Technical Briefs

A Model of the Recruitment-Derecruitment and Volume of Lung Units in an Excised Lung as it is Inflated-Deflated Between Minimum and Maximum Lung Volume

[+] Author and Article Information
D. G. Frazer

e-mail: dgf1@cdc.gov

W. G. Lindsley

e-mail: wdl7@cdc.gov

W. McKinney

e-mail: wdm9@cdc.gov

J. S. Reynolds

e-mail: jsr0@cdc.gov
1095 Willowdale Road,
Morgantown, WV 26505

G. N. Franz

Department of Physiology and Pharmacology,
WVU Robert C. Byrd Health Sciences Center,
Morgantown, WV 26505
e-mail: gnfranz@msn.com

M. Jackson

1095 Willowdale Road,
Morgantown, WV 26505
e-mail: moj8@cdc.gov

W. T. Goldsmith

WVU School of Public Health,
1095 Willowdale Road,
Morgantown, WV 26505
e-mail: wbg4@cdc.gov

The findings and conclusions in this report are those of the authors and do not necessarily represent the views of the National Institute for Occupational Safety and Health.

Contributed by the Bioengineering Division of ASME for publication in the Journal of Biomechanical Engineering. Manuscript received August 21, 2012; final manuscript received December 6, 2012; accepted manuscript posted January 10, 2013; published online February 11, 2013. Assoc. Editor: Tim David.

J Biomech Eng 135(3), 034503 (Feb 11, 2013) (5 pages) Paper No: BIO-12-1367; doi: 10.1115/1.4023372 History: Received August 21, 2012; Revised December 06, 2012; Accepted January 10, 2013

The role of the recruitment-derecruitment of small structures in the lung (lung units) as the lung increases and decreases in volume has been debated. The objective of this study was to develop a model to estimate the change in the number and volume of open lung units as an excised lung is inflated-deflated between minimum and maximum lung volume. The model was formulated based on the observation that the compliance of the slowly changing quasi-static pressure-volume (P-V) curve of an excised rat lung can differ from the compliance of a faster changing small sinusoidal pressure volume perturbations superimposed on the curve. In those regions of the curve where differences in compliance occur, the lung tissue properties exhibit nonlinear characteristics, which cannot be linearized using “incremental” or “small signal” analysis. The model attributes the differences between the perturbation and quasi-static compliance to an additional nonlinear compliance term that results from the sequential opening and closing of lung units. Using this approach, it was possible to calculate the normalized average volume and the normalized number of open units as the lung is slowly inflated-deflated. Results indicate that the normalized average volume and the normalized number of open units are not linearly related to normalized lung volume, and at equal lung volumes the normalized number of open units is greater and the normalized average lung unit volume is smaller during lung deflation when compared to lung inflation. In summary, a model was developed to describe the recruitment-derecruitment process in excised lungs based on the differences between small signal perturbation compliance and quasi-static compliance. Values of normalized lung unit volume and the normalized number of open lung units were shown to be nonlinear functions of both transpulmonary pressure and normalized lung volume.

Copyright © 2013 by ASME
Topics: Lung , Pressure
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Fig. 1

Quasi-static pressure-volume curve of an excised lung recorded between 0.0 and 30 cm H2O. The opening, open, and closing regions of the curve are indicated. The solid curve represents the PL-V*L curve for open lung units. When the end-expiratory pressure falls below the ‘closing’ pressure, lung units begin to close. To reopen a closed unit, pulmonary pressure must rise to the ‘opening’ pressure of that lung unit.

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Fig. 2

An example of a small volume perturbation superimposed on a quasi-static PL-VL curve of an excised rat lung during lung inflation. Note the difference between perturbation loop compliance (DE) and quasi-static compliance (AEF) in this region of the curve (further illustrated in Subpanel B). The recorded (points A-B) results from the opening of lung units during lung inflation, and then the contraction and re-expansion of lung units as pressure and volume decrease then increase forming a PL-VL loop (points B-C-D-E). Points (E-F) represent the opening of additional lung units during the following volume perturbation. The volume versus time signal is illustrated in Subpanel A.

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Fig. 3

A block diagram of a method using SIMULINK (MathWorks, Inc., Natick, MA) to calculate both the normalized volume of a lung unit and the normalized number of open lung units as the lung was inflated-deflated between transpulmonary pressures of 0.0 and 30 cm H2O. F1 represented the compliance of a PL-VL loop as a function of PL, (F1 = NO* dVU*/dPL), and F2 represented the difference between the normalized compliance of the of the quasi-static PL-VL curve and the loop compliance as a function of PL (F2 = dVL*/dPL – NO* dVU*/dPL). dPL/dt was the rate at which transpulmonary pressure changed with respect to time as the PL-VL* curve was recorded.

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Fig. 4

Diagram of the system used to record quasi-static PL-VL curves of excised rat lungs with small volume perturbations superimposed on the curve. Lung volume perturbations were performed with a computer controlled syringe pump.

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Fig. 5

The ventilation pattern of the lung as volume perturbations were superimposed on a slow quasi-static inflation rate generated by a digitally controlled syringe pump during lung inflation.

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Fig. 6

A typical PL-VL relationship showing small deflation/inflation volume perturbations superimposed on a quasi-static curve of an excised rat lung.

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Fig. 7

An example of the calculated values of normalized lung unit volume (VU*) and the normalized number of open lung units (NO*) plotted as a function of transpulmonary pressure. The quasi-static PL-V*L curve is also shown.

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Fig. 8

The normalized number of open lung units (NO*) and the average volume of a normalized lung unit (VU*), versus normalized lung volume (VL*) during lung inflation and deflation. Note that VL* = NO* · VU* during inflation and deflation.




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