Pérez Fontán J J, Heldt G P, Targett R C, Willis M M, Gregory G A
Crit Care Med. 1986 Jan;14(1):39-47. doi: 10.1097/00003246-198601000-00010.
Mechanical ventilation at rapid rates may cause gas trapping by decreasing the time available for expiration. The volume of gas trapped within the lungs depends not only on the expiratory time, but also on the rate of emptying of the lungs. Because newborn infants are frequently ventilated at rapid rates, we examined the factors determining the rate of lung emptying, and thereby the development of gas trapping, at ventilatory rates of 40 to 120 breath/min in anesthetized and paralyzed rabbits. We found that flow and volume were related nonlinearly during the first segment of expiration, and linearly during the second segment. Only the second segment could therefore be described with a single time constant. The expiratory resistance of the respiratory system and endotracheal tube was up to 4.5 times greater than the inspiratory resistance, a finding explained by the higher transmural airway pressures during inspiration and by the dynamic effects of the increase in cross-sectional airway area at the junction of endotracheal tube and trachea. This high expiratory resistance lengthened the time constant of the second segment of expiration which, combined with the delay caused by the slow opening of the exhalation valve during the first segment, promoted gas trapping when the expiratory time was shortened.
快速机械通气可能会因缩短呼气时间而导致气体潴留。肺内潴留气体的量不仅取决于呼气时间，还取决于肺的排空速率。由于新生儿常接受快速通气，我们在麻醉和麻痹的兔身上，研究了在40至120次/分钟的通气速率下，决定肺排空速率进而决定气体潴留发展的因素。我们发现，在呼气的第一阶段，流量与容积呈非线性关系，而在第二阶段呈线性关系。因此，只有第二阶段能用单一的时间常数来描述。呼吸系统和气管内导管的呼气阻力比吸气阻力大4.5倍，这一发现可通过吸气时较高的跨壁气道压力以及气管内导管与气管交界处气道横截面积增加的动态效应来解释。这种高呼气阻力延长了呼气第二阶段的时间常数，再加上第一阶段呼气阀缓慢开启所造成的延迟，当呼气时间缩短时就会促进气体潴留。
