Upper airway resistance: species-related differences

Table of Contents

Zusammenfassung

Den Strömungswiderständen der oberen Atemwege wird in der Tiermedizin hauptsächlich beim Sportpferd sowei beim brachyzephalen Hund eine große Bedeutung beigemessen. Aufgrund anatomischer Besonderheiten des Nasen- und Rachenraums und/oder pathologischer Veränderungen können bei beiden Spezies signifikante Veränderungen der Leistungstoleranz (Performance) und/oder des Wohlbefindens auftreten. Bei anderen Tierarten wird den physiologischen Eigenschaften und den pathologischen Veränderungen der unteren Atemwege meist eine größere Bedeutung beigemessen.

Abstract

In veterinary medicine, upper airway resistance deserves a particular attention in equines athletes and brachycephalic dogs. Due to the anatomical peculiarities of the upper airway and/or pathological conditions, significant alterations of performance and/or well being might occur in horses and dogs. Physiological specificities and pathological changes of the lower respiratory tract deserve a major attention in other species.

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].

References

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