Stress Relaxation, another cause of “Pseudo Auto-Peep”?

doi:10.21203/rs.3.rs-4359013/v1

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Stress Relaxation, another cause of “Pseudo Auto-Peep”?

https://doi.org/10.21203/rs.3.rs-4359013/v1

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In critically ill patients receiving mechanical ventilation, a positive end-expiratory pressure greater than the applied externally (auto-PEEP) reflects dynamic hyperinflation of the lungs. The term “Pseudo Auto-Peep” (Bilen & Cohen, 1993) ¹ was coined to describe a progressive increase in expiratory plateau pressure in prolonged occlusion, attributed to a retrograde flow of extraluminal gas into the airways. However, the mechanism has not been studied closely. We intend to describe another mechanism for this phenomenon, explained by stress relaxation due to inequalities in expiratory time constant (τE), manifested in inspiration and expiration.

Pulmonology

Auto-PEEP

Pseudo auto-PEEP

Stress index

Time constants.

In mechanical ventilation, a final expiratory volume greater than the relaxation volume generates an end-expiratory pressure known as “auto-PEEP.” Airway dynamic hyperinflation could be the pathophysiological explanation for this phenomenon, as could expiratory dynamic collapse, insufficient expiratory time, or increased expiratory time constant (τE), which have important consequences on cardiovascular function and respiratory mechanics ².

Usually, it is detected by end-expiratory occlusion (static auto-PEEP) or end-expiratory flow amputation (dynamic auto-PEEP). Another term associated with this is “Occult positive end-expiratory pressure ”³, due to severe airway obstruction, as in patients with asthma, which can be observed using the end-inspiratory occlusion maneuver. Bilen et al. ¹ describe another related phenomenon, termed “pseudo-auto-PEEP,” caused by backward gas flow from the extrapulmonary to the alveolar space, and it is illustrated by the progressive rise of the end-expiratory pressure during prolonged expiratory occlusion.

In this manuscript, our aim is to describe a pathophysiological mechanism of a phenomenon reproducing findings similar to those described by Bilen et al. ¹ (Figure 1A). We hypothesize that stress relaxation caused by regional inequalities of τE generated an anterograde contra-pulmonary flow from fast emptying areas to slow ones (“pendelluft”). This phenomenon is manifested in both inspiratory and expiratory occlusion, as suggested by a fit of the pressure released in relaxation stress to a logarithmic function. (Figure 1B).

A 42-year-old female patient with a medical history of pulmonary interstitial disease was admitted to the ICU after surgical intervention of a right unilateral pulmonary transplant. She had multiple postoperative complications: hemothorax, bacterial, fungal pneumonia, and bronchial prosthesis due to stenosis of the transplanted lung, requiring support with venous-arterial extracorporeal membrane oxygenation during the perioperative period. The patient required prolonged mechanical ventilation as well as a prolonged stay in the ICU. To evaluate respiratory mechanics, advanced monitoring (Tracheal, esophageal pressures) and electrical impedance tomography (EIT), were carried out during controlled ventilation, with neuromuscular blockade. Respiratory mechanic parameters were obtained by multiple linear regression, and the results showed a high elastance and resistance, Ers: 43.18 cmH₂O/L, EL 40.09 cmH₂O/L, and total resistances: 15.83 cmH₂O/L/s. An end-expiratory occlusion prolonged maneuver showed a progressive increase in pressure until reaching the plateau (6 sec). This finding corresponds to the stress relaxation during inspiratory occlusion (Figure 1A). Values of transpulmonary pressure correspond to a logarithmic function of the release pressure in stress relaxation, and the end-expiratory occlusion pressure revealed equivalent log functions but inverse, i.e., 0.525*ln(x)] and -0.514*ln, respectively (Figure 1B). EIT monitoring shows similar findings to the obtained with flow-volume loop (Figure 1C), as relevant inequalities in the time constants with fast-emptying native left lung, which corresponded to the radiological images (Figures 2).

A specific form of auto-PEEP is the so-called "Pseudo auto-PEEP." It was described by Bilen, 1993 ¹ and attributed to the retrograde flow of extraluminal gas concerning the airway into it. To our knowledge, there have been no additional publications regarding this concept. We provide a representative case, which shows similar findings that can be explained by relaxation stress due to inequalities of time constants and complement the original description.

The equilibrium state of the inspiratory and expiratory plateau pressure depends on the relaxation stress due to thoracic-pulmonary viscoelastic resistance, surface tension, and differences in regional time constants ⁶. Stress relaxation has a non-linear

pressure-dependent viscoelastic properties of the respiratory system with a focus on differences in energy distribution ⁷.

Our case represents a phenomenon equivalent to "Pseudo auto-PEEP" explained by relaxation stress due to extreme inequalities in time constants. The flow-volume loop could suggest an obstructive mechanism due to the biphasic morphology ⁸ (Figure 1C). However, this is explained by two different lung emptying compartments. In Figure 1A, we can see how the end-expiratory flow reaches a level near 0, and EIT monitoring shows a remarkable difference in lung emptying between the two lungs. As the radiological images show (Figure 2), there is a predominantly unilateral pathology, with notable involvement of the native lung. Finally, Figure 1B shows how stress relaxation has an equivalent logarithmic fitting of both the inspiratory and expiratory phases, manifested in the progressive increase in end-expiratory pressure during the prolonged pause, which simulates the phenomenon called "Pseudo auto-PEEP." In addition, this finding could be used as a parameter to evaluate the severity of lung injury ⁶.

The clinical and physiologic consequences of "Pseudo auto-PEEP" remain uncertain; however, not recognizing it may affect the respiratory mechanics calculation. Although our purpose has not been to evaluate the inspiratory effort, this phenomenon could increase the muscle's workload.

The study procedures and data collected for this report were approved by the Ethics Committee of the University Hospital, Reina Sofia, Cordoba, Spain (Refer.: Musc-Txp23). The patient could not execute the permission to publish patient information, so the surrogates (family legally responsible) were informed about the study with enough detail that the patient was admitted, understood the entire procedure, and signed a written informed consent form before inclusion in the study. All methods were performed according to the guidelines and regulations of the Declaration of Helsinki.

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The authors declare no competing interests.

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Stress Relaxation, another cause of “Pseudo Auto-Peep”?

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