Frey U, Suki B, Kraemer R, Jackson A C
Department of Biomedical Engineering, Boston University Massachusetts 02215, USA.
J Appl Physiol (1985). 1997 Mar;82(3):1018-23. doi: 10.1152/jappl.1997.82.3.1018.
Respiratory input impedance (Zin) over a wide range of frequencies (f) has been shown to be useful in determining airway resistance (Raw) and tissue resistance in dogs or airway wall properties in human adults. Zin measurements are noninvasive and, therefore, potentially useful in investigation of airway mechanics in infants. However, accurate measurements of Zin at these f values with the use of forced oscillatory techniques (FOT) in infants are difficult because of their relatively high Raw and large compliance of the face mask. If pseudorandom noise pressure oscillations generated by a loudspeaker are applied at the airway opening (FOT), the power of the resulting flow decreases inversely with f because of capacitive shunting into the volume of the gas in the speaker chamber and in the face mask. We studied whether high-frequency respiratory Zin can be measured by using rapid flow interruption [high-speed interrupter technique (HIT)], in which we expect the flow amplitude in the respiratory system to be higher than in the FOT. We compared Zin measured by HIT with Zin measured by FOT in a dried dog lung and in five healthy adult subjects. The impedance was calculated from two pressure signals measured between the mouth and the HIT valve. The impedance could be assessed from 32 to 800 Hz. Its real part at low f as well as the f and amplitude of the first and second acoustic resonance, measured by FOT and by HIT, were not significantly different. The power spectrum of oscillatory flow when the HIT was used showed amplitudes that were at least 100 times greater than those when FOT was used, increasing at f > 400 Hz. In conclusion, the HIT enables the measurement of high-frequency Zin data ranging from 32 to 800 Hz with particularly high flow amplitudes and, therefore, possibly better signal-to-noise ratio. This is particularly important in systems with high Raw, e.g., in infants, when measurements have to be performed through a face mask.
在很宽的频率（f）范围内，呼吸输入阻抗（Zin）已被证明有助于确定犬类的气道阻力（Raw）和组织阻力，或成人的气道壁特性。Zin测量是非侵入性的，因此在婴儿气道力学研究中可能有用。然而，由于婴儿相对较高的Raw和面罩较大的顺应性，使用强迫振荡技术（FOT）在这些f值下准确测量Zin很困难。如果由扬声器产生的伪随机噪声压力振荡施加在气道开口处（FOT），由于电容性分流到扬声器腔室和面罩中的气体体积中，产生的气流功率会随着f呈反比下降。我们研究了是否可以使用快速气流中断[高速中断技术（HIT）]来测量高频呼吸Zin，我们预计呼吸系统中的气流幅度会高于FOT中的气流幅度。我们在干燥的犬肺和五名健康成年受试者中比较了通过HIT测量的Zin和通过FOT测量的Zin。阻抗是根据在口腔和HIT阀之间测量的两个压力信号计算得出的。阻抗可以在32至800Hz范围内进行评估。通过FOT和HIT测量的其在低频f时的实部以及第一和第二声共振的f和幅度没有显著差异。使用HIT时振荡气流的功率谱显示的幅度至少比使用FOT时大100倍，在f>400Hz时增加。总之，HIT能够测量32至800Hz范围内的高频Zin数据，气流幅度特别高，因此可能具有更好的信噪比。这在具有高Raw的系统中尤为重要，例如在婴儿中，当必须通过面罩进行测量时。
