Australian Regenerative Medicine Institute

Amplitude-modulated Ventilation: Discussion

The first inequality is a formal quantitative statement of the notion that a region of deformed parenchyma subject to external shear will maintain its patency as long as it maintains its ability to resist further shear. The second inequality states that if the sum of the outward stabilizing forces generated by tethering from surrounding lung tissue (the 4/3|i term) and the intrinsic ability of the tissue to resist normal deformation (the к term) remains positive, the alveoli within that region of parenchyma will remain stable. Although this approach does represent a simplification of the actual mechanics of the lung, the model has been used to generate reliable predictions for alveolar stability in normal animal lungs and is consistent with results obtained using other approaches. Shear and bulk moduli have been measured for normal animal lungs as a function of transpulmonary pressure, and predict alveolar stability in response to a shear or normal force perturbation. Figure 4 shows the relationship between shear and bulk moduli and transpulmonary pressure and illustrates how an increase in mean pulmonary pressure might help to stabilize previously unstable alveoli. Although the relationships shown here are for normal lungs, they illustrate that an increase in transpulmonary pressure does tend to increase both \i and K, thereby making previously marginally stable units more stable. canadian neightbor pharmacy

To utilize our tissue and surface film measurements in damaged and normal lungs to predict the effects of a deep lung inflation on lung compliance, expressions for the ability of tissue to resist shear and normal force deformation as functions of tissue and surface film properties are needed. These expressions have been derived from energy conservation considerations and are available in the literature for the static lung. If one considers a dynamic situation where inertial and viscous terms are small, then similar expressions can be used to relate surface film and parenchymal properties to the lung’s ability to resist shear and normal deformation, although the terms shear and bulk moduli can no longer formally be applied. For the purpose of this discussion we will designate their dynamic equivalents as jldyn and Kdyn: ^dyn(t) = {0.4 + 0.1 B(t)) Po(t) + 0.4 Py(t)
Kdyn = 1/3 (B(t)-2) Po(t) + 1/3 (3B'(t)-l) Ptft) where Po = F(t)L(t)/3V(t) Ptft) = 2y(t)SA(t)/3V(t) B(t) = (8F(t)/8L(t)}(L(t)/F(t)) B'(t) = (&y(t)/8V(t)}V(t)/7(t) and F is the recoil force in the tissues is response to the given level of volume distention and tissue stretch, L is the length of the tissue stretch beyond that which would be present in a stress-free state, and V is lung volume.


Figure 4. Relationship between tissue properties and transpleural pressures for normal lung tissue. Shear and bulk moduli as a function of transpulmonary pressure as measured in normal canine lungs under quasistatic conditions. Both shear and bulk modulus increase with increasing transpulmonary pressure, indicative of greater alveolar stability.

Category: Respiratory Symptoms

Tags: gas exchanging, lung compliance, lung inflation, lung tissue, parenchyma, transpulmonary pressure