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  • 1990-1994  (2)
  • 1
    Electronic Resource
    Electronic Resource
    Lung tissue rheology and 1/f noise (1994)
    Springer
    Annals of biomedical engineering 22 (1994), S. 674-681 
    Details
    ISSN: 1573-9686
    Keywords: Stress adaptation ; Fractals ; Viscoelasticity ; Lung tissue impedance
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine , Technology
    Notes: Abstract The mechanical properties of lung tissue are important contributors to both the elastic and dissipative properties of the entire organ at normal breathing frequencies. A number of detailed studies have shown that the stress adaptation in the tissue of the lung following a step change in volume is very accurately described by the functiont −k for some small positive constantk. We applied step increases in length to lung parenchymal strips and found the ensuing stress recovery to be extremely accurately described byt −k over almost 3 decades of time, despite the quasi-static stress-length characteristics of the strips being highly nonlinear. The corresponding complex impedance of lung tissue was found to have a magnitude that varied inversely with frequency. We note that this is highly reminiscent of a phenomenon known as 1/f noise, which has been shown to occur ubiquitously throughout the natural world. 1/f noise has been postulated to be a reflection of the complexity of the system that produces it, something like a central limit theorem for dynamic systems. We have therefore developed the hypothesis that thet −k nature of lung tissue stress adaptation follows from the fact that lung tissue itself is composed of innumerable components that interact in an extremely rich and varied manner. Thus, although the constantk is no doubt determined by the particular constituents of the tissue, we postulate that the actual functional form of the stress adaptation is not.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF02368292
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  • 2
    Electronic Resource
    Electronic Resource
    Influence of the viscoelastic properties of the respiratory system on the energetically optimum breathing frequency (1993)
    Springer
    Annals of biomedical engineering 21 (1993), S. 489-499 
    Details
    ISSN: 1573-9686
    Keywords: Work of breathing ; Inspiratory pressure-time integral ; Respiratory modeling ; Dogs ; Humans
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine , Technology
    Notes: Abstract We hypothesized that the viscoelastic properties of the respiratory system should have significant implications for the energetically optimal frequency of breathing, in view of the fact that these properties cause marked dependencies of overall system resistance and elastance on frequency. To test our hypothesis we simulated two models of canine and human respiratory system mechanics during sinusoidal breathing and calculated the inspiratory work ( $$\dot W$$ ) and pressure-time integral (PTI) per minute under both resting and exercise conditions. The two models were a two-compartment viscoelastic model and a single-compartment model. Requiring minute alveolar ventilation to be fixed, we found that both models predicted almost identical optimum breathing frequencies. The calculated PTI was very insensitive to increases in breathing frequency above the optimal frequencies, while $$\dot W$$ was found to increase slowly with frequency above its optimum. In contrast, both $$\dot W$$ and PTI increased sharply as frequency decreased below their respective optima. A sensitivity analysis showed that the model predictions were very insensitive to the elastance and resistance values chosen to characterize tissue viscoelasticity. We conclude that the $$\dot W$$ criterion for choosing the frequency of breathing is compatible with observations in nature, whereas the optimal frequency predictions of the PTI are rather too high. Both criteria allow for a fairly wide margin of choice in frequency above the optimum values without incurring excessive additional energy expenditure. Furthermore, contrary to our expectations, the viscoelastic properties of the respiratory system tissues do not pose a noticeable problem to the respiratory controller in terms of energy expenditure.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00000004
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