These investigations were concerned with the evaluation of the pulmonary circulation under mild gravitational stresses, such as water immersion and passive tilt to the upright position. Both anesthetized animals and conscious humans were studied. We extended our previous observations (made under Air Force Contract F41609-71-R-0039) that the standard USAF anti-G suit might be more efficient if inflated from below upwards, rather than from the abdominal bladder. Profiles of pressure inflation of the standard suit, during this contract year, confirmed our hypothesis that abdominal bladder pressure was higher than calf or leg pressure during the inflation period. A modified anti-G suit was designed with pressure switches that enabled sequential filling from below upward. This suit was tested under 90° of passive tilt, and a higher cardiac output was produced after inflation of this modified suit than with the standard USAF anti-G suit. Such a modification might improve G tolerance and be useful for therapy of hypotensive states where venous return is reduced. Utilizing the rebreathing technique previously developed for pulmonary circulatory parameters under AF Contract F41609-71-R-0039 - for which we also wrote the computer analysis program in Fortran - we measured the effects on pulmonary circulatory parameters of water immersion to the neck (NI). We thus demonstrated that, in the seated subject, such NI caused a significant increase in pulmonary capillary blood flow and diffusing capacity which persisted throughout the 4-hr immersion period. The lack of change in pulmonary tissue volume suggested that the central vascular engorgement so induced is not accompanied by extravasation of fluid into the pulmonary interstitial space. Our study thus supports the concept that, in the seated subject, the renal or hormonal effects of NI are mediated by a redistribution of blood volume, with a resultant central hypervolemia. We developed a bronchofiberscopic technique for sampling lobar oxygen, carbon dioxide, and nitrogen; as well as a computer program for calculating instantaneous respiratory exchange ratio. This ratio was proportional to that for ventilation to perfusion; and studies in anesthetized dogs gave expected values with change in posture and occlusion of a lobar pulmonary artery. Finally, we measured, for the first time, the fractionation of pulmonary blood volume in the intact animal. Of total pulmonary blood volume, pulmonary arterial blood volume constituted 28%; capillary blood volume, 22%; and venous blood volume, 50%.
|Title of host publication||Unknown Host Publication Title|
|State||Published - Dec 1 1976|
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