These forces are visually evident on excised healthy lungs that quickly collapse to their minimal volume if the chest is opened to outside air, creating a pneumothorax. Such lungs normally retain air only in large airways and a few clusters of alveoli, having undergone atelectasis. The collapsed lungs are suspended by the trachea within a vacuum jar that can be evacuated to simulate negative intrapleural pressure P IP in vivo Fig.
Curves are nonlinear and approach total lung capacity at the most negative pressures. See the text for additional details. When negative pressure is first applied to the collapsed lungs, their volume does not change, that is, their compliance is zero. Then, as the lungs achieve a critical opening pressure P CO their compliance rapidly increases and the lungs fill easily.
Thus, P CO is the minimal pressure required to open or recruit alveoli and small airways that were previously atelectatic. Then, as the vacuum is slowly released stepwise toward 0 cm H 2 O, normal lungs deflate toward residual volume RV.
Basford, Jeffrey R. Prange, Henry D. Butler, James P. Clements, John A. Morgan, Thomas E. Lachmann, B. Robertson, and J. Notter, Robert H. Van Golde, L. Batenburg, and B. King, Richard J. Composition and physiological correlations. Morley, Colin, and Alec Bangham. Karger Publishers, Chander, A. Whitsett, Jeffrey A. Wert, and Timothy E. Properties, production and regulation of lung surfactant. Previous chapter: Structure and function of the alveolus Next chapter: Roles of the chest wall and diaphragm in respiratory mechanics.
However, as the seconds pass, you may note that the measured volume of the lung decreases. This is due to the fact that the gas contained within is being absorbed into the pulmonary circulation.
Therefore, in the living human organism, there is never going to be a situation where a truly static pressure-volume relationship can be recorded, and Harris recommends the term "quasi-static" to describe them. In terms of exam relevance, apart from the abovestated definition, one may safely expect to be asked to draw a diagram to represent the pressure and volume relationhsip of the human lung.
If so, one could do worse than reproduce the famous relationship described by Rahn et al in , which was for some reason the first of such efforts. They acquired normal men, occluded their nostrils with cork stoppers, and measured their airway pressures at different fractions of their vital capacity the subjects exhaled fully and then inspired a known volume of gas from the spirometer before performing a breath hold.
With these manoeuvres, the following relationship was demonstrated:. The diagram above is identical to Figure 6 from the original paper, but it was gentrified slightly to modernise it for consumption by modern readers nobody calls that volume "residual air" any more. It demonstrates the classical lung compliance curve, where the compliance is poor at low and high volumes, but optimal just above the FRC, i. Obviously, when you pump gas into a person's chest, the pressure-volume relationship is going to be a complex combination of several factors.
Of these, the dominant players will be the chest wall and the tissues of the lung itself. When asked to desribe this concept, a CICM trainee would likely be expected to regurgitate this equation:. Where, predictably, C RS is the compliance of the respiratory system as a whole, C L is the compliance of the lung and C CW is the compliance of the chest wall.
The compliance of the lungs and chest wall are related to the elastic properties of these structures, which are discussed in a chapter all of their own. Under normal conditions i. The upshot of this is that inflation and deflation have different pressure-volume relationships, and the difference between them is called "hysteresis", a term etymologically related to "lag" or shortcoming" which describes the dependence of a system's state upon its history.
If one were completely unprepared for the questions "define hysteresis", one could easily break down and blather something like "the inspratory thing does not look like the expiratory thing", so it would probably be worth investing some time in memorising a more solid definition. The property of dissipating energy receives the name of hysteresis. A pithier, more memorable definition is available from a much less reputable source :.
It makes logical sense to expect something like this in a dynamic PV loop because of the effects of resistance more on that later , but it is seen even in static compliance measurements.
Here, a diagram from Harris demonstrates the hysteresis in a static PV loop using the supersyringe method. The added labels demonstrate that, for the same change in pressure, the expiratory compliance is lower:. In contrast to static compliance, the term "dynamic compliance" sounds like it refers to something vigorous and mobile. The definition of static compliance is easily repurposed to suit:.
Dynamic compliance is change in volume divided by change in pressure, measured in the presence of gas flow. Or, rather, it would be more accurate to say that, in the measurement of dynamic compliance, no effort is made to interrupt the natural rhythm of breathing with any sort of supersyringe. Instead, the pressures used to calculate dynamic compliance are those pressures at which flow naturally "stops" for an instant , which for a mechanically ventilated patients are vaguely corresponding to peak inspiratory pressure PIP and end-expiratory pressure PEEP.
So, a more accurate definition would be,. Dynamic compliance is change in volume divided by change in pressure, measured during normal breathing, between points of apparent zero flow at the beginning and end of inspiration.
However, even when air flow has stopped at the mouth or in the ventilator circuit does not mean that it has stopped inside the lung, and at these points of apparent zero flow there is still some pendelluft going on inside the lung. Now, at this stage it is also important though probably not relevant for exam purposes to point out that in fact the definition of dynamic compliance used here and in many other resources is not entirely accurate.
Even though that is what the examiners want you to think, the inclusion of resistance in the definition makes dynamic compliance something of a misnomer. Or rather, it would be more accurate to say that the equation,.
Moreover, in any case the measurement of dynamic compliance which is usually performed by the mechanical ventilator during routine function is determined from constructing a pressure-volume loop during ventilation. That loop allows the ventilator to determine where the gas flow is zero, i. The gradient of the line connecting these points is the dynamic compliance. The point of zero gas flow, however, is usually not the peak inspiratory pressure, but something closer to P 1 , the drop in pressure which occurs at the end of inspiration:.
Fiegel, J. Lung stability during respiratory maneuvers is due to the presence of the pulmonary surfactant at the air-liquid interface of the lungs.
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