Interpreting the Shape of the Pressure-volume Loop
Pressure-volume loop in spontaneous breathing
Behold, the pressure-volume loop.
It demonstrates how compliance changes as volume increases. Observe this spontaneously breathing individual.
Inspiration creates a negative pressure, which gradually trends to zero as the lungs fill to the full capacity of the tidal volume. At expiration, the elastic recoil of the chest wall and lung tissue creates a positive pressure, which decreases towards zero as the volume is exhaled.
If PEEP is applied, the entire loop shifts right, as the baseline pressure becomes the PEEP. This loop can be observed in patients on CPAP.
Anatomy of the pressure-volume loop
This is a pressure-volume loop of a patient on a volume-control mode of ventilation, which in normal ventilator (pressure-flow-volume over time) waveforms looks like this.
The mode is mandatory; the control variable is volume, and the flow is constant.
The curve of optimal compliance
This is the part of the loop during which the compliance is linear; it is thought to represent the ideal pressure, at which the alveoli are all happily open and distending gradually as the pressure rises.
Lower inflection point (Pflex)
It is sometimes said that this is the airway pressure at which there resistance of the airways is overcome. In situations where airway resistance is very high, this inflection point is dragged to the right.
However, airway resistance plays a role throughout the inspiratory (and expiratory) curves, because there is flow. So long as there is gas flowing, and there are narrow tubes for it to flow though, their resistance will influence the pressure of that gas.
Thus, the lower inflection point mainly represents the critical opening pressure of the alveoli. The initial rapid rise in pressure is a reflection of alveolar recruitment. It takes more pressure to re-inflate a collapsed alveolus than it takes to distend a deflated one.
In ARDS, it has been thought that this lower inflection point suggests the PEEP you should set. Given that constant collapsing and re-opening of alveoli causes VILI, one should aim to constantly keep all of the alveoli at a positive pressure slightly above their critical opening pressure, so as to prevent de-recruitment. However in practice, the lower inflection point tends to over-estimate the ideal PEEP. Not only that, but it seems clinicians are very poor at determining where this inflection point is, when shown a PV loop. We tend to vary by as much as 11cm H2O.
But then... Is derecruitment not an expiratory phenomenon? Then why are we using an inspiratory limb of the pressure-volume loop to determine the optimal pressure to prevent de-recruitment?
Upper inflection point
This feature of the expiratory limb represents the elastic recoil of the lung tissue and chest wall which occurs when the ventilator cycles to expiration and the pressure drops back to PEEP.
Obviously, at some stage during this curve alveolar derecruitment begins. But where?
It appears that the derecruitment occurs throughout this curve.
it is generally held that the rapid drop in pressure at the beginning of this curve corresponds to the deflation of the most hyperinflated lung units (which by virtue of being hyperinflated contribute the greatest deflation pressure).
Thus, one might come to the conclusion that the most appropriate PEEP for these patients lies somewhere between the lower inflection point and the upper inflection point, corresponding to the best compromise between recruitment and hyperinflation.
The utility of all this
The difficulty in discerning the inflection points notwithstanding, there are other barriers to using these curves to guide your management of the noncompliant lung. The problem with ARDS is its heterogeneity; some bits of lung are aerated well and have surfactant, others are collapsed, and others are packed full of impenetrable hemorrhagic pus. Some bronchioles are patent, some are clogged with exudate, and some are squished by the weight of waterlogged lung above them. This heterogeneity leads to a smoothing of the classical S-shaped inspiratory pressure-volume curve. Go looking for an inflection point in this mess, and your estimates will vary by 11cmH2O just like the intensivists mentioned above.
The beak-shaped part of the curve, which lends it its penguin-like shape, is the region of pressure where rising pressure does not lead to increasing volume. Put simply, the lung is overstretched, at breaking point.
One can draw the conclusion that if it is tidal volume and minute ventilation you are after, then this part of the curve represents "wasted" pressure, which does not buy you any extra litres.
The idealised pressure-volume loop of volume-controlled ventilation
This is the pressure volume loop of some sort of well-behaved ideal patient, on a mandatory volume-controlled mode.
In this situation, the flow is delivered at a constant rate, which causes pressure to increase in a predictable pattern. There is an initial rapid rise in pressure as collapsed alveoli are recruited; then, there is a smooth increase in pressure up to the peak.
The idealised pressure-volume loop of pressure-controlled ventilation
This is another idealised scenario. In this setting, the pressure is controlled, and changes in the rate of flow are used to maintain it at a certain level.
In this setting one does not expect to derive a lot of useful information from the inspiratory part of the curve. It is impossible to tell what is happening in terms of alveolar recruitment because the flow is constantly changing (usually, on a decelerating ramp).
However, the useful feature of this is the satisfying flatness of the right side of the curve. Note how the penguins beak is missing. There is no alveolar overdistension in this scenario; in fact one can imagine the whole alveolar volume distending peacefully up to a certain volume.
A more realistic and more familiar pressure-volume loop
This is the pressure volume loop of a pressure-supported breath from a patient on SIMV-PRVC, which features decelerating flow, a constant inspiratory pressure, and which is patient-triggered.
That's right. Where is that lower inflection point? Nothing is where it is supposed to be. The curvature of the inspiratory component is influenced by the flow rate, which is maximal at the onset of the breath; the decelerating flow rate makes it difficult to derive any useful information about the lower inflection point from this loop. All you can do is focus at the two points of the respiratory cycle where the flow is zero. Of these, the more interesting point is the end of inspiration: when you have both a volume and a pressure, from which you can estimate compliance.