Measuring Dynamic Compliance and Static Compliance.
At each breath, the respiratory system must overcome two opposing forces to be able to move air in and out of the lungs: the resistance to airflow and the distensibility (or compliance) of the respiratory system. Physiological and/or pathological factors may alter these mechanical characteristics and induce significant changes in the effort required to breathe or tolerance to exercise.
During flexiVent experiments, it is possible to measure the mechanical properties of the respiratory system and to evaluate the impact a given change will have on specific parameters. While a measure of resistance can only be performed in presence of airflow (or under dynamic conditions), measurements of compliance can be obtained under dynamic or static conditions. Assessing compliance in presence or absence of airflow, can allow for an exhaustive evaluation of the respiratory system or of its response under particular physiologic or pathophysiologic states.
A measure of the dynamic compliance of the respiratory system (also often simply called compliance) is obtained with the SnapShot perturbation, which studies the lung under conditions of tidal breathing.
Since this measure is typically performed under closed chest conditions, the compliance obtained generally reflects the overall stiffness the entire respiratory system has to overcome on a breath-by-breath basis and will include the compliance of the lungs, the chest walls, as well as that of the airways.
A measure of the static compliance (Cst) is obtained following the construction of a pressure-volume curve. In this maneuver, the respiratory system is typically inflated and deflated in a step-wise manner, with at each plateau, a period of no gas flow for equilibration.
If performed under closed-chest conditions, Cst reflects the elastic properties of the respiratory system (i.e. lungs and chest walls) at rest. Measurements of static compliance can also be performed under open chest conditions, where it will now reflect intrinsic distensibility the lungs.
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