Effects of breathing route, temperature and volume of inspired gas, and airway anesthesia on the response of respiratory output to varying inspiratory flow

The determinants of the response of the respiratory output to inspiratory flow rates (Vover dotI) were examined in awake normal subjects. Subjects were connected to a volume-cycle ventilator in the assist/control mode, and Vover dotI was increased in steps from 30 to 90 L/min and then back to 30 L/min. Vover dotI pattern was square, and all breaths were subject-triggered. In six subjects the effects of breathing route (nasal or mouth) and temperature and volume of inspired gas (Protocol A) and in 8 subjects the effects of airway anesthesia (upper and lower airways; Protocol B) on the response of respiratory output to varying Vover dotI were studied. In Protocol B, in order to calculate muscle pressure during inspiration (Pmus), respiratory system mechanics were measured using the interrupter method at end-inspiration. Independent of conditions studied, breathing frequency increased significantly and end-tidal concentration of CO2 decreased as Vover dotI increased. The response was graded and reversible and not affected by breathing route, temperature and volume of inspired gas, and airway anesthesia. With and without airway anesthesia (Protocol B), neural inspiratory and expiratory time and neural duty cycle, estimated from Pmus waveform, decreased significantly as Vover dotI increased. At all conditions studied, the rate of change in airway pressure prior to triggering the ventilator tended to increase as Vover dotI increased. The changes in timing and drive were nearly complete within the first two breaths after transition, with no evidence of adaptation during a given Vover dotI period. We conclude that Vover dotI exerts an excitatory effect on respiratory output which is independent of breathing route, temperature and volume of inspirate, and airway anesthesia. The response most likely is neural in origin, mediated through receptors not accessible to anesthesia, such as those located in the chest wall or below the airway mucosa.