Heliox enhances carbon dioxide clearance from lungs of normal rabbits during low bias flow oscillation

Design of peripheral airways for efficient gas exchange

Peripheral airways combine branched tubes for ventilation with the gas exchanging alveoli in the pulmonary acini, defined as the complex of airways supplied by one first order respiratory or transitional bronchiole. In this part, the replenishment of oxygen at the alveolar surface occurs by a combination of convective air flow with diffusion of oxygen in the air. The transition between convection and diffusion depends on the morphometric properties of the airways. The design of the peripheral airways in the acinus of the human lung is described quantitatively on the basis of measurements obtained on casts of the acinar airways. Comparable data for rat and rabbit are also discussed. On the basis of this morphometric information, a typical path model for human acinar airways is derived. These studies also form the basis for advanced modeling studies of gas exchange and ventilation. In particular the problems occurring because of diffusional screening and the design conditions for minimizing this effect are discussed.

Effect of low-bias flow oscillation with partial liquid ventilation on fluoroscopic image analysis, gas exchange, and lung injury

OBJECTIVE

To evaluate the effect of low-bias flow oscillation (LBFO) with partial liquid ventilation (PLV) on perfluorochemical evaporation, histopathology, and oxidative tissue damage in an animal model of acute lung injury.

DESIGN

Prospective, randomized animal study.

SETTING

Research laboratory of a health sciences university.

SUBJECTS

Twelve New Zealand White rabbits.

INTERVENTIONS

Juvenile rabbits were anesthetized, paralyzed, and ventilated through a tracheostomy with either high-frequency oscillatory ventilation or LBFO. Lung injury was induced by repeated saline lavage, after which perflubron was instilled through a side port of the endotracheal tube. Lateral fluoroscopic images were performed at baseline and at various postfill intervals of animals in the high-frequency oscillatory ventilation-PLV and LBFO-PLV groups. The images were digitalized for computer analysis of the Lung Lucency Index, a surrogate marker of perflubron evaporation. Histopathologic evaluation was performed using a lung-injury scoring system. Malondialdehyde was measured in lung homogenates to assess oxidative damage.

MEASUREMENTS AND MAIN RESULTS

There were no significant differences in gas exchange and ventilator settings between groups throughout the experiment. At 300 mins, the high-frequency oscillatory ventilation-PLV group had a significantly higher Lung Lucency Index compared with the LBFO-PLV group in both dependent and nondependent lung regions (a high Lung Lucency Index correlates with increased perflubron loss). Malondialdehyde measurements were not different between groups. Animals treated with LBFO-PLV had a lower histopathologic lung-injury score compared with high-frequency oscillatory ventilation-PLV.

CONCLUSION

LBFO-PLV is a viable mode of ventilation in a model of acute lung injury and is associated with significant preservation of perflubron in comparison with high-frequency oscillatory ventilation-PLV. The lower evaporative losses during LBFO-PLV were associated with improved histology scores.

Diffusional screening in real 3D human acini--a theoretical study

Gas exchange at the acinar level involves several physico-chemical phenomena within a complex geometry. A gas transport model, which takes into account both the diffusion into the acinus and the diffusion across the alveolar membrane, is used to understand gas mixing in realistic systems. It is first shown that the behaviour of the system, computed on model geometries in 3D, only depends on the topological structure of the acinus. Taking advantage of this property, a new efficient method based on random walks on a lattice is used to compute gas diffusion in structures taken from real morphological data. This approach shows that, at rest, the human acinus efficiency is only 30-40%. These results provide a new evidence of the existence of diffusional screening at the acinar level. This implies permanent spatial inhomogeneity of oxygen and carbon dioxide partial pressure. The notion of an "alveolar gas" is reinterpreted as a spatial average of the gas distribution. This model casts new light on the respiratory properties of other gas mixtures, such as helium-oxygen.

Heliox administration in the pediatric intensive care unit: an evidence-based review

OBJECTIVE

To provide a comprehensive, evidence-based review of helium-oxygen gas mixtures (heliox) in the management of pediatric respiratory diseases.

DATA SOURCE

A thorough, computerized bibliographic search of the preclinical and clinical literature regarding the properties of helium and its application in pediatric respiratory disease states.

DATA SYNTHESIS

After an overview of the potential benefits and technical aspects of helium-oxygen gas mixtures, the role of heliox is addressed for asthma, aerosolized medication delivery, upper airway obstruction, postextubation stridor, croup, bronchiolitis, and high-frequency ventilation. The available data are objectively classified based on the value of the therapy or intervention as determined by the study design from which the data are obtained.

CONCLUSIONS

Heliox administration is most effective during conditions involving density-dependent increases in airway resistance, especially when used early in an acute disease process. Any beneficial effect of heliox should become evident in a relatively short period of time. The medical literature supports the use of heliox to relieve respiratory distress, decrease the work of breathing, and improve gas exchange. No adverse effects of heliox have been reported. However, heliox must be administered with vigilance and continuous monitoring to avoid technical complications.

Low bias flow oscillation with heliox in oleic acid-induced lung injury

OBJECTIVE

To evaluate Co(2) clearance in oleic acid-induced lung injury in rabbits receiving high-frequency oscillatory ventilation with helium-oxygen mixtures through a low bias flow oscillation system designed to conserve expensive gases.

DESIGN

A prospective, controlled, interventional, in vivo animal laboratory study.

SETTING

Research laboratory of a health sciences university.

SUBJECTS

Eight New Zealand White Rabbits.

INTERVENTIONS

Lung injury (Pao(2)/Fio(2) of <250) was induced by intravenous infusion of oleic acid. Low bias flow oscillation was performed with a modified high-frequency oscillatory ventilation circuit that uses low bias flow (100 mL/kg/min) and a soda lime canister to clear CO(2). Low bias flow oscillation-heliox trials were performed with 40%, 50%, 60%, and 70% helium (balanced with oxygen) for 20 mins. Each heliox trial was preceded by a 20-min paired control trial with 40% oxygen/60% nitrogen.

MEASUREMENTS AND MAIN RESULTS

Helium concentrations of 40%, 50%, 60%, and 70% decreased Paco(2) by 13% (47 +/- 7 to 41 +/- 8 torr), 17% (50 +/- 7 to 41 +/- 6 torr), 22% (49 +/- 5 to 38 +/- 7 torr), and 26% (48 +/- 7 to 35 +/- 9 torr), respectively. The gradient between partial pressure of alveolar oxygen and Pao(2) was not affected by 60% helium; however, absolute Pao(2) increased by 15%. Fluid and inotropic requirements were similar in both control and heliox low bias flow oscillation trials.

CONCLUSION

Helium concentrations greater than 40% increase Co(2) clearance from oleic acid-injured lungs of rabbits during low bias flow oscillation. The low bias flow oscillation system makes this possible using 1% of the gas volume required during high-frequency oscillatory ventilation.

Development of Aerosol Drug Delivery with Helium Oxygen Gas Mixtures

Aerosol drug delivery using helium-oxygen gas mixtures (heliox) is considered in terms of flow physics, atomization, and aerosol mechanics. Theoretical considerations are then related to past studies of the physiological effects of the inhalation of heliox and its potential use as a drug delivery medium. Past clinical trials of heliox investigating this use are reviewed and technical recommendations made for its successful development. It is proposed that improved peripheral lung drug delivery with heliox is highly dependent on proper administration, especially the inclusion of proper reservoir system for the gas.