Influence of intrathoracic vagotomy on the cough reflex in the anesthetized cat

Abstract

Recurrent laryngeal afferent fibers are primarily responsible for cough in response to mechanical or chemical stimulation of the upper trachea and larynx in the guinea pig. Lower airway slowly adapting receptors have been proposed to have a permissive effect on the cough reflex. We hypothesized that vagotomy below the recurrent laryngeal nerve branch would depress mechanically or chemically induced cough. In anesthetized, bilaterally thoracotomized, artificially ventilated cats, thoracic vagotomy nearly eliminated cough induced by mechanical stimulation of the intrathoracic airway, significantly depressed mechanically stimulated laryngeal cough, and eliminated capsaicin-induced cough. These results support an important role of lower airway sensory feedback in the production of tracheobronchial and laryngeal cough in the cat. Further, at least some of this feedback is due to excitation from pulmonary volume-sensitive sensory receptors.

Keywords: Cough; Pulmonary stretch receptor; Recurrent laryngeal afferent.

Fig. 1.. Vagotomy decreases cough excitability.

Raw and integrated EMG data from diaphragm (Dia) and abdominal (Abd) muscle during (A) tracheobronchial (TB), (B) laryngeal (LAR) and (C) capsaicin-induced cough from control (post-thoracotomy/pre-vagotomy) and intrathoracic vagotomized conditions are shown from the same animal. EMG scales are the same between control and vagotomy conditions and between (A), (B) and (C). * indicates cough; ^s indicates swallow.

Fig. 2.. Tracheobronchial (TB) and capsaicin-induced induced cough were abolished post-vagotomy.

While not abolished, laryngeal-induced cough (LAR) was attenuated in the vagotomy condition. Changes in cough frequency of (A) TB cough and changes in cough number due to (B) laryngeal stimulation or (C) capsaicin due to vagotomy distal to the recurrent laryngeal nerve are compared to control (post-thoracotomy/pre-vagotomy). α = 0.05, power = 1.0. TB – control: n = 20, vagotomy: n = 11, unpaired t-test, * p < 0.001; LAR control and vagotomy: n = 5, paired t-test, * p = 0.002; Capsaicin control and vagotomy: n = 6, paired t-test, * p < 0.001.

Fig. 3.. Thoracotomy did not alter diaphragm magnitude during TB-induced cough, but abdominal muscle magnitude was significantly decreased relative to control.

* p < 0.01, α = 0.05, power = 0.915, paired t-test against normalized control, n = 15.

Fig. 4.. Cough frequency decreased with decreasing positive end-expiratory pressure (PEEP).

After thoracotomy, PEEP was used to maintain lung inflation. Cough frequency was significantly reduced at 0 cmH₂O PEEP relative to 5 and 2 cmH₂O (* p < 0.05, α = 0.05, power = 0.999, repeated-measures ANOVA, Holm-Sidak post hoc, n = 9). There was a nonsignificant trend for cough frequency to be reduced between the 5 and 2 cmH₂O conditions († p = 0.06).