Mechanical ventilators are used to support or replace spontaneous breathing in patients and animals suffering from inadequate respiratory functions. Ventilators are also often used as part of long-term anesthesia procedures, due to the depression of the respiratory system by certain anesthesia agents, and when neuromuscular blocking (NMB) agents are used. Mechanical ventilators are selected based on the size and weight of the animal. Ventilators retired from human use are usually reutilized for laboratory animals. For small animals, such as rats and mice, neonatal ventilators can be used since they can deliver smaller, more precise volumes and pressures.
Despite their life-saving application, mechanical ventilators come with their associated risks and complications. Mechanical ventilators have the potential to cause lung injury referred to as ventilator-induced lung injury (VILI), lead to the rapid type of disuse atrophy in respiratory-related muscles, barotrauma, and impairment of mucociliary motility in the airways, among other complications. The interest in the risks and complications arising from ventilator use and the relationship between ventilator settings and certain biological responses has allowed the discovery of four mechanisms involved in ventilator-induced lung injury. VILI can result from the application of a local pressure that force cells and tissues to acquire an unnatural shape (regional overdistension), the repeated recruitment and de-recruitment of unstable lung causing epithelial airspace lining abrasion by interfacial forces (low-volume injury), large alveolar surface area oscillations associated with surfactant aggregate conversion (inactivation of surfactant) and interdependence mechanisms that raise cell and tissue stress between neighboring structures with differing mechanical properties.
A retrospective cohort study of 97 United States’ intensive care units (ICU) from 2005 to 2007 led to the observation that at any given hour the mean percentage of ICU patients receiving mechanical ventilation was 39.5% (± 15.2%). Further, the study also found that at least an average of 29.0% (± 15.9%) of ICU patients relied on a ventilator (Wunsch et al., 2013). Assisted ventilation also plays an important role in the survival of preterm newborns and infants born with respiratory diseases. In comparison to adults, the sensitivity of lung tissues and the small volume of the lungs of neonates and infants serve additional challenges and complications associated with mechanical ventilation. Additionally, mechanical ventilation in this group raises their risk of developing long-term clinical complications that include chronic lung disease, pulmonary hypertension, prolonged supplemental oxygen constraint, nutritional issues, and developmental delay. Given their life-saving application, constant research and development of mechanical ventilators are needed to understand their associated risks and complications.
Animal models provide a practical means to investigate the risks and complications of assisted ventilation using mechanical ventilators. Among the animal models of lung injury and airway diseases, the rodent models have seen the greatest popularity. This popularity is mostly due to their accessibility and their well-described respiratory functions. In comparison to larger animals, the rodents offer lower husbandry costs, faster regeneration, and shorter breeding cycles. The availability of transgenic animals also makes them an enticing option for research of lung diseases. Further, mechanical ventilators are often a part of many animal-based types of research that require surgery.