Lung Mechanics: Theory and Practice III Impedance

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Lung Mechanics Theory and Practice IIIImpedance

• Jason H.T. Bates, PhD, DSc
• Research Professor of Medicine
• Vermont Lung Center
• University of Vermont College of Medicine

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A model prediction about passive expiration
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Relaxed expiration in the anesthetized, paralyzed
dog is bi-exponential. (Bates et al. J Appl
Physiol 61 873-880, 1986)
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Two exponentials implies two compartments
parallel model series model combination
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Studies with the alveolar capsule in dogs (Bates
et al. J Appl Physiol 65 408-414, 1988), mice
(Tomioka et al. J Appl Physiol 93 263-270, 2002)
and other species showed that the normal lung
ventilated at normal breathing frequencies is
best described by a uniformly ventilated
compartment surrounded by viscoelastic tissue.
viscoelastic tissue
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Lung tissue has a resistance as well as an
elastance, because it dissipates energy when it
is deformed
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R and E depend on frequency
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Impedance
f frequency R resistance X reactance E
elastance
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Linear dynamic systems
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A linear dynamic system treats the individual
sinewaves in the input as if they are alone. Each
input sinewave is scaled by an amplitude factor
and shifted by a phase factor that depend only on
frequency.
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The impedance of the single-compartment linear
model
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The impedance of the single-compartment linear
model including the inertance of the air in the
central airways
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Alveolar capsule measurements demonstrate that
Raw estimated using the constant-phase impedance
model matches a direct measurement of Raw.
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Summary 1

• Normal lungs are significantly better described
  by a two-compartment model than a
  single-compartment model
• Tissue viscoelasticity is the main contributor to
  the two-compartment behavior in the normal lung
• Lung tissue exhibits resistive as well as elastic
  behavior when distorted

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Summary 2

• Lung resistance and elastance depend on frequency
• Impedance is a complex quantity that
  characterizes mechanical behavior over a range of
  frequency
• The constant-phase model describes the impedance
  of normal lung tissue accurately

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Summary 3

• Parallel heterogeneity (differences in regional
  alveolar ventilation) cause R and E to increase
  at low frequencies
• Serial heterogeneity (shunting of applied flow
  oscillations into distension of airway walls)
  also causes R and E to increase at low frequencies