Mechanisms and clinicophysiological implications of the sniff- and gasp-like aspiration reflex.

Mechanical stimulation of the pharyngeal mucosa in cats and some other mammals evokes the 'aspiration reflex' (AR) characterized by rapid and strong inspiratory efforts not followed by active expirations. It resembles other spasmodic inspiratory acts such as the sniff, the gasp, and the sigh in several aspects, e.g. reflex or semireflex triggering from the upper airways, the sudden onset and termination of such inspirations, the massive recruitment, steep rise and high-peak amplitude of inspiratory unit activity, analogous ventilatory pattern, and contribution to arousal. The similarity of these spasmodic acts is manifested mainly in enhanced speed and volume of inhalation, although of different intensity, which is determined by the varying degree of forced inspiratory activity and a concomitant inhibition of expiratory activity. The extent of the inspiratory dilation of the glottis and the timing and range of late-inspiratory and/or postinspiratory glottal narrowing modulate the depth of aspiration. Thus, the inhalation can be moderate as in sniffing, which provides a transfer of odorants to the olfactory mucosa. In AR the airstream is presumably strong enough to tear off the mechanical particles from the naso- and oropharynx and to convey them into the hypopharynx to allow their subsequent elimination by reflex swallowing or coughing. Prolonged glottal opening allows either the transfer of some additional air to the bronchi by sighing to prevent the development of atelectasis, or redistribution of a larger amount of fresh air into the lungs by gasping to support autoresuscitation. Should aspiration be a common effective component in these spasmodic processes, then the easily elicitable AR could be beneficial as a simple model for studying their properties in health and disease.