Lung Mechanics Have You Stressed Out?

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The Pre-brief

The recent RECOVERY RS trial has been expertly reviewed in several blogs, see Rebel EM, TheBottomLine, and St. Emlyn’s Blog.  This randomized controlled study showed a decreased rate of intubation with the use of CPAP as compared to HFNC.  When reading through this study, thoughts of mechanical power and the equation of motion once again rose to the surface.  The equation of motion defines the pressure (force/area) required to drive ventilation.  This equation is ultimately a keystone of understanding lung mechanics and mechanical power. 

Closely related are the concepts of stress and strain. These terms are borrowed from material science and warrant their own discussion.

Stress Defined

Stress can be defined as the restoring force experienced by a stretched elastic structure per unit area of that structure. Hence, stress is force per unit of area, i.e. pressure.  With regards to the lung parenchyma, the pressure that most directly correlates lung stress is the pressure across the lung parenchyma. From the perspective of a single alveolus, this can be referred to as transalveolar pressure (alveolar pressure – pleural pressure).

From the whole-lung perspective this is often labeled as ‘elastic recoil pressure of the lung: Pel(L). Practically, transpulmonary pressure reflects Pel(L) in zero-flow conditions. Hence, end-inspiratory transpulmonary pressure (Pplat – Ppl) and end-expiratory transpulmonary pressure (total PEEP – Ppl) are the best measures of lung parenchymal stress during end-inspiration and end-expiration respectively. A prevailing concept in mechanical ventilation is to minimize parenchymal lung stress. 

While conveniently simplified, we know there are several problems with this model that need to be accounted for:

  1. The lung is never homogenous, especially in our ARDS patients. Mechanisms such as “stress raisers” may amplify local alveolar stress to several times higher than the average stress measured with transpulmonary pressures.
  2. Although we use esophageal pressure (Pes) as a surrogate for pleural pressure, there are several practical challenges with this. Importantly, Pes is a better reflector of Ppl in the dependent lung regions and overestimates Ppl in the non-dependent regions.
  3. In cases of severe small airway disease (e.g. asthma), there may be complete airway collapse in some lung units, especially during expiration (waterfall effect). In this example, an end-expiratory hold will not see the alveolar pressure of these lung units sometimes labeled as occult autoPEEP). 

In addition to parenchymal stress as an isolated value, stress amplitude also contributes to total lung injury as markers of deforming stress.  These can be inferred from the pressure waveform and inspiratory hold maneuvers.

Strain Defined

To the engineer, strain is defined as the ratio of the increase in length of a structure divided by its resting length.  In terms of lung mechanics, this is the difference between end-inspiration and functional residual capacity relative to FRC.  Conceptualized as the tidal volume normalized to residual lung volume, often simplified to tidal volume alone.  Strain rate, alternatively, is the speed at which the given strain occurs, represented as flow rate.

The concepts of stress and strain, and subsequently, volume, plateau pressures, PEEP, and transpulmonary pressures contribute to the total amount of work being done by the respiratory system.  With the addition of peak pressure and respiratory rate we come to complete a simplified model of mechanical power, validated by none other than Gattatoni.

This simplified formula is easy to assess at the bedside and gives the practitioner yet another construct to assess the potential ventilator-induced injury and subsequently a route to minimize stress and strain.  

While these models are validated in patients undergoing mechanical ventilation, similar concepts of stress and strain can be applied to those on NIPPV such as CPAP and HFNC.  While not measurable at the bedside, we often get a qualitative impression of stress and strain.  Imagine the purse-lipped breather, with a respiratory rate of 34, taking huge breaths with extremely high inspiratory effort – we can see how this image translates to high stress and strain and possibly need for mechanical ventilation to reduce the injury to the lungs. So when interpreting data such as the RECOVERY RS trial it warrants to keep these concepts in mind; perhaps the benefit of CPAP was in one way or another, a reduction of mechanical power and lung injury. 

The Debrief

  • Stress on the lung parenchyma is the restoring force per unit of area; it has units of pressure. Practically, the best surrogates of parenchymal stress are end-inspiratory and end-expiratory transpulmonary pressures.
  • Strain is the degree of deformation of the lung and is often reduced to tidal volume.
  • Appreciation of these basic concepts allows one to have a deeper understanding of the various pathophysiological aspects of VILI.

References

  1. Hubmayr RD, Kallet RH. Understanding Pulmonary Stress-Strain Relationships in Severe ARDS and Its Implications for Designing a Safer Approach to Setting the Ventilator. Respir Care. 2018 Feb;63(2):219-226. doi: 10.4187/respcare.05900. PMID: 29367383.
  2. Gattinoni L, Carlesso E, Caironi P. Stress and strain within the lung. Curr Opin Crit Care. 2012 Feb;18(1):42-7. doi: 10.1097/MCC.0b013e32834f17d9. PMID: 22157254.
  3. Giosa L, Busana M, Pasticci I, Bonifazi M, Macrì MM, Romitti F, Vassalli F, Chiumello D, Quintel M, Marini JJ, Gattinoni L. Mechanical power at a glance: a simple surrogate for volume-controlled ventilation. Intensive Care Med Exp. 2019 Nov 27;7(1):61. doi: 10.1186/s40635-019-0276-8. PMID: 31773328; PMCID: PMC6879677.

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