An orthotropic viscoelastic material model for passive myocardium: theory and algorithmic treatment

This contribution presents a novel constitutive model in order to simulate an orthotropic rate-dependent behaviour of the passive myocardium at finite strains. The motivation for the consideration of orthotropic viscous effects in a constitutive level lies in the disagreement between theoretical predictions and experimentally observed results. In view of experimental observations, the material is deemed as nearly incompressible, hyperelastic, orthotropic and viscous. The viscoelastic response is formulated by means of a rheological model consisting of a spring coupled with a Maxwell element in parallel. In this context, the isochoric free energy function is decomposed into elastic equilibrium and viscous non-equilibrium parts. The baseline elastic response is modelled by the orthotropic model of Holzapfel and Ogden [Holzapfel GA, Ogden RW. 2009. Constitutive modelling of passive myocardium: a structurally based framework for material characterization. Philos Trans Roy Soc A Math Phys Eng Sci. 367:3445-3475]. The essential aspect of the proposed model is the account of distinct relaxation mechanisms for each orientation direction. To this end, the non-equilibrium response of the free energy function is constructed in the logarithmic strain space and additively decomposed into three anisotropic parts, denoting fibre, sheet and normal directions each accompanied by a distinct dissipation potential governing the evolution of viscous strains associated with each orientation direction. The evolution equations governing the viscous flow have an energy-activated nonlinear form. The energy storage in the Maxwell branches has a quadratic form leading to a linear stress-strain response in the logarithmic strain space. On the numerical side, the algorithmic aspects suitable for the implicit finite element method are discussed in a Lagrangian setting. The model shows excellent agreement compared to experimental data obtained from the literature. Furthermore, the finite element simulations of a heart cycle carried out with the proposed model show significant deviations in the strain field relative to the elastic solution

Self-Harm in Children and Adolescents who Presented at Emergency Units During the COVID-19 Pandemic: An International Retrospective Cohort Study

Objective: To compare psychiatric emergencies and self-harm at emergency departments (EDs) 1 year into the pandemic, to early pandemic and pre-pandemic, and to examine the changes in the characteristics of self-harm presentations. Method: This retrospective cohort study expanded on the Pandemic-Related Emergency Psychiatric Presentations (PREP-kids) study. Routine record data in March to April of 2019, 2020, and 2021 from 62 EDs in 25 countries were included. ED presentations made by children and adolescents for any mental health reasons were analyzed. Results: Altogether, 8,174 psychiatric presentations were recorded (63.5% female; mean [SD] age, 14.3 [2.6] years), 3,742 of which were self-harm presentations. Rate of psychiatric ED presentations in March to April 2021 was twice as high as in March to April 2020 (incidence rate ratio [IRR], 1.93; 95% CI, 1.60-2.33), and 50% higher than in March to April 2019 (IRR, 1.51; 95% CI, 1.25-1.81). Rate of self-harm presentations doubled between March to April 2020 and March to April 2021 (IRR, 1.98; 95% CI, 1.68-2.34), and was overall 1.7 times higher than in March to April 2019 (IRR, 1.70; 95% CI, 1.44-2.00). Comparing self-harm characteristics in March to April 2021 with March to April 2019, self-harm contributed to a higher proportion of all psychiatric presentations (odds ratio [OR], 1.30; 95% CI, 1.05-1.62), whereas female representation in self-harm presentations doubled (OR, 1.98; 95% CI, 1.45-2.72) and follow-up appointments were offered 4 times as often (OR, 4.46; 95% CI, 2.32-8.58). Conclusion: Increased pediatric ED visits for both self-harm and psychiatric reasons were observed, suggesting potential deterioration in child mental health. Self-harm in girls possibly increased and needs to be prioritized. Clinical services should continue using follow-up appointments to support discharge from EDs. Diversity & inclusion statement: One or more of the authors of this paper self-identifies as a member of one or more historically underrepresented racial and/or ethnic groups in science. We actively worked to promote inclusion of historically underrepresented racial and/or ethnic groups in science in our author group. While citing references scientifically relevant for this work, we also actively worked to promote inclusion of historically underrepresented racial and/or ethnic groups in science in our reference list. The author list of this paper includes contributors from the location and/or community where the research was conducted who participated in the data collection, design, analysis, and/or interpretation of the work