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The mechanical advantage of negative intra‐thoracic pressure during swallow

In humans and animals, it is established there is a phrenic nerve burst and contraction of muscles which create negative intrathoracic pressure (costal/crural diaphragm and parasternal) during the pharyngeal phase of swallow. We speculated this swallow activity would change with conditions which create the necessity for more rapid pharyngeal clearance, such as increased respiratory drive. Swallowing was induced in anesthetized spontaneously breathing cats by injection of water into the oropharynx, and respiratory drive was altered by inhalation of 5% and 10% CO2. Electromyograms (EMGs) were recorded from pharyngeal, laryngeal, and chest wall muscles, and intra‐esophageal pressure was recorded from the midthoracic esophagus. The magnitude of negative esophageal pressure during swallow increased by 44% during 5% CO2 and 45% during 10% CO2 compared to baseline. Parasternal and post‐swallow cricopharyngeal (upper esophageal sphincter) EMG activity also increased during swallows that occurred under hypercapnic conditions. Effective pharyngeal swallow may require multiple mechanical events including: positive pressure from oral and pharyngeal muscle contraction propelling the bolus, and negative intra‐thoracic pressure drawing the bolus into the esophagus. Supported by NIH HL89104, HL103415, HL89071, HL109025.