Pleural Pressure Gradient Research Articles

Reversal of the pleural pressure gradient during electrophrenic stimulation.

Reversal of the pleural pressure gradient during electrophrenic stimulation.

Bulk elastic properties of excised lungs and the effect of a transpulmonary pressure gradient

Isolated dog lungs and lobes were studied while suspended in air and while immersed in a low density fluidised bed to simulate a pleural pressure gradient. Pressure-volume loops (vital capacity, VC) were recorded during 2 minute cycles between transpulmonary pressures of -5 and + 30 cm water. Volume history dependent features were investigated by cycling to intermediate pressures from either minimum volume (−5 cm H 2O, MV) or total lung capacity (+30 cm H 2O, TLC). Cycling to pressures below 2.5 cm water led to “closure” of lung units and pronounced changes in subsequent inflation curves. Deflation curves were also modified. The upper portions of VC pressure-volume loops could be fitted accurately by exponential regressions of the form VC% = V max V TLC (1−e −kP) . The points where the lower portions of the curves departed from the exponential curve were considered to represent completion of “opening” and initial “closing” of lung units during inflation and deflation respectively. “Opening” and “closing” pressures for lower lobes were 2 cm water greater than for upper lobes with the lungs in air, and the difference increased to 4.1 cm water in the fluidised bed. Studies conducted on primate lungs showed similar behaviour.

Transpulmonary pressure gradient and ventilation distribution in excised lungs

Isolated dog lungs and lobes, and whole monkey lungs, were studied while suspended in air, and while immersed in a low density fluidised bed to simulate a pleural pressure gradient. Gas distribution was investigated during two minute cycles between transpulmonary pressures of −5 and +30 cm water (minimum volume, MV and total lung capacity, TLC respectively), by (1) injecting boluses of xenon-133 during inflation and scanning the lung at total lung capacity, and (2) monitoring the xenon content of gas leaving the lung during the subsequent deflation. With a bolus given at MV there was preferential distribution to the upper lung and the washout typically showed a rising “plateau” with a steeper terminal portion (phase IV). These features were exaggerated in the presence of a gradient of transpulmonary pressure when the bolus still went preferentially to the superior lung, even when the lung was inverted. Individual lobes also distributed boluses unevenly and gave washouts with rising plateaux and phase IVs, implying inhomogeneity which must also contribute to whole lung behaviour. A two-compartment mathematical model of the lung was used to demonstrate that washout plateaux could be level, or rise or fall steadily, depending upon the relative compliance of the two compartments. However, the terminal phase IV must always rise if the initial concentration of tracer is greater in the upper compartment. Macaque lungs showed similar features, but, when suspended in air, boluses were uniformly distributed and washout plateaux were relatively flat.

A non-linear theory of the distribution of pulmonary ventilation

A theory for the distribution of ventilation in a two compartment model of the lung is developed, neglecting inertial forces but allowing resistance and compliance to vary with lung volume and resistance to vary with flow rate, all in a non-linear manner. Numerical solutions to the basic equation are found for the distribution of inspired gas between the upper and lower lobes of erect human lungs. Results are presented to show how a bolus of labelled gas inspired at various volumes and flow rates would be distributed. The theory predicts the preferential distribution of the bolus to the lower lobes, if inspired slowly, because these lobes are more compliant than the upper as a result of the gradient of pleural pressure. With increasing inspiratory flow rate more of the inspired bolus is distributed to the upper lobes. Reversal, where ventilation per alveolus in the upper and lower lobes is equal after a given inspired volume occurs at approximately 2 litres/sec when the inspired volume is 400 ml. Above this flow there is a preferential distribution of gas to the upper lobes. This reversal is a direct result of the volume-dependence of resistance, the more expanded upper airways having a lower resistance. The computations are made for a wide range of physiological and anatomical parameters, so that the influence of various factors, such as overall lung volume, airway resistance, and pressure-volume relations, on the distribution of inspired gas, can be explored. The flow rate at which reversal occurs is influenced to some extent by all parameters but is most sensitive to changes in lung volume. The results are in good agreement with recent experiments on the distribution of gas using boli of radio-active Xenon and external counters at the chest wall. It is stressed that the theory can readily be extended to calculate the distribution of ventilation in situations other than uniform inspiration, for example during cyclic breathing at various frequencies, if the appropriate anatomical and physiological variables for expiration can be determined.

