The sheep as a pre-clinical model for testing intra-aortic percutaneous mechanical circulatory support devices

Animal Models for Mechanical Circulatory Support: A Research Review

Heart failure is a clinical syndrome that has become a leading public health problem worldwide. Globally, nearly 64 million individuals are currently affected by heart failure, causing considerable medical, financial, and social challenges. One therapeutic option for patients with advanced heart failure is mechanical circulatory support (MCS) which is widely used for short-term or long-term management. MCS with various ventricular assist devices (VADs) has gained traction in end-stage heart failure treatment as a bridge-to-recovery, -decision, -transplant or -destination therapy. Due to limitations in studying VADs in humans, animal studies have substantially contributed to the development and advancement of MCS devices. Large animals have provided an avenue for developing and testing new VADs and improving surgical strategies for VAD implantation and for evaluating the effects and complications of MCS on hemodynamics and organ function. VAD modeling by utilizing rodents and small animals has been successfully implemented for investigating molecular mechanisms of cardiac unloading after the implantation of MCS. This review will cover the animal research that has resulted in significant advances in the development of MCS devices and the therapeutic care of advanced heart failure.

Biomechanical effects of hemin and sildenafil treatments on the aortic wall of chronic-hypoxic lambs

Introduction: Gestation under chronic hypoxia causes pulmonary hypertension, cardiovascular remodeling, and increased aortic stiffness in the offspring. To mitigate the neonatal cardiovascular risk, pharmacological treatments (such as hemin and sildenafil) have been proposed to improve pulmonary vasodilation. However, little is known about the effects of these treatments on the aorta. Therefore, we studied the effect of hemin and sildenafil treatments in the aorta of lambs gestated and raised at highlands, thereby subjected to chronic hypoxia. Methods: Several biomechanical tests were conducted in the descending thoracic aorta (DTA) and the distal abdominal aorta (DAA), assessing 3 groups of study of hypoxic animals: non-treated (Control) and treated either with hemin or sildenafil. Based on them, the stiffness level has been quantified in both zones, along with the physiological strain in the unloaded aortic duct. Furthermore, a morphological study by histology was conducted in the DTA. Results: Biomechanical results indicate that treatments trigger an increment of axial pre-stress and circumferential residual stress levels in DTA and DAA of lambs exposed to high-altitude chronic hypoxia, which reveals a vasodilatation improvement along with an anti-hypertensive response under this characteristic environmental condition. In addition, histological findings do not reveal significant differences in either structure or microstructural content. Discussion: The biomechanics approach emerges as a valuable study perspective, providing insights to explain the physiological mechanisms of vascular function. According to established results, alterations in the function of the aortic wall may not necessarily be explained by morphostructural changes, but rather by the characteristic mechanical state of the microstructural components that are part of the studied tissue. In this sense, the reported biomechanical changes are beneficial in mitigating the adverse effects of hypobaric hypoxia exposure during gestation and early postnatal life.

Transdermal wires for improved integration in vivo

Alternative engineering approaches have led the design of implants with controlled physical features to minimize adverse effects in biological tissues. Similar efforts have focused on optimizing the design features of percutaneous VAD drivelines with the aim to prevent infection, omitting however a thorough look on the implant-skin interactions that govern local tissue reactions. Here, we utilized an integrated approach for the biophysical modification of transdermal implants and their evaluation by chronic sheep implantation in comparison to the standard of care VAD drivelines. We developed a novel method for the transfer of breath topographical features on thin wires with modular size. We examined the impact of implant's diameter, surface topography, and chemistry on macroscopic, histological, and physical markers of inflammation, fibrosis, and mechanical adhesion. All implants demonstrated infection-free performance. The fibrotic response was enhanced by the increasing diameter of implants but not influenced by their surface properties. The implants of small diameter promoted mild inflammatory responses with improved mechanical adhesion and restricted epidermal downgrowth, in both silicone and polyurethane coated transdermal wires. On the contrary, the VAD drivelines with larger diameter triggered severe inflammatory reactions with frequent epidermal downgrowth. We validated these effects by quantifying the infiltration of macrophages and the level of vascularization in the fibrotic zone, highlighting the critical role of size reduction for the benign integration of transdermal implants with skin. This insight on how the biophysical properties of implants impact local tissue reactions could enable new solutions on the transdermal transmission of power, signal, and mass in a broad range of medical devices.

Assessment of Large Animal Vascular Dimensions for Intra-Aortic Device Research and Development: A Systematic Review

Animal studies are often required to evaluate new cardiovascular medical devices before they reach the market. Moreover, first-generation novel devices including aortic endovascular prostheses and circulatory support devices are often larger than later iterations or tested in a limited range of sizes. One of the challenges in evaluating these devices is finding a model that is both accessible and anatomically similar to humans, as there is a paucity of data on vascular dimensions in large animals. We set out to complete a comprehensive review of available reports on vascular dimensions in swine, ovine, and bovine models, with a particular focus on the descending aorta and ilio-femoral arteries. We searched Embase and MEDLINE databases for reports of descending aorta and peripheral vascular dimension in large animal models. Data from swine, ovine, and bovine models were separated by weight into 3 categories: 40 to 60 kg, 61 to 80 kg, and >80 kg. We also incorporate our computed tomography angiography data from 4 large sheep and 9 calves into this review. Swine, sheep, and calf >80 kg may serve as the best models to maximize aortic diameter resemblance to humans. If device implantation can be achieved in aortas of smaller dimensions, care should be taken to ensure access site suitability such as the common femoral artery in these smaller animals.

Physiologic Effects of Prolonged Terminal Anesthesia in Sheep (Ovis gmelini aries)

The ruminant alimentary tract and its effects on blood homeostasis complicate prolonged terminal studies conducted under general anesthesia in sheep. We therefore studied 15 healthy female white alpine sheep that were undergoing prolonged anesthesia (> 30 h) for an unrelated terminal study. In the current study, all sheep developed a decreased hematocrit and hemoglobin concentration after induction of anesthesia, which fell further, along with a significant decrease in white blood cell count, over the course of anesthesia. Sheep also showed an initial hyponatremia, a persistent hypokalemia, hypocalcemia, and a progressive hyperchloremia. A significant drop in blood pH developed over time despite normal values of blood lactate and a marked decline in partial pressure of carbon dioxide over the course of the experiment. The latter consequently reduced the efficacy of mechanical ventilation, as reflected in a reduced oxygen partial pressure. A significant increase in lactate dehydrogenase and creatinine kinase was observed. Arterial blood pressure and heart rate significantly decreased over time, but remained within normotensive and normocardic limits. Central venous pressure rose significantly over the course of anesthesia. In conclusion, prolonged anesthesia in sheep is associated with a wide range of complex physi- ologic changes. An in-depth understanding of all metabolic compensatory mechanisms and their underlying cause during prolonged anesthesia is necessary for interpreting data from the primary study, with special considerations to account for ruminant-specific physiology.