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The USAF Military Airlift Command is charged with air evacuation of injured and ill patients in times of war and peace. In recent years the types of patients eligible for transport have expanded to include those with illnesses or injuries that require full-time mechanical support of respiratory functions with closed-system positive-pressure mechanical ventilators. The clinical condition of some patients meets the criteria of the Adult Respiratory Distress Syndrome, which is characterized by low pulmonary compliance and poor gas exchange. Respiratory support of such patients in a sea-level environment may require a combination of high-pressure mechanical ventilation and high inspired oxygen tensions. During air evacuation, these patients are routinely exposed to cabin pressures equivalent to 8,000 ft standard atmosphere (564 torr); and during rapid cabin decompression, to pressures equivalent to 40,000 ft standard atmosphere (141 torr). Concern that such environmental situations will exhaust already strained machine and physiological reserves, with potential damage to transported patients, has prompted this evaluation of the effects of altitude on the mechanical performance of ventilators and the functioning of already damaged physiological gas-exchange systems.

The researchers developed a new non-invasive technique for measurement of cardiac output that can be applied in the presence of ventilation-perfusion maldistribution, and determined the extent to which the time required for diffusion of inspired gas in the air spaces limits gas exchange and contributes to unequal distribution of ventilation. Cardiac output is measured on the basis of the rise of alveolar carbon dioxide during rebreathing. To use this method it is necessary to measure only the volume of the rebreathing bag before the maneuver and monitor the carbon dioxide fraction at the mouth during 10 seconds of rebreathing. The technique does not postulate equality of alveolar and arterial carbon dioxide tensions, takes into account carbon dioxide storage in lung tissue, and can be applied at any inspired oxygen tension with or without addition of an inert tracer gas. It has been validated on human subjects exercising at different speeds and grades on a treadmill. (Modified author abstract)

The primary purpose of the present study was to elucidate the effects of perfusion, solubility and diffusivity on inert gas tissue washout patterns. The motivation for this work was a preliminary study in this laboratory in which inert gas washouts using two gases having similar blood tissue partition coefficients but different diffusivities were performed. Because no significant differences in the washout curves of the two gases were noted it was concluded that inert gas washout is perfusion rather than diffusion limited. It was hoped that the analysis described would suggest the magnitude of the diffusivity differences that are required to produce differences in the washout curves. Obtained data indicate that gas pairs with diffusivity ratios of one to three could be used to assess the importance of diffusion limitation when tissue hypoxia exists. (Modified author abstract)