Ventilatory components of lungs in relation to sex and age.

Ventilatory components of lungs were studied by multiple breath nitrogen washout in a group of 225 subjects, 123 males and 102 females ranging in age from 7 years to 30 years. Open-circuit, end-tidal nitrogen concentrations were recorded by a rapidly responding nitrogen meter. Lungs with two exponential components were found in 21.3 per cent of the subjects. The percentage of two component lungs increased with age, probably because of an unequal change of alveolar volume and airway conductance due to unequal rates of growth in different parts of lung that might have resulted in an increased diversity of time constants among the numerous pathways. Another possibility is the effect of gravity. As the lung grows, the gradient of pleural pressure between apex and base becomes greater, and the static pressure-volume curve of the lungs might no longer be a single component. In different age groups between 10 years to 30 years, the occurrence of single and double component lungs in males and females did not diff...

A non-linear theory of the distribution of pulmonary ventilation.

Abstract A theory for the distribution of ventilation in a two compartment model of the lung is developed, neglecting inertial forces but allowing resistance and compliance to vary with lung volume and resistance to vary with flow rate, all in a non-linear manner. Numerical solutions to the basic equation are found for the distribution of inspired gas between the upper and lower lobes of erect human lungs. Results are presented to show how a bolus of labelled gas inspired at various volumes and flow rates would be distributed. The theory predicts the preferential distribution of the bolus to the lower lobes, if inspired slowly, because these lobes are more compliant than the upper as a result of the gradient of pleural pressure. With increasing inspiratory flow rate more of the inspired bolus is distributed to the upper lobes. Reversal, where ventilation per alveolus in the upper and lower lobes is equal after a given inspired volume occurs at approximately 2 litres/sec when the inspired volume is 400 ml. Above this flow there is a preferential distribution of gas to the upper lobes. This reversal is a direct result of the volume-dependence of resistance, the more expanded upper airways having a lower resistance. The computations are made for a wide range of physiological and anatomical parameters, so that the influence of various factors, such as overall lung volume, airway resistance, and pressure-volume relations, on the distribution of inspired gas, can be explored. The flow rate at which reversal occurs is influenced to some extent by all parameters but is most sensitive to changes in lung volume. The results are in good agreement with recent experiments on the distribution of gas using boli of radio-active Xenon and external counters at the chest wall. It is stressed that the theory can readily be extended to calculate the distribution of ventilation in situations other than uniform inspiration, for example during cyclic breathing at various frequencies, if the appropriate anatomical and physiological variables for expiration can be determined.

The effect of the abdomen on the vertical gradient of pleural surface pressure

The effect of the abdomen on the vertical gradient of pleural surface pressure in the horizontal postures has been quantitated by determining the topography of pleural surface pressure after evisceration. In supine, prone and lateral rabbits the overall vertical gradient of transpulmonary pressure after evisceration decreased 2–3 times, and more than the lung density. In prone rabbits suspended from the vertebral column the overall vertical gradient after evisceration decreased while the lung density increased. The lines relating lung height to pleural surface pressure after evisceration became similar in all the horizontal postures. In the head-up rabbits and dogs the overall vertical gradient of transpulmonary pressure was about twice the lung density. These results indicate that the vertical gradient of transpulmonary pressure does not depend essentially upon the lung density and that, in the horizontal postures, it is markedly affected by the vertical gradient of abdominal pressure.

Regional lung volume and pleural pressure gradient estimated from lung density in dogs.

Regional lung volume and pleural pressure gradient estimated from lung density in dogs.