Role of the Mechanical Properties of Tracheobronchial Airways in Determining the Respiratory Resistance Time Course

Abstract

A physiologically based simulation model of breathing mechanics was considered in an attempt to interpret and explain the time course of input respiratory resistance during the breathing cycle, observed in recent studies on ventilated patients. The model assumes a flow-dependent Rohrer resistance for the upper extrathoracic airways and volume-dependent resistance and elastance for the intermediate airways. A volume-dependent resistance describes the dissipative pressure loss in the lower airways, and two constant elastances represent lung and chest wall elasticity. Simulated mouth flow and pressure signals obtained in a variety of well-controlled conditions were used to analyze total respiratory resistance and elastance estimated by an on-line algorithm based on a time-varying parameter model. These estimates were compared with those provided by classical estimation algorithms based on time-invariant models with two, three, and four parameters. The results show that the four-parameter model is difficult to identify, while the three-parameter one offers no substantial advantage for estimating input resistance with respect to the more simple two-parameter model. In contrast, the time-varying approach provides good on-line estimates of the simulated end-expiration and end-inspiration resistances. These values provide further information of potential clinical utility, with respect to time-invariant models. For example, the results show that the difference between the end-expiration and end-inspiration resistance increases when obstructions shift from the upper to the lower airways. The similarity of the results obtained with measured and simulated data indicates that, in spite of its simplicity, the simulation model describes important physiological mechanisms underlying changes in respiratory input resistance, specifically the mechanical properties of intermediate airways.

Comparison of variable and conventional ventilation in a sheep saline lavage lung injury model*

“…The second index was the absolute difference between E L at 2 Hz and E L at 0.2 Hz (E het ). Computational modeling studies (25)(26)(27)(28) have shown that the frequency dependence of E L is indicative of mechanical time constant heterogeneities in the lung. Thus, E het , as an index of frequency dependence, characterizes disease heterogeneity.…”

Heterogeneous Airway Versus Tissue Mechanics and Their Relation to Gas Exchange Function During Mechanical Ventilation

“…The structure-function correlates of the features were based on published anatomically consistent computational models. 1,9,22,43 Specifically, both the absolute difference between R at 0.2 Hz and R at 8 Hz and the difference between E at 2 Hz and E at 0.2 Hz are indices of frequency dependence of R and E, and thus reflect the heterogeneity of disease. It is important to note that other mechanisms such as tissue viscoelasticity and airway wall shunting can contribute to both of these absolute differences; however, we assume that the preponderance of the increase in frequency dependence is due to disease heterogeneity.…”

Mathematical Modeling of Respiratory System Mechanics in the Newborn Lamb

“…where Ru is the upper airways resistance, K 1 is the resistance component for laminar flow, K 2 is the resistance component that describes turbulence andV is the flow [3] [15] [31]. The smaller inner diameters of newborn upper airways, combined with higher respiratory rates, provoke significant differences in newborn airflow behavior with respect to adults.…”

“…the difference between intraluminal pressure of the intermediate airways and pleural pressure [3]. While this equation has been determined from adult data, similar curves, describing pulmonary pressure-volume relationships, have shown to be useful in the study of liquid ventilation in newborn lambs [35].…”

“…Figure 1 depicts the electrical analog of the proposed non-linear, lumpedparameter model of respiratory mechanics. The global structure is based on a generic two-compartment model, which includes chest wall dynamics [2] [3] [21] [22]. This two-compartment resolution was chosen since it integrates a description of the interaction between upper airways, the lung and chest wall dynamics, while requiring low computational resources.…”