RESPIRATORY PHASE RESETTING AND AIR-FLOW CHANGES INDUCED BY SWALLOWING IN HUMANS

1. Relationships between the timing of respiration and deglutition wer e studied in thirty awake healthy subjects at rest. Deglutition was mo nitored by submental electromyography, pharyngeal manometry and videof luoroscopy. Respiration was recorded by measurement of oronasal airflo w and chest wall movement. Three types of deglutition were studied: in jected bolus swallows, spontaneous swallows, and visually cued swallow s of boluses previously placed in the mouth. 2. The effect of each swa llow on respiratory rhythm was characterized by measurement of cophase , defined as the interval between the onset of deglutitive submental E MG activity to the onset of subsequent rescheduled inspirations. Copha se was determined for swallows initiated at different phases of the re spiratory cycle. In all subjects deglutition caused phase resetting of respiratory rhythm. Cophase was largest for swallows initiated near t he inspiratory-expiratory (I-E) transition and smallest for swallows i nitiated near the expiratory-inspiratory (E-I) transition. The pattern of respiratory resetting by deglutition was topologically classified as type 0. This pattern was shown for swallows induced by bolus inject ion or visual cue, and for spontaneous swallows. 3. The incidence of s pontaneous deglutition was influenced by the position of the swallow i n the respiratory cycle. Few spontaneous swallows were initiated near the E-I transition whereas most occurred from late inspiration to mid- expiration. 4. Deglutition caused an abrupt decrease in airflow leadin g to an interval of apnoea, followed by a period of expiration. The du ration of deglutition apnoea for spontaneous swallows was shorter than that for 5 ml bolus swallows, and was unaffected by the respiratory p hase of swallow initiation. The period of expiration after swallowing was longest for swallows initiated at the I-E transition, and shortest for E-I swallows. 5. The intervals between bolus injection and the on set of deglutition apnoea, and the timing of swallowing events, were n ot significantly altered by the phase in the respiratory cycle at whic h swallowing was exhibited. 6. To quantify the relationship between bo lus flow and respiration, we determined the latencies between cessatio n of inspiratory airflow and arrival of the bolus at the larynx (alpha ), and between laryngeal bolus departure and resumption of inspiratory airflow (delta). Both values were dependent upon the respiratory phas e of swallowing. The lowest values for alpha and delta were found for early-inspiratory and late-expiratory swallows, respectively. 7. We co nclude that swallowing causes respiratory phase resetting with a patte rn that is characteristic of the strong perturbations of an attractor- cycle oscillator. The threshold for initiation of swallowing in awake subjects is influenced by, but not strongly coupled to, the phase of r espiration. We propose that respiratory timing, in addition to anatomi cal barriers within the upper airway, influences the vulnerability for aspiration during deglutition. Swallows initiated near the E-I transi tion may be the most likely to result in bolus aspiration, especially in pathological conditions that weaken the impact of swallowing on res piratory rhythm or slow the transport of the bolus through the pharynx .