In pulmonary mechanics we have an hysteresis curve. Note I said *AN* hysteresis curve instead of *A* hysteresis curve. My momma raised me right. Our home was one where infinitives were not something to mindlessly split, Oxford commas were always in play and prepositions were not something to cavalierly end a sentence with.
Much to the dismay of RTs everywhere, brace yourselves, hysteresis is not just a respiratory phenomenon. Hysteresis is a general term for any system in which the state of the system is dependent upon the history of the system. This dependency creates a a differential when charting state variables, say ‘x’ and ‘y’, of a system cartesianally where the values of y are different depending on whether x is rising or falling.
“Cartesianally”. My momma also was a strong advocate for neologism. And she portmantotally loved portmanteaus.
Hysteresis comes in many forms: magnetic, mechanical, biological; and can be found in almost all disciplines from physics to economics. The specific form of hysteresis we observe in pulmonary measures is mechanical, specifically an elastic form. Essentially, the lung is an imperfect elastic organ and it requires more energy to fill (inspiration), than it does to empty (expiration). Inspiration requires active effort from the diaphragm. Expiration is, or can be, passive. Therefore the volume during a specific pressure is going to be lower during inspiration that it will be during expiration.
The end result *cartesianally* is this:
In respiratory therapy we talk a lot about lung compliance. Compliance is the relationship of a change in volume to a change in pressure (ml/cm H2O) which is represented by the slope of the inspiratory and expiratory curve (whether you like it or not, congratulations, you just dipped your toes into the mythical waters of calculus). The differences in expiratory and inspiratory compliance over the course of one breath cycle are what create this hysteresis effect.
We speak of two forms of compliance in respiratory therapy, dynamic and static. Dynamic compliance is, definitionally, the compliance of the lung at any given moment during the movement of air through the lung, but generally we measure dynamic compliance (Cdyn) at peak inspiratory pressure (PIP), which is the pressure at the end of inhalation.
Cdyn = Vt / (PIP – PEEP)
If, at the end of inspiration, we perform an inspiratory hold, giving the volume of air in the lungs a chance to overcome resistance and equalize. This is the Plateau Pressure or Pplat. We use Pplat to calculate static compliance (Cstat).
Cstat = Vt / (Pplat – PEEP).
Static compliance can be reflective of disease states. Increased Cstat occurs in obstructive diseases such as Emphysema and lowered Cstat occurs in restrictive disease such as fibrosis or pneumonia.
If static compliance stays consistent and dynamic compliance goes down, meaning your PIP increases but your Pplat stays constant, then the practitioner should consider causes that are increasing airway resistance: bronchospasms, retained secretions, a family member standing *unintentionally* on the inspiratory circuit while greedily clutching a will…
Also, this hysteresis curve is one of the many ways in which we as practitioners choose optimal PEEP. PEEP (positive end-expiratory pressure) is a back pressure we can add during ventilation to stint open the alveoli of the lungs and help prevent alveolar collapse and atelectasis on exhalation. The lower inflection point (LIP) of the inspiratory curve, where the change in pressure slows in relation to the volume taken in, representing increased compliance, marks the point at which the alveoli have opened. This is purported to be a perfect place to preset the PEEP preventing atelectasis and atelectrauma. On the graph shown, optimal PEEP would be right around 8 cm H2O.
The pressure at which the alveoli start to become hyperextended and susceptible to volutrauma occurs at the upper inflection point (UIP), where the compliance begins to drop. This curve shows us that we should target a tidal volume where the peak pressures don’t exceed more than 20 cm H2O.
The Hamilton G5 mechanical vent now comes equipped with a Pressure Volume tool that can be used on sedated patients performing an alveolar recruitment maneuver which steps up the PEEP by chosen increments over time monitoring the changes in Cstat and, at the end, recommending PEEP and other settings for the patient.
Most mechanical ventilators, even ones without this tool, will graph this Pressure/Volume hysteresis curve for you real time and it is useful for making a multitude of clinical decisions. If you can handle the pressure. 🙂