Ventilatory Compensation for Continuous Inspiratory Resistive and Elastic Loads during Halothane Anesthesia in Humans

Inspiratory mechanical loads were applied to the airway continuously for 5 min in healthy young adult volunteers maintained in a near steady-state of halothane anesthesia 1.1 MAC. The loads, both flow resistive and elastic in nature, had been selected to reduce the first loaded tidal volume approximately 10, 30 or 50%--these being designated "small," "medium," and "large" loads, respectively. The actual magnitudes of resistive load were 8 +/- 1, 21 +/- 3, and 48 +/- 6 cmH2O X l-1 X s, and of elastic load 6 +/- 1, 18 +/- 1, and 41 +/- 5 cmH2O X l-1 (mean +/- SEM). All loads caused an immediate reduction of ventilation proportional to the size of the load. This was followed by a gradual recovery of ventilation toward control values over approximately 2 min and then nearly stable ventilation for the rest of the loading period. Respiratory frequency was unchanged throughout. At 5 min of loading, ventilation and PaCO2 had been nearly steady for 3 min and O2 uptake and CO2 output at the airway were unchanged from control, suggesting the establishment of a near steady respiratory state. With the small and medium loads of both types, ventilation and PaCO2 in this near steady-state were not detectably different from control. With the large loads, however, ventilation was significantly reduced and PaCO2 slightly increased. The