In this study, we present a rodent model to demonstrate age-dependent differences in ventilation after infection with A/Wuhan/359/95. As expected, and reported for animal models as well as humans, our studies show that cotton rat airway tidal volume increases with age, reflecting changes in lung structure, primarily increased diameter of airways and alveoli. For the same reasons, there was a decrease in airway resistance with age. While tidal volume and body weight increased with age, these parameters did not increase at the same rate, resulting in a decrease in TV:weight ratio with age. There was a similar decrease in lung elastance. This may reflect age-dependent physiologic changes such as airway and diaphragm smooth muscle stiffening and developmental changes in the chest wall [4
] that result in limited airway dilation in adult animals compared to infants [17
]. While this may seem a disadvantage, the benefit is immense as the subsequent work of breathing is reduced due to efficient lung recoil against the stiffened chest wall.
To evaluate the impact of age-dependent changes in the airway on respiratory disease, we compared ventilatory parameters in young and adult cotton rats after influenza infection. Tachypnea and lower airway obstruction were evident in both infant and adult cotton rats early after infection and resolved soon after the virus was cleared from the lower respiratory tract, suggesting a direct effect of viral replication on ventilation that correlated with significant epithelial cell damage and the presence of airway exudates. These alterations in ventilation are consistent with clinical changes seen in influenza-infected human infants, in whom tachypnea is a characteristic finding that can be directly observed. These results also validate respiratory rate as an objective measure of respiratory tract disease especially in influenza infection.
Our study confirms the relative ease and non-invasive use of WBP in evaluating ventilation in cotton rats. Influenza clearly induced changes in ventilation as shown by the parameters that were measured. While tachypnea was induced by influenza in both adult and infant animals, there was a decrease in TV in adults but not in infant animals. Consequently, the MV in infected adult animals was unchanged whereas significan MV increases were measured in infected infant animals. Since the total volume of air exchanged per minute is the product of TV and the number of breaths per minute (f
), this difference in MV between the two age groups suggests that unlike adults, infant cotton rats have the ability to accommodate the same volume of air in their lungs during infection in spite of the influenza-induced obstruction. This difference is likely due to greater lung elastance in very young animals. In addition, the immature central respiratory drive or neuromuscular transmission of infants [20
] may contribute to the differences in MV recorded after influenza infection in infant and adult cotton rats. An example of the contribution of sensorineural stimulation that results in an altered breathing pattern is that of respiratory syncytial virus (RSV)-induced apnea in weanling rats [21
]. This observation is age-dependent and correlates with number of vagal C fibers [22
]. These observations model RSV associated apnea in young infants. Although apnea can be observed after influenza infection [23
], it is not a common clinical sign of disease. The equivalent increase in respiratory rate following influenza infection in infant and adult cotton rats suggests that CO2
sensors and synaptic interactions that control respiratory rate are similar in these age groups. It is therefore logical to propose that structural/mechanical attributes of the infant lung allow a constant tidal volume in influenza-infected infant cotton rats even though there is extensive epithelial cell damage and airway obstruction. Our results show an age-dependent decrease in dynamic lung elastance (ability of lungs to distend); this correlates with the inability of adult influenza-infected cotton rats to maintain equivalent tidal volume in the presence of airway obstruction.
Lung elastance is inversely related to ventilatory efficiency as the alveoli not only expand more easily but also are susceptible to collapse, particularly during forced expiration [19
]. The elastance of infant lungs is thought to contribute to sudden infant death syndrome [24
]. Therefore, even though it may seem advantageous for infant lungs to accommodate a larger volume when there is airway obstruction, this has risks. We predict that this may also contribute to disease severity in influenza-infected infants. While the stiffness of the chest wall in older children and adults may prevent an increase in TV, this benefits them in that expiration can occur passively as a consequence of lung recoil against the chest wall, limiting the energy required for air exchange and reducing muscle fatigue [25
]. We therefore hypothesize that tachypneic infants are likely to experience respiratory muscle fatigue, with severe consequences unless they receive mechanical ventilation.
Both infant and adult cotton rats had increased Penh
values following infection. However, in addition to the contribution of an increase in pause (resulting from decreased RT), the disproportionate increase in PEF (resulting in an increased PEF/PIF ratio) also contributed to the increase in Penh
in influenza-infected infant, but not adult, cotton rats. In adults, the volume of air retained at the end of expiration is determined by the outward recoil of the chest wall and the lung's inward recoil, whereas young infants use diaphragm and laryngeal adductor activities to control expiratory flow [27
]. Once the chest wall stiffens and can resist the inward lung recoil force, expiration is maintained passively [4
]. Based on these mechanisms, it is likely that the control of expiration in infant cotton rats is different from adult animals and therefore the volume of air, presence of obstruction, as well as decreased compliance, contribute to the increase in PEF observed.
The most obvious and clinically relevant change in ventilation following influenza infection in cotton rats is the tachypnea seen in both age groups. Tachypnea as well as increased Penh
coincided with epithelial cell damage. Epithelial cell damage is the consequence of virus-induced apoptosis and necrosis and is therefore proportional to the inoculating dose [10
]. When infant and adult animals were infected with the same virus dose per 100 g, tachypnea was evident for a longer period of time in adults than infants. This may be the result of the four times greater amount of virus that adults received. On the other hand, when both age groups were infected with an identical dose of virus (5 × 106
/animal), the adult animals cleared the virus faster than the infant animals. The tachypnea as well as epithelial damage in the adults was less severe and resolved by day 3 p.i. showing that the quantity of the viral load is proportional to the amount of epithelial damage and the increase in respiratory rate.
In contrast to epithelial cell damage, interstitial pneumonitis, peribronchiolitis, perivasculitis and alveolitis (not shown) persisted after tachypnea had resolved. Our findings of influenza-induced tachypnea therefore suggest that epithelial cell damage rather than inflammation plays a crucial role in inducing pulmonary ventilation deterioration. In contrast, tachypnea that is induced in mice following infection with a high dose of RSV coincides with the presence of inflammatory cells. This tachypnea is partially resolved in animals that are deficient in IFN-γ [29
], suggesting that the inflammatory response causes the increased respiratory rate following RSV infection.
Age-dependent differences in respiratory parameters support the need for different treatment strategies in influenza-infected infant and adults. Since greater lung elastance measured in infants is inversely related to ventilatory efficiency and could result in alveoli collapse, it may be particularly important to limit influenza infection and reduce the extent and duration of tachypnea in this very young age group.