A Review of Recent Findings About Stress-Relaxation in the Respiratory System Tissues

A Comparative Study of Ex-Vivo Murine Pulmonary Mechanics Under Positive- and Negative-Pressure Ventilation

Increased ventilator use during the COVID-19 pandemic resurrected persistent questions regarding mechanical ventilation including the difference between physiological and artificial breathing induced by ventilators (i.e., positive- versus negative-pressure ventilation, PPV vs NPV). To address this controversy, we compare murine specimens subjected to PPV and NPV in ex vivo quasi-static loading and quantify pulmonary mechanics via measures of quasi-static and dynamic compliances, transpulmonary pressure, and energetics when varying inflation frequency and volume. Each investigated mechanical parameter yields instance(s) of significant variability between ventilation modes. Most notably, inflation compliance, percent relaxation, and peak pressure are found to be consistently dependent on the ventilation mode. Maximum inflation volume and frequency note varied dependencies contingent on the ventilation mode. Contradictory to limited previous clinical investigations of oxygenation and end-inspiratory measures, the mechanics-focused comprehensive findings presented here indicate lung properties are dependent on loading mode, and importantly, these dependencies differ between smaller versus larger mammalian species despite identical custom-designed PPV/NPV ventilator usage. Results indicate that past contradictory findings regarding ventilation mode comparisons in the field may be linked to the chosen animal model. Understanding the differing fundamental mechanics between PPV and NPV may provide insights for improving ventilation strategies and design to prevent associated lung injuries.

Strain-specific differences in lung tissue viscoelasticity of mechanically ventilated infant Sprague-Dawley and Wistar rats

Rats are often used in ventilator-induced lung injury (VILI) models. However, strain-specific susceptibility for VILI has not been elucidated yet. The aim of this study was to demonstrate strain-specific differences in VILI in infant Sprague-Dawley and Wistar rats. VILI was compared in 2-wk-old pups after 8 h of protective or injurious ventilation. Pups were ventilated with tidal volumes (V_T) of ∼7 mL/kg and positive end-expiratory pressures (PEEP) of 6 cmH₂O (V_T7 PEEP6) or with V_T of ∼21 mL/kg and PEEP 2 cmH₂O (V_T21 PEEP2). Interleukin-6, macrophage inflammatory protein-2 (MIP-2), inflammatory cells, and albumin in bronchoalveolar lavage fluid (BALF); histology; and low-frequency forced oscillation technique (LFOT) and pressure-volume (PV) maneuvers were assessed. Alveolar macrophages, neutrophils, and MIP-2 derived from BALF revealed more pronounced VILI after V_T21 PEEP2 in both strains. LFOT and PV analyses demonstrated rat strain-specific differences both at baseline and particularly in response to V_T21 PEEP2 ventilation. Sprague-Dawley rats showed higher airway and tissue resistance and elastance values with no difference in hysteresivity between ventilation strategies. Wister rats challenged by V_T21 PEEP2 experienced significantly more energy dissipation when compared with V_T7 PEEP6 ventilation. In conclusion, both rat strains are useful for VILI models. The degree of VILI severity depends on ventilation strategy and selected strain. However, fundamental and time-dependent differences in respiratory system mechanics exist and reflect different lung tissue viscoelasticity. Hence, strain-specific characteristics of the respiratory system need to be considered when planning and interpreting VILI studies with infant rats.

Cells respond to deletion of CAV1 by increasing synthesis of extracellular matrix

A range of cellular functions have been attributed to caveolae, flask-like invaginations of the plasma membrane. Here, we have used RNA-seq to achieve quantitative transcriptional profiling of primary embryonic fibroblasts from caveolin 1 knockout mice (CAV1-/- MEFs), and thereby to gain hypothesis-free insight into how these cells respond to the absence of caveolae. Components of the extracellular matrix were decisively over-represented within the set of genes displaying altered expression in CAV1-/- MEFs when compared to congenic wild-type controls. This was confirmed biochemically and by imaging for selected examples. Up-regulation of components of the extracellular matrix was also observed in a second cell line, NIH-3T3 cells genome edited to delete CAV1. Up-regulation of components of the extracellular matrix was detected in vivo by assessing collagen deposition and compliance of CAV1-/- lungs. We discuss the implications of these findings in terms of the cellular function of caveolae.

