Medical Gases, American Air Liquide, Delaware Research and Technology Center (DRTC), 200 GBC Drive, Newark, DE 19702, USA.
Biomed Eng Online. 2012 May 30;11:27. doi: 10.1186/1475-925X-11-27.
Expiratory time constants are used to quantify emptying of the lung as a whole, and emptying of individual lung compartments. Breathing low-density helium/oxygen mixtures may modify regional time constants so as to redistribute ventilation, potentially reducing gas trapping and hyperinflation for patients with obstructive lung disease. In the present work, bench and mathematical models of the lung were used to study the influence of heterogeneous patterns of obstruction on compartmental and whole-lung time constants.
A two-compartment mechanical test lung was used with the resistance in one compartment held constant, and a series of increasing resistances placed in the opposite compartment. Measurements were made over a range of lung compliances during ventilation with air or with a 78/22% mixture of helium/oxygen. The resistance imposed by the breathing circuit was assessed for both gases. Experimental results were compared with predictions of a mathematical model applied to the test lung and breathing circuit. In addition, compartmental and whole-lung time constants were compared with those reported by the ventilator.
Time constants were greater for larger minute ventilation, and were reduced by substituting helium/oxygen in place of air. Notably, where time constants were long due to high lung compliance (i.e. low elasticity), helium/oxygen improved expiratory flow even for a low level of resistance representative of healthy, adult airways. In such circumstances, the resistance imposed by the external breathing circuit was significant. Mathematical predictions were in agreement with experimental results. Time constants reported by the ventilator were well-correlated with those determined for the whole-lung and for the low-resistance compartment, but poorly correlated with time constants determined for the high-resistance compartment.
It was concluded that breathing a low-density gas mixture, such as helium/oxygen, can improve expiratory flow from an obstructed lung compartment, but that such improvements will not necessarily affect time constants measured by the ventilator. Further research is required to determine if alternative measurements made at the ventilator level are predictive of regional changes in ventilation. It is anticipated that such efforts will be aided by continued development of mathematical models to include pertinent physiological and pathophysiological phenomena that are difficult to reproduce in mechanical test systems.
呼气时间常数用于量化整个肺部和各个肺区的排空情况。呼吸低密度氦/氧混合物可能会改变局部时间常数，从而重新分布通气，可能减少阻塞性肺疾病患者的气体陷闭和过度充气。在本工作中，使用了肺部的实验和数学模型来研究不均匀阻塞模式对分腔和全肺时间常数的影响。
使用具有一个腔室阻力恒定、另一个腔室阻力逐渐增加的两腔机械测试肺。在通气时用空气或 78/22%氦/氧混合物测量一系列肺顺应性范围内的测量值。评估了两种气体的呼吸回路阻力。将数学模型应用于测试肺和呼吸回路的实验结果进行了比较。此外，将分腔和全肺时间常数与通气机报告的时间常数进行了比较。
时间常数随分钟通气量的增加而增大，用氦/氧替代空气可减小时间常数。值得注意的是，由于肺顺应性高（即弹性低）导致时间常数较长的情况下，即使代表健康成人气道的低阻力水平，氦/氧也能改善呼气流量。在这种情况下，外部呼吸回路的阻力是显著的。数学预测与实验结果相符。通气机报告的时间常数与全肺和低阻力腔室的时间常数高度相关，但与高阻力腔室的时间常数相关性较差。
认为呼吸低密度气体混合物（如氦/氧）可以改善阻塞性肺区的呼气流量，但这些改善不一定会影响通气机测量的时间常数。需要进一步研究，以确定通气机上的替代测量是否可预测通气的区域性变化。预计，通过不断开发数学模型来纳入难以在机械测试系统中重现的相关生理和病理生理现象，将有助于这些努力。
