Introduction
The measurement of lung function and determination of lower and
upper airway resistances in animal species was first performed in the equine
species. Horses are obligate nasal breathers and their respiratory system is
the performance limiting system (even in healthy conditions!). Indeed, the
equine athlete naturally develops exercise-induced hypoxemia, which seems to be
related to an insufficient oxygen transfer throughout the capillary-alveolar
barrier as well as to inappropriate ventilation due to strongly increased
airflow resistance (more than 100 % increase) [1]. The investigation methods developed for equines have
been extended to other animal species where the main reasons for measuring
upper and lower airway resistance are investigation of pathophysiology and
therapy of respiratory diseases. As certain animal peculiarities might model
respiratory disorders in man, there is some interest for animals’ upper
airway in human medicine.
Measurement techniques of upper airway resistance in animals:
conventional pulmonary function tests and impulse oscillometry
The first measurements of upper and lower airway resistance
(respectively UAR and LAR) were performed by use of so called conventional
pulmonary function tests where transpulmonary, tracheal and naso-pharyngeal
pressures and airflow need to be recorded [2]. Impulse
oscillometry has also been validated for several species in respiratory
veterinary research and appears as a more sensitive and less invasive
technique: a spectrum of frequencies is taken into consideration and
calculation models of upper and lower (or central and peripheral) airway
resistance are available [3].
Upper airway resistance in animal species
As a consequence of anatomical peculiarities of their upper airways,
horses show the highest UAR:LAR ratio and rest and at exercise ([Table 1]), associated with a considerable work of
breathing [2].
Table 1 Ratio between upper
airway resistance (UAR) and lower airway resistance (LAR) in animal
species.
| Animal species
| UAR : LAR
ratio (%) in normal conditions (mature animals)
| Ratio change in particular
conditions
| References
|
| Horse
| 80 : 20
| Exercise
82 : 18
|
[1]
[2]
|
| Cattle
| 70 : 30
| Young animals
60 : 40
|
[7]
[8]
|
| Sheep
| 70 : 30
| –
| Reinhold; unpublished
|
| Swine
| 60 : 40
| Young animals
55 : 45
|
[9]
|
The functional impact of several affections of UA, i. e.
laryngeal paresis or paralysis, soft dorsal palate displacement, guttural pouch
mycosis or empyema etc. can be quantified by use of an impulse oscillometry
system (IOS) [4] or measurement of pressure changes
[5]. Although the IOS technique offers several
advantages in terms of sensitivity and non-invasiveness, it can not be used in
exercising horses.
Cattle, and especially hypermuscled breeds such as Belgian White
Blue, slowly adapt their respiratory function after birth. Their anatomical and
physiological peculiarities of the respiratory system, i. e. a
relatively small respiratory tract, an important development of pleural septa,
the absence of collateral ventilation, and an important capacity of hypoxic
vasoconstriction, explain why this species is prone to respiratory affections
of the upper and lower airways [6]. Comparative
assessment of respiratory function between growing Friesian and BWB calves has
further provided evidence that the UAR:LAR ratio decreases with age due to
decreased LAR and that both, UAR and LAR are higher in BWB ([Table 1]) [7]
[8].
Similar growth-related changes are described in pigs; the relative
impact of lower airway resistance remains however higher than in other species
([Table 1]) [9].
Although cats have been used during early respiratory research for
investigating nervous control of respiration, upper airways have been poorly
investigated from a clinical point of view. In dogs, especially brachycephalic
dogs presenting considerable nasal airflow resistance, resistive properties of
upper airways are under ongoing investigation [10].
