In the present study, we described variations in respiratory parameters measured in clinically healthy rabbits during the adaptation process to an enclosed environment, and, secondarily, provided normal values for barometric plethysmography in this animal species.
Animal models are of paramount importance for a comprehensive study of human diseases. Compared with other animal species, the rabbit has been scarcely employed as a model of human respiratory diseases. Certainly, its use has some limitations, such as the higher cost and lesser availability of reagents, but rabbits have proved to be suitable for the study of certain disorders. For example, the rabbit asthma model shares some important pathophysiological mechanisms with the human disease, such as the IgE production after sensitization and the development of the early- and late-phase responses to an antigenic challenge, with the advantage that these animals are large enough as to better study the pulmonary mechanics . Unfortunately, respiratory parameters of rabbits have been scantily investigated.
Traditionally, it has been claimed that respiratory frequency of rabbits varies around 30–80 breaths/min [15–17]. In sharp contrast with this concept, in our study we found that adult New Zealand rabbits had a much higher frequency during plethysmography, reaching approximately 340–370 breaths/min, and this is clearly illustrated in Fig. 1C. Some authors have also reported relatively higher breath rates than the previously assumed. For example, Schroedder et al.  and Finzi  described and average of ~ 182 and ~ 185 breaths/min in adult New Zealand rabbits, while Richter et al.  reported an average respiratory frequency of 198 breaths/min in newborn rabbits (hybrids of New Zealand and Dendermore). By contrast, Maskrey and Nicol  used barometric plethysmography (like us) and their results clearly showed a high basal breath frequency (235 ± 46 breaths/min, mean ± SEM) in the first 30 min of recording, which increased up to 297 ± 48 breaths/min in the second 30-min period. These breath frequency and time-trend fully agree with our results and were made in control rabbits at 35 °C ambient temperature. Moreover, in order to verify this high respiratory rate, we introduced a rabbit into a home-made plexiglass chamber that contained the body excluding the head with a latex seal at the neck. This hermetic chamber allowed us to record changes in the inbox pressure caused exclusively by thoracic respiratory movements, and we confirmed that rabbits reached indeed such high breath frequency (Fig. 3).
The high level of respiratory frequency observed in spontaneously breathing rabbits was comparable to respiratory frequencies reached in humans during the ventilator mode known as high frequency oscillation . In this sense, the rabbit might be considered as a natural model of this type of mechanical ventilation. In fact, Piva et al.  found that in rabbits anesthetized and deprived from surfactant by repeated bronchoalveolar lavage, the PaO2 and oxygenation index could be restored to normal levels when animals were submitted to high frequency ventilatory oscillation (300 to 900 breaths/min). Thus, it seems that these high rates of breathing are within physiological ranges for rabbits.
Perhaps the most used parameter in barometric plethysmography is Penh, a unitless index that has been employed as a surrogate of lung resistance and airway obstruction [24, 25]. We found that Penh in rabbits had a progressive decrease in the first 60 min of recording. This is contrary to Penh changes occurring in guinea pigs during barometric plethysmography . Thus, in guinea pigs, Penh values were relatively low immediately after the animal entered the chamber, but these were followed by a progressive increase in the next 90 min. The initially low Penh values in guinea pigs were probably due to the stress generated by the new environment and the ensuing release of catecholamines and nitric oxide, as they were avoided by administration of propranolol or L-NAME . Contrasting with this adrenergic response of guinea pigs to stress, it has been demonstrated that in rabbits a stressful maneuver is accompanied by parasympathetic activation causing, for example, transient bradycardia and hypotension that can be abolished by a muscarinic antagonist [4, 5]. Thus, guinea pigs and rabbits seem to trigger different physiological responses to stressful situations.
Figure 4 describes the possible sequence of events occurring in the rabbit during the first minutes of confinement inside the plethysmographic chamber. The stress generated by the new environment probably triggers an increment in the parasympathetic tone, causing an end-inspiratory glottis closure through activation of laryngeal nerves and a tracheobronchial narrowing through activation of vagus nerves. Both phenomena would lead to prolongation of TB and Ti, and to an increment in Penh, respectively. Combination of these alterations, in turn, would be reflected by a decrease in respiratory frequency. On the other hand, the stressful situation would enhance the metabolic demands and thus increase MV. It is reasonable to suppose that all these respiratory parameters will return to more physiological (normal) values once the rabbit is adapted to the confinement inside the plethysmographic chamber.
In addition to the “short-term” adaptation described above, we had also hypothesized that rabbits will display a “long-term” adaptation during the course of the 5 days of barometric plethysmography, i.e., that at day 5 they would be less stressed than on day 1. However, we did not identify consistent differences in the time trend pattern among the 5 days of recording. Thus, it is possible that 5 days were not sufficient for the rabbit to become familiarized with the plethysmographic chamber.
A potential limitation of our study is that validity of barometric plethysmography, and especially Penh, has been questioned by some authors [26, 27]. However, this last parameter has proved to be correlated with acute changes in lung resistance during a cholinergic challenge in some animal species such as guinea pigs and mice [24, 25]. Thus, barometric plethysmography seems to be an appropriate tool to measure acute modifications of pulmonary mechanics, such as those expected to occur during confinement of rabbits inside the plethysmograph box.