Volume-limited ventilation

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). 

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. 

Volume-controlled ventilation—the patient receives a specific volume of gas delivered to the lungs at set time intervals with pressure limit. 

Other trends in mechanical ventilation equipment such as hybrid ventilators, which provide various pressure and volume-limited modes as well as alarm systems (34) are addressed elsewhere in this text, as are developments in mask technology. Such advances include softened silicone sealing gaskets, gel seals, very compact masks, and others that minimize skin seal pressure. 

The role of bacterial infections in COPD exacerbations is controversial, and there are limited data on the efficacy of antibiotics in treating COPD exacerbations. Recent studies suggest that bacteria cause 40% to 50% of acute exacerbations.31 Antibiotics should be used in patients with COPD exacerbations who have either of the following characteristics (1) at least two of three cardinal symptoms increased dyspnea, sputum volume, or sputum purulence or (2) a severe exacerbation requiring mechanical ventilation.2... 

Enclosed fires may exhibit fire growth characteristics as shown in Figure 5-5. Unlike gaseous or liquid fuels, there may be a considerable fire growth period in which temperatures and overall heat release is low and the fire is localized. As the fire becomes fully developed, the entire room volume can become engulfed in flames, finally, as air is depleted or fuel is consumed, a decay period occurs. In many cases, an enclosure fire will be starved for air ("ventilation-limited"), and the available airflow becomes the limiting factor for the fuelburning rate. 

The hottest fires may be associated with those cases where the fire is big enough to give flames to fill at least half the structure volume, cases where it is stoichiometric or just under ventilated, and cases where the hot gas layer is 10 ft (3 m) or more deep. Heavier fuels would be less likely to give the hottest fires, asthey may not receive enough heat feedback to vaporize the liquid and therefore they may be self limiting in terms of the burn rate. Where these conditions may be encountered, heat fluxes of 1320-1584 BTU/ft (250 to 300 kW/m ) may be experienced. In certain circumstances, (which are not yet fully understood) highly efficient combustion can occur with fluxes of 1848-2112 BTU/ft (350-400 kW/m2) and temperatures of 2,500°F (1,400°C). 

Human exposure limits through air exposure are to be applied at a plant that manufactures Heptachlor, C10H5CI7. Heptachlor is a chemical that is currently restricted in use to termite control, because of exposure concerns. These limits are given in ppm(v), but the release to the air is determined in g/s. Applying the volume flow with the current ventilation system and the volume of the plant in a complete mix reactor assumption resulted in a vapor phase concentration of 0.1 M-g/m. What is the vapor phase concentration in ppm(v) ... 

Controlling ventilation rates to limit volumes of combustible mixtures present in a facility after a fuel release. 

Benzene can have both chronic and acute toxic effects. The risk of acute effects is low, since acute symptoms occur only at 1000 ppm or higher. Chronic vapor inhalation at the level of 25 to 50 ppm can cause changes in blood chemistry, and co/ rmoa7 exposure at 100 ppm can cause severe blood disorders. The OSHA exposure limits for benzene vapor are 1 ppm as an 8-hour time-weighted average and a ceiling of 50 ppm for no more than 10 min. In order to reach the level of 10 ppm in a laboratory of 750 m volume, 23 g of liquid benzene would have to evaporate into a closed atmosphere. Thus the hazards associated with the infrequent use of liquid benzene in a well-ventilated laboratory are very low. 

Spirometry has its limitations, however. It can measure only ventilated volumes. It cannot measure lung capacities that contain the residual volume. Measurements of TLC, FRC, and RV have diagnostic value in defining lung overdistension or restrictive pulmonary disease the body plethysmograph can determine these absolute lung volumes. 

Both respiration rate and tidal volume (the amount of air moved per breath) increase with exercise, but above the anaerobic threshold the tidal volume no longer increases (remains at about 2 to 2.51). From that point, increases in ventilation require greater increases in respiration rate. A similar limitation occurs for stroke volume in the heart (limited to about 120 ml). 

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