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The minimization of muscular energy expenditure during inspiration in linear models of the respiratory system (1986)

Bates, J. H. T.

Springer

Biological cybernetics 54 (1986), S. 195-200

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ISSN: 1432-0770

Source: Springer Online Journal Archives 1860-2000

Topics: Biology , Computer Science , Physics

Notes: Abstract The consequences of requiring a general linear model of respiratory mechanics to inspire a fixed volume in a fixed time in a way that minimizes various measures of muscular energy expenditure are examined. For such a model no volume profile minimizes the time-integral of the applied pressure developed by the respiratory muscles, although this integral is not independent of the profile. Minimizing the mechanical work done by the respiratory muscles, on the other hand, requires that the inspiratory flow be constant. These results support the hypothesis that neither the pressure integral nor the mechanical work are individually minimized over an inspiration by an animal or man at rest. Minimization of a weighted sum of the pressure integral and the work done may be a more physiologically reasonable criterion by which the respiratory muscles direct inspiration. This combined cost function predicts a non-constant optimum velocity profile.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF00356858

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2

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Effect of valve closure time on the determination of respiratory resistance by flow interruption (1987)

Bates, J. H. T. ; Hunter, I. W. ; Sly, P. D. ; [et al.]

Springer

Medical & biological engineering & computing 25 (1987), S. 136-140

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ISSN: 1741-0444

Keywords: Airways resistance ; Linear models of respiratory system

Source: Springer Online Journal Archives 1860-2000

Topics: Biology , Chemistry and Pharmacology , Medicine

Notes: Abstract The resistance of the respiratory system to flow may be conveniently assessed by the flow-interruption technique, in which the flow of gas at the mouth of a subject is suddenly interrupted, while the pressure just distal to the point of interruption, is recorded. There is a rapid change in pressure immediately upon interruption, presumably giving the resistive pressure drop across the pulmonary airways. This is followed by a further slow change in pressure reflecting stress relaxation in the respiratory system and possibly gas redistribution between different regions of the lung. The diagnostic potential of the post-occlusion pressure signal is dependent on the airway opening being occluded effectively instantaneously. We present a description of an occlusion valve we have designed and built for performing rapid airway occlusions. We are able to measure the closing characteristics of our valve precisely, and show that its finite closure time of 12 ms causes the initial rapid drop in pressure to be underestimated by about 7 per cent. A simple numerical correction scheme allows us to estimate this pressure drop correctly to within one or two per cent.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF02442841

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3

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Correcting for the thermodynamic characteristics of a body plethysmograph (1989)

Bates, J. H. T.

Springer

Annals of biomedical engineering 17 (1989), S. 647-655

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ISSN: 1573-9686

Keywords: Frequency response ; Thoracic gas volume ; Digital correction procedure

Source: Springer Online Journal Archives 1860-2000

Topics: Medicine , Technology

Notes: Abstract A constant volume body plethysmograph is used to measure changes in total body volume by relating pressure changes measured inside a closed chamber to changes in its volume. The relationship between pressure and volume, however, is complicated by the fact that rapid changes in pressure are accompanied by changes in the temperature of the gas in the plethysmograph. The rate at which the temperature of the gas subsequently approaches that of the environment affects the frequency response of the plethysmograph as a volume measuring instrument. In this paper it is shown that the transient response in plethysmograph pressure to a step change in volume must be a multi-component function, due to the gas near the walls of the plethysmograph approaching thermal equilibrium with the environment at a faster rate than gas near the center. The step response of a 200 L plethysmograph was determined experimentally and found to be well described empirically by a sum of two decaying exponential functions of time. The fitted exponentials were used to construct an algorithm for digitally correcting a measured pressure signal for the effects of the plethysmograph response. When applied to a hypothetical volume waveform over a single breath, an almost perfect correction was obtained.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF02367468

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4

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Lung tissue rheology and 1/f noise (1994)

Bates, J. H. T. ; Maksym, G. N. ; Navajas, D. ; [et al.]

Springer

Annals of biomedical engineering 22 (1994), S. 674-681

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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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Influence of the viscoelastic properties of the respiratory system on the energetically optimum breathing frequency (1993)

Bates, J. H. T. ; Milic-Emili, J.

Springer

Annals of biomedical engineering 21 (1993), S. 489-499

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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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A model of transient oscillatory pressure-flow relationships of canine airways (1995)

Suki, B. ; Davey, B. L. K. ; Sato, J. ; [et al.]

Springer

Annals of biomedical engineering 23 (1995), S. 682-690

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ISSN: 1573-9686

Keywords: Tissue viscance and elastance ; Tissue impedance ; Hysteresivity ; Frequency and volume dependence of tissue mechanics ; Vagal tone

Source: Springer Online Journal Archives 1860-2000

Topics: Medicine , Technology

Notes: Abstract In a previous paper (27) we developed a lumped parameter model of canine pulmonary airway mechanics featuring airway wall elasticity, gas inertance, and laminar and turbulent gas flow. The model accurately accounted for the steadystate pressure-flow data we obtained during sinusoidal cycling of the lung following a period of apnea. In the present paper, we extend the model to account for the transient decrease in the amplitude of the trans-airway pressure swings that we observed immediately following the apnea, which we have shown to be due to a vagally mediated bronchodilatation reflex. The extended model accounts for this transient in terms of a sudden change in airway smooth muscle tone acting on the viscoelastic properties of the airway wall and tissues mechanically coupled to it. Consequently, this model is able to temporarily store a volume of gas in the conducting airway tree as its volume changes cyclically with that of the whole lung. This means that the flow entering the airway tree from the trachea at any instant ( $$\dot V$$ ) is not precisely equal to that entering the alveoli ( $$\dot V_{alv} $$ ) even when the gas is considered incompressible. We found that assuming $$\dot V$$ to be equal to $$\dot V_{alv} $$ can lead to errors in estimating respiratory tissue impedance of as much as 10%. However, tissue hysteresivity remained almost unaffected, suggesting that the hysteretic properties of respiratory system tissues and airway wall are well matched.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF00000014

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7

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General method for describing and extrapolating monotonic transients and its application to respiratory mechanics (1987)

Bates, J. H. T. ; Sly, P. D. ; Okubo, S.

Springer

Medical & biological engineering & computing 25 (1987), S. 131-135

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ISSN: 1741-0444

Keywords: Flow interruption technique ; Overdamped system ; Respiratory system resistance

Source: Springer Online Journal Archives 1860-2000

Topics: Biology , Chemistry and Pharmacology , Medicine

Notes: Abstract The transient response of an overdamped system is a monotonically changing output. We present a rapid method for fitting smooth curves to suc responses that is capable of describing multicomponent transients. Confidence intervals on the fitted curves are also obtained. In addition, the curves can be made to extrapolate monotonically. We demonstrate the method by applying it to the flow interruption method commonly used for determining respiratory resistance. If the flow of gas through a resistance is suddenly interrupted the pressure just distal to the point of occlusion immediately changes by an amount reflecting the resistive pressure drop across the resistance that existed just prior to interruption. The pressure and flow signals measured during interruption contain artefacts due to the finite closure time of the occluding mechanism and the ringing of inertive elements within the system. By extrapolating the signals observed before and after interruption to the midpoint of interruption we are able to estimate the resistance of a test system to within a few per cent.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF02442840

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