Laryngeal reflex apnea is blunted during and after hindlimb muscle contraction in sheep

This study was carried out on seven chloralose-anesthetized sheep and was designed to investigate the role of muscular afferent fiber stimulation on the duration of reflex apnea triggered by laryngeal stimulation (LS). In six animals, injection of distilled water onto the laryngeal mucosa provoked a 15.7 ± 1.0 s (mean ± SE) apnea associated with a rise in systemic blood pressure (+7 ± 0.8 Torr). Electrically induced contractions (EIC) of the hindlimb muscles doubled the metabolic rate and ventilation and reduced the duration of the apnea produced by LS to 7.4 ± 1.0 s (P < 0.01). Apnea duration was still reduced during the first minute after the cessation of EIC (7.2 ± 1.1 s, P < 0.01) but returned to control after a 5-min recovery period (16.7 ± 1.6 s). The apnea triggered by LS was also reduced during EIC when the venous return was impeded by occluding the inferior vena cava (5.2 ± 1.1 s, P < 0.01), despite a profound hypocapnia (20.7 ± 0.3 Torr). The duration of apnea was not significantly affected (14.2 ± 1.4 s) by breathing a 6% CO₂-14% O₂ in N₂ gas mixture that roughly mimicked the alveolar gas composition when the apnea turned off. These results suggest that chemical drive has a negligible role in the fast reinitiation of breathing after LS during muscular stimulation. Stimulation of muscle afferent fibers does, however, appear to be a potent source of ventilatory reflexes capable of counteracting the inhibition of breathing resulting from laryngeal stimulation. Conversely, it is postulated that any reduction in somatic afferent traffic during this type of reflex apnea, including that resulting from the LS-induced systemic vasoconstriction, may delay the termination of apnea.