implanted subcutaneously in rabbits, (o) 1 device/rabbit, ( ) 3 devices/rabbit. (From Ref. 2.)... 

This device is based on multiple parallel bi-lamination using bifurcation cascade for generating multiple thin fluid olamellae [25]. The first feed stream is split into multiple sub-streams via a bifurcation cascade in a similar way this is done for the second feed stream in another level. The corresponding sub-streams enter via nozzles into the first level. Here, the end of the channels of the bifurcation cascade and the nozzles lie next to each other. Thereby, bi-laminated sub-streams are formed and enter many parallel channels of an inverse-bifurcation cascade. These are recombined to multilayered stream in one main channel which has a serpentine shape, i.e. comprises extended length. 

A new branched carbazole derivative with phenyl ethylene moieties attached, l,3,5-tris(2-(9-ethylcarbazyl-3)ethylene)benzene (TECEB, 41) (Scheme 3.15), was prepared as a HTM for OLEDs [86], TECEB has a HOMO energy level of —5.2 eV and hole-drift mobility of 1(T 4 cm2/(V s), comparable to NPD. The device performance (maximum luminance of about 10,000 cd/m2 and current efficiency of 3.27 cd/A) in a standard HTL/tris-(8-hydroxyquino-line) aluminum double-layer device is also comparable to NPD, but TECEB has a higher Tg (130°C) and its ease of synthesis is superior to NPD. Distyryl units linked to a TPD derivative, A, A"-bis(4-(2,2-diphenylethenyl)-phenyl)-jY,jV -di(p-tolyl)-bendidine (DPS, 42) (Scheme 3.15), reported by Yamashita and coworkers, showed good hole transport properties and improved thermal stability compared with the parent TPD [87]. 

If the tunnel junction of Fig. 1 a is simply immersed in an electrolyte, the polarization between the tip and the sample will promote an electrolysis. A bi-potentiostat is necessary to ensure real tunneling between the sample and the tip. Such a device, classically used in electrochemistry, enables to split the tunnel junction into two sol-id/liquid interfaces, independently polarized against a reference of potential (Fig. 1 b). Using this configuration, also referred to as the four-electrode configuration and introduced very early by several groups, it is possible to avoid any electrochemical transfer between the sample and the tip [25,26]. The reference potential is an electrode whose potential is well defined and constant with respect to the vacuum level. The sample is biased against the reference electrode to monitor reactions at the surface, just as in a classical electrochemical cell. The tip potential is adjusted... 

Marks and coworkers used a siloxane-derived hole-transport material, 4,4/-bis[(p-trichlorosilylpropylphenyl)phenylamino]biphenyl (TPD-Si2), to modify the ITO anode surface via formation of a self-assembly monolayer (SAM) [72]. Due to the close Ip of TPD-Si2 (5.5 eV) to the HOMO level of PFO (5.9 eV), the PFO-based device with ITO anode treated with TPD-Si2 SAM exhibited a maximum brightness and external quantum efficiency of 7000 cd m-2 and 0.35%, respectively, which were about two orders of magnitude larger than those for the bare ITO device (work function of ITO 4.7 eV). 

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