Human Versus Porcine Localized Strain Mechanics

Rationale: COVID-19 has inspired numerous studies on ventilated induced lung injury (VILI). To investigate the strains stimulating lung damage, animal lungs are often used as surrogate models for scarce human specimens. Such studies are restricted to bulk pressure and volume investigations instead of examining regional, real-time, and evolutionary pulmonary behavior offered by digital image correlation (DIC) techniques. Here we subject porcine lungs and a human lung case study to shared global loads and compare local strain distributions as yielded by DIC to assess the applicability of animal models to represent human pulmonary mechanics. Methods: One anonymized human cadaveric lung (854g) and four comparable sized Yorkshire Farm pig lungs (784-1218g) were tested ex vivo within 36hrs postmortem (no IACUC or IRB approval required). Specimens were ventilated at 15 breaths per minute to 675, 900 and 1350ml (6, 8, and 12mL/kg) using a recently established custom-designed electromechanical breathing apparatus interfaced with high resolution and high speed DIC cameras. Lungs were preloaded to 5cmH20 and preconditioned three times for reproducibility. The resulting local deformations associated with global ventilation loads were analyzed throughout the inflation cycle. Results: Similar peak inflation pressures were observed between human and porcine specimens (21-35 and 21±2 - 26±2cmH20 respectively) for the shared applied volumes. Despite comparable global mechanics, the topological strain distributions of human lungs were relatively reduced: when the applied global volume was doubled from 675 to 1350ml, the local averaged strain across the specimen surface increased from 19 to 28%, while porcine strains showed a greater increase from 21±4 to 42±4%. Also from 675 to 1350ml, the human lung surface strains were prominently homogenous with a range of 43 to 58%, compared to the observed heterogeneous porcine strain contours, quantified with a range of 92±15 to 124±24%. The maximum strain values of the human lung were also smaller than porcine specimens (58 versus 106±17%). Conclusion: Collateral ventilation and respective monopodial versus bipodial bronchial networks may explain the discrepancies noted between porcine and human lung strains. While a single human lung specimen is statistically inconclusive, pairing new DIC applications with conventional global metrics offers the ability to characterize the localized strain distribution of the breathing lung and evaluate the anisotropic and heterogenous deformation profiles correlated with VILI, previously uncharacterized to date. These results have implications for understanding the role of amplified strains in translational animal to human lung studies.

Investigating the Mechanics of Positive- versus Negative-Pressure Ventilation

Rationale: In response to the COVID-19 pandemic, an increasing number of studies seek to investigate the differences between artificially ventilated lung behavior via positive-pressure ventilation (PPV), and physiological negative-pressure ventilation (NPV) in order to prevent barotrauma and ventilated induced lung injury (VILI). However, these studies have been limited to comparing downstream biochemical inflammatory markers, instead of PPV/NPV energetics and strain mechanics. In this fundamental study, we subject each lung specimen to both PPV and NPV under matched loading conditions and characterize the pressure-volume-strain behavior. Methods: PPV and NPV was applied to ex-vivo porcine lungs using our novel validated custom-designed dual-piston ventilator (Yorkshire farm pigs, n=3, >6months old);PPV pushed air into the lung, and NPV removing air from the sealed tank enclosure housing the tissue, imitating the role of the diaphragm. After preconditioning, a preload/positive end-expiratory pressure (PEEP) of ±5cmH20 was applied followed by various tidal inflation volumes of 675, 900, and 1350mL (corresponding to 6, 8, and 12 mL/kg) at 15 breaths per minute. Each lung was both positively and negatively ventilated, matching the peak pressures and actual displaced lung volume (measured by accounting for air compressibility in real time) to enable direct comparisons. Furthermore, the specimens were speckled to interface global pressure-volume bulk behaviors with local strain measures using new digital image correlation techniques established for lung tissue. Results: The classical pressure-volume loop was analyzed, and the energy loss (measured from the normalized hysteresis response) averaged 23% higher in PPV (0.54±0.06) compared to NPV (0.44±0.06) across all tidal volumes. The bulk material compliancy, measured from the initial inflation slope representing alveolar recruitment and airway resistance, was 2.7-4.5 times greater in NPV (13.18±4.25 mL/cmH2O) than PPV (3.52±1.06 mL/cmH2O), depending on the tidal volume. The strain contours for all inflation volumes of NPV demonstrated uniform stretch topology and histograms revealed reduced strain ranges and means when compared to PPV. Conclusion: Findings highlight loading and deformation-based mechanisms for pulmonary damage and inflammation. The altered response of the same specimen under a positive versus negative pressure gradient demonstrates how artificial ventilation (PPV) underutilizes material elastic recoil, induces a stiffer bulk response, and causes localized regions of high strains compared to natural breathing (NPV). These results have implications for understanding the role of strain-induced damage in VILI and for reevaluating the physiological pressure-volume response in pathologies (e.g. pulmonary emphysema and fibrosis), which have only been based on PPV (not NPV) to date.

Bring it on Covid-19: being an older person in developing countries during a pandemic

Purpose: This study aims to explore the respective risk perception toward the Covid-19 pandemic among the elderly in two developing countries and their lifestyle change and adaptation behavior as a result of that. Design/methodology/approach: In total, 22 in-depth interviews were conducted among people aged 60 to 85 in Iran and India. The interview guideline was based on two constructs inspired by the study of Gerhold (2020) – risk perception of the pandemic and change in lifestyle and adaptation to the pandemic situation. Findings: The results showed that although the majority of the interviewees initially experienced fear and anxiety during the stay-at-home order and the pandemic situation at large, they quickly adapted to the new situation with behavioral and attitudinal changes in ways relevant to the new normal. The results confirmed the maturation hypothesis proposed by Knight et al. (2000). Originality/value: This paper is an original study of the researchers and among the first contributing to the literature of the elderly studies in developing countries. It contributes to the knowledge required for future behavioral studies concerning the Covid-19 pandemic by evaluating the experiences, risk perception and adaptation approaches of the older people in India and Iran. © 2020, Emerald Publishing Limited.