Understanding the Role of Inertia in Mechanical Test Lungs

July 23, 2014
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It’s no secret—mechanical systems move differently than biological systems particularly when it comes to inertia.  This concept has a number of significant implications when working with a mechanical test lung.  During ventilation the inertia of the lung chamber must first be overcome before it can start to expand.  Prior to this point any gas delivered to the lung has been delivered into a chamber with static compliance.  Only when the lung chamber begins to move do we see the dynamic compliance changes that would be expected of a normal patient.  This results in a sharp peak in pressure data taken at the beginning of inspiration as the lung chamber overcomes inertia and at the end of expiration as the chamber returns to rest.  Static compliance compensation is rarely included in ventilators and other medical instruments since it does not represent a normal patient scenario.

The most common physical symptom of this problem occurs during the expiratory phase.  When a test lung with considerable inertia returns to rest at the end of a breath it can cause a pressure “bounce” in the airway that is sometimes strong enough to trigger an assisted breath from an IMV/SIMV compatible machine.  While the false peaks in the pressure wave are rarely large enough to trigger limit warnings in a ventilator, this second breath caused by the bounce can skew tests measuring rate parameters such as breath rate and minute volume.

While frustrating, this problem is not unsolvable.  Some mechanical test lungs are equipped with counterbalances to help minimize the inertia of the lung chamber while advanced breath parsing in instrumentation allows knowledgeable users to eliminate most of the false peaks that come up in testing.  Tests themselves are often adjusted to get rid of these inaccuracies.  Almost all modern ventilation tests call for a PEEP of at least 5 cmH2O which (while being representative of actual ventilation practices) helps to reduce the inertia of mechanical and biological lungs alike.  Decreasing I:E ratio can also help as it allows for a more gradual progression between the breath phases.

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