HYPERCAPNIA ENHANCES THE DEVELOPMENT OF COUGHING DURING CONTINUOUS-INFUSION OF WATER INTO THE PHARYNX

We investigated the effects of increasing CO2 ventilatory drive on the coordination of respiration and reflex swallowing elicited by continu ous infusion of distilled water into the pharynx (2.5 ml/min) in 11 no rmal subjects. Ventilation was monitored using a pneumotachograph and swallowing was recorded by submental electromyogram. The CO2 ventilato ry drive was increased by addition of external dead space, while venti lation, the frequency of swallows, and the timing of swallows in relat ion to the phases of the respiratory cycle were measured at steady-sta te conditions. We found that the CO2 ventilatory response is not influ enced by continuous reflex swallowing but that hypercapnia influences the timing and frequency of these swallows. Signs of aspiration were n ever observed during continuous infusion of water at eucapnia, but sev en of 11 subjects showed laryngeal irritation and/or pending aspiratio n during hypercapnia, and the incidence of laryngeal irritation was hi gher the greater the PCO2. Detailed analysis of laryngeal irritations consisting of single coughs in seven subjects revealed that the majori ty of laryngeal irritations occurred when swallows coincided with expi ratory-inspiratory transition or when swallows coincided with inspirat ion, whereas laryngeal irritation after an expiratory swallow was neve r observed. These results suggest that the automatic respiratory contr ol system is not influenced by continuous swallowing but that the coor dination of swallowing and respiration may be compromised during hyper capnia.