Lung ventilation and perfusion

Hoffman EA, Chon D (2005) Computed tomography studies of lung ventilation and perfusion. Proc Am Thorac Soc 2 492-498, 506... 

Bronchiolar smooth muscle is sensitive to changes in carbon dioxide levels. Excess carbon dioxide causes bronchodilation and reduced carbon dioxide causes bronchoconstriction. Pulmonary vascular smooth muscle is sensitive to changes in oxygen levels excess oxygen causes vasodilation and insufficient oxygen (hypoxia) causes vasoconstriction. The changes in bronchiolar and vascular smooth muscle tone alter the amount of ventilation and perfusion in a lung unit to return the V/Q ratio to one. 

The V/Q term describes the imbalance between ventilation (V) and perfusion (Q) in different areas of the lung. Given that alveolar ventilation is 4.5 l.min and pulmonary arterial blood flow is 5.0 l.min 1, the overall V/Q ratio is 0.9. Both ventilation and perfusion increase from top to bottom of the lung, but perfusion by much more than ventilation. 

The basic principles to consider in establishing an experimental system for lung perfusion experiments are considered with regard to the apparatus and the mode of ventilation and perfusion. 

The ventilated and perfused human lung lobe was used as described by Linder and co-workers [74], A twofold difference in the appearance of drug and metabolites in the perfusate was found for the two formulations. Small fractions of the applied dose of BDP were immediately detectable in the perfusate and the amount of the major metabolite, beclomethasone-17-propionate (17-BMP), increased over the experimental period. These observations were similar to the clinical observations that BDP is detected rapidly in the plasma after inhalation and that the appearance of the active metabolite 17-BMP occurs rapidly. The kinetic differences between the formulations were explained on the basis of particle size effects with the conclusion that the discriminatory value of this system to examine the lung pharmacokinetics of inhaled medicines in the absence of systemic effects such as hepatic metabolism was apparent. 

Ewing P, Blomgren B, Ryrfeldt A, Gerde P (2006) Increasing exposure levels cause an abrupt change in the absorption and metabolism on acutely inhaled benzo(a)pyrene in the isolated, ventilated and perfused lung of the rat. Toxicol Sci 91 332-340. 

For gas exchange to occur properly in the lung, air must be dehvered to the alveoK via the conducting airways, gas must diffuse from the alveoli to the capillaries through extremely thin walls, and the same gas must be removed to the cardiac atrium by blood flow. This three-step process involves (1) alveolar ventilation, (2) the process of diffusion, and (3) ventilatory perfusion, which involves pulmonary blood flow. Obviously, an alveolus that is ventilated but not perfused cannot exchange gas. Similarly, a perfused alveolus that is not properly ventilated cannot exchange gas. The most efficient gas exchange occurs when ventilation and perfusion are matched. 

The overall effect of nonuniform ventilation and perfusion is that both decrease as one progresses vertically upward in the upright lung. But perfusion decreases more rapidly so that the dimensionless ratio of ventilation to perfusion, VJQ, decreases upward, and can vary fin>m approximately 0.5 at the lung s bottom to 3 or more at the lung s top. Extremes of this ratio are ventilated regions with no blood flow, called dead space, where VJQ — , and perfused regions with no ventilation, called... 

FIGURE 4.12 Schematic for ventilation and perfusion in the lung and tissues. Dashed line indicates control volume for mass balance. Assume instantaneous and homogenous mixing. Subscripts indicate gas concentrations for systemic (s) or pulmonary (p) end capillary C , venous C alveolar C , and arterial C . 

Kreck TC, Krueger MA, Altemeier WA, Sinclair SE, Robertson HT, Shade ED, Hildebrandt J, Lamm WJ, Frazer DA, Polissar NL, Hlastala MP. Determination of regional ventilation and perfusion in the lung using xenon and computed tomography. J Appl Physiol 2001 91(4) 1741-1749. 

This broad span of diseases suggests that a more thorough understanding of the relationship between tissue structure and function may lead to a vast array of potential therapeutic interventions. Presently, there are numerous therapies in use or in development that focus on targeting cellular mechanotransduction and/or cell and tissue mechanics. These mechanical therapies include treatments such as tissue expansion methods, muscle relaxants, cardiac perfusion, botox, lung ventilation, and tissue engineering... 

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