The influence of CO2 on spatiotemporal features of mechanically induced cough in anesthetized cats

Cough results in a dramatic increase in ventilation in part due to large increases in tidal volume. This increase in ventilation can cause severe hyperventilation during repetitive cough bouts. The purpose of this study was to quantify how reversal of hyperventilation during repetitive coughing affects the cough motor pattern. Prior studies have shown that increasing CO2 well above eupneic levels can suppress cough in anesthetized humans (Nishino 1989) but the effects of hypocapnia on the behavior are less well understood. Cough was elicited in anesthetized, spontaneously breathing adult cats (n=9) via a flexible cannula inserted into the trachea for 60 seconds. Animals were subjected to both a control condition, where cough‐induced hyperventilation was not perturbed (ETCO2 12±1, arterial CO2 17±1 mm Hg), and an “isocapnic” condition, where inspired CO2 was increased so that ETCO2 during cough approximated eupneic levels (ETCO2 32±1, arterial CO2 28±1 mm Hg). There was no change in cough number between control (9±1) and isocapnic (11±1) conditions. Further, the magnitudes of diaphragm, internal oblique and parasternal EMGs were unchanged under isocapnic relative to control conditions. There also was no significant change in intrathoracic pressures between the two conditions (control 53±12 cm H2O vs isocapnic 47±12 cm H2O). However, there was a decrease in the intensity of the maximal cough effort during the addition of CO2 (control 71±13 cm H2O vs isocapnic 62±13 cm H2O, p<0.05). There was a trend for inspiratory time during cough to be slightly reduced with added CO2 (p=0.07). The E1 phase of cough (control 0.5±0.03 s vs isocapnic 0.5±0.02 s) and E2 phase of cough (control 2±0.5 s vs isocapnic 2±0.3 s) were unchanged under isocapnic conditions. There also was a significantly shorter decrease in the time to first breath following the cessation of coughing in the isocapnic trials (control 16±4 s vs isocapnic 5±1 s, p<0.02). The results indicate that CO2 at or below the eupneic range does not strongly alter the motor pattern of coughing in anesthetized cats. Cough‐induced hyperventilation appears to be solely a consequence of the acceleration of pulmonary gas to accomplish ejection of foreign materials from the airways, rather than an important regulatory feature that modulates the behavior.