Peslin R, Duvivier C, Vassiliou M, Gallina C
Institut National de la Santé et de la Recherche Médicale, Université H. Poincaré Nancy I, France.
J Appl Physiol (1985). 1995 Dec;79(6):1958-65. doi: 10.1152/jappl.1995.79.6.1958.
Measurements of airway resistance (Raw) by body plethysmography during unconditioned air breathing are implicitly based on the assumption that the warming and humidification of air in the airways are instantaneous. Simulation with a simple model suggests that Raw may be frequency dependent and substantially underestimated at most breathing frequencies if the time constant of gas conditioning (theta) is between 0.01 and 0.3 s. We measured the frequency dependence of the real (Re) and imaginary parts of the relationship between the plethysmographic signal and airway flow from 0.5 to 3 Hz in six healthy subjects in several situations. During breathing of unconditioned air through a heated pneumotachograph, Re increased by 47 +/- 70% between 2 and 3 Hz; the data were consistent with a theta of 0.087 +/- 0.023 s. Additional dead spaces moderately increased theta: 0.105 +/- 0.031 and 0.120 +/- 0.027 s with 50-cm-long polyvinyl chloride and copper tubes, respectively. During breathing of saturated air conditioned at 34, 36, 38, and 40 degrees C, Re exhibited a much smaller positive frequency dependence, most of which was probably due to a flow dependence of Raw. We conclude that unless the inspired gas is conditioned, plethysmographic Raw is likely to be substantially underestimated, particularly when measured during spontaneous breathing.
在未调节空气呼吸期间，通过体容积描记法测量气道阻力（Raw）隐含地基于这样一种假设，即气道中空气的加热和加湿是瞬间完成的。用一个简单模型进行的模拟表明，如果气体调节时间常数（θ）在0.01至0.3秒之间，Raw可能与频率相关，并且在大多数呼吸频率下会被大幅低估。我们在几种情况下，对6名健康受试者从0.5至3赫兹测量了体容积描记信号与气道流量之间关系的实部（Re）和虚部的频率依赖性。在通过加热式呼吸流速计呼吸未调节空气期间，Re在2至3赫兹之间增加了47±70%；数据与θ为0.087±0.023秒一致。额外的死腔适度增加了θ：分别使用50厘米长的聚氯乙烯管和铜管时，θ为0.105±0.031秒和0.120±0.027秒。在呼吸34、36、38和40摄氏度的饱和调节空气期间，Re表现出小得多的正频率依赖性，其中大部分可能归因于Raw的流量依赖性。我们得出结论，除非对吸入气体进行调节，否则体容积描记法测量的Raw可能会被大幅低估，尤其是在自发呼吸期间进行测量时。