The effects of nifedipine on respiratory mechanics investigated by theend-inflation occlusion method in the rat

CONTEXT

Calcium channel blockers may theoretically exhibit relaxing effects not only on vascular smooth muscle but also on airway smooth muscle.

OBJECTIVE

To investigate possible effects of nifedipine on respiratory mechanics in the rat.

METHODS

Respiratory system mechanical parameters were measured by the end-inflation occlusion method in the rat in vivo before and after the intraperitoneal administration of nifedipine.

RESULTS

We found that nifedipine affects respiratory mechanics, inducing a reduction of airway resistance and of respiratory system elastance, probably because of a relaxing action on airway and parenchimal smooth muscle cells.

CONCLUSION

Should these results be further confirmed by human investigations, a possible role of nifedipine in pharmacological respiratory system's diseases treatment may be suggested.

A review of the effects of some endocrinological factors on respiratory mechanics

CONTEXT

Endocrinological factors have been recently described to affect respiratory mechanics.

OBJECTIVE

To review recent literature data, most of all obtained by the end-inflation occlusion method, describing the effects of molecules of endocrinological interest such as endothelin, erythropoietin and renin-angiotensin, on respiratory mechanics parameters.

METHODS

The papers considered in this review were found by inserting in Pubmed/Medline the following indexing terms: hormones, endothelin, erythropoietin, angiotensin and respiratory mechanics.

RESULTS

It was found that the above cited molecules, beside their well known physiological main effects, exhibit influences on respiratory mechanics, most of all on the airflow resistance, which was described to be increased by endothelin and angiotensin, and decreased by erythropoietin.

CONCLUSIONS

A number of molecules of biological interest exhibit unexpected influences on respiratory mechanics. The clinical effects depend on the consequences of modified inspiratory pressure values the respiratory muscles have to perform for a given breathing pattern.

The Effects of Prone with Respect to Supine Position on Stress Relaxation, Respiratory Mechanics, and the Work of Breathing Measured by the End-Inflation Occlusion Method in the Rat

PURPOSE

The working hypothesis is that the prone position with respect to supine may change the geometric configuration of the lungs inside the chest wall, thus their reciprocal mechanical interactions, leading to possible effects on stress relaxation phenomena and respiratory mechanics.

METHOD

The effects of changing body posture from supine to prone on respiratory system mechanics, particularly on stress relaxation, were investigated in the rat by the end-inflation occlusion method.

RESULTS

In the prone with respect to supine position, an increment of the frictional resistance of the airway (from 0.13 ± 0.01 to 0.19 ± 0.02 cm H2O/l sec(-1), p < 0.05) and a decrement of the stress relaxation-linked pressure dissipation (from 0.51 ± 0.05 to 0.45 ± 0.05 cm H2O/l sec(-1), p < 0.01) were found. Respiratory system elastance and total resistive pressure dissipation did not change significantly. Accordingly, a significant increase of the frictional "ohmic" mechanical inspiratory work of breathing and a decrease of the visco-elastic work of inspiration were demonstrated, while no significant changes occurred for the total mechanical work of breathing and its total resistive and elastic components.

CONCLUSION

It is concluded that postural changes affect the visco-elastic characteristics of the respiratory system and the related stress relaxation phenomena by influencing the disposition and relation of the lungs inside the chest wall and their relative geometrical configuration, and the interaction phenomena of the constitutive parenchymal structures, i.e., elastin and collagen fibers. Since the prone position resulted in no serious or disadvantageous respiratory system mechanical derangement, it is suggested it may be usefully applied in nursing or for therapeutic goals.