Changes Induced by Severe Hypoxia in Respiratory Defence Reflexes in Anaesthetized Cats

Abstract

The authors studied the effects of progressive isobaric hypoxia (FO₂ = 0.11, 0.07, 0.06, 0.05, 0.04 and 0.03) on respiratory defence reflexes elicited by mechanical stimulation of the airways in 10 pentobarbital-anaesthetized cats. The intensity of the expiration reflex was already significantly reduced at a hypoxia level of only FO₂ = 0.11; the intensity of cough also showed a tendency to decrease. The number of efforts and the intensity of both tracheobronchial (TB) and laryngopharyngeal (LPh) cough fell significantly in hypoxia of FO₂ = 0.07. On an average, cough could no longer be elicited at PaO₂ = 2.0 ± 0.06 kPa. The aspiration reflex was found to be the most resistant to hypoxia, and could still be elicited just before hypoxic apnoea developed. During hypoxia with FO₂ ranging from 0.21 to 0.06, minute ventilation increased, while the intensity of respiratory defence reflexes diminished. TB cough could not be elicited during hypoxia until the breathing was depressed. It can be seen from the results that the regulatory mechanisms responsible for the intensity of respiratory defence reflexes are less resistant to the consequences of an inadequate systemic oxygen supply than the mechanisms controlling ventilation.

Brainstem mechanisms underlying the cough reflex and its regulation

“…Some specific neural mechanisms involved in the generation or modulation of the cough reflex Modulation or, in some instances, even generation of the cough reflex depend on the cerebellar nucleus interpositus (Xu et al, 1997), nasal mucosa trigeminal afferents (Plevkova et al, 2009;Poussel et al, 2012), esophageal vagal afferents (Hennel et al, 2015), afferents from the external acoustic meatus (auricular branch of the vagus nerve, Arnold's or Alderman's nerve) that mediates the Arnold's ear-cough reflex (e.g. Todisco 1982;Canning et al, 2014;Murray et al, 2016), pharyngeal afferents (for review see Canning et al, 2014), chemoreceptor (Tatar et al, 1986(Tatar et al, , 1987Nishino et al, 1989a;Hanacek et al, 2006) and baroreceptor (Poliacek et al, 2011) afferents. Pharyngeal afferents can activate not only the gag reflex (Bassi et al, 2004), but also coughing or the "urge-to-cough"; the receptors implicated are probably innervated by vagal and glossopharyngeal nerves or by trigeminal branches .…”

Neurogenesis of cough, other airway defensive behaviors and breathing: A holarchical system?

“…These two types of cough differ in their sensitivity to antitussive agents (Korpas and Tomori, 1979) as well as the suppressive effects of poikilocapnic hypoxia (Tatar et al, 1986). We therefore have developed the hypothesis that separate holons control laryngeal and tracheobronchial cough.…”

“…The effects of chemoreceptive feedback also are different for cough and breathing. Mild to moderate poikilocapnic hypoxia inhibits tracheobronchial cough (Tatar et al, 1986) and moderate to severe hypercapnia has a slight suppressive effect on this behavior (Nishino et al, 1989). These stimuli elicit well-known increases in ventilation.…”

Capsaicin exposure elicits complex airway defensive motor patterns in normal humans in a concentration-dependent manner

“…They are affected differently by the same stimuli suggesting they may follow different motor path-ways and/or be regulated differently. For example, hypercapnia [3,27] and anaesthesia [27] depress ER's more than CR's, whereas slow-wave sleep inhibits CR's but not ER's [32,33]. Furthermore, they likely serve different functions; CR clears the airway while ER prevents entry of material into the airway [11].…”

“…For instance, antitussives such as codeine reduce cough strength (as determined by rectus abdominis EMG signal amplitude) as well as cough number [2]. Hypoxic hypercapnia, a hallmark of several respiratory diseases, reduces cough strength (as determined by intrapleural pressure) but not cough number [3]. Cough in patients with motor neural disease, such as Parkinson's disease, are reduced in strength but higher in frequency [4,5].…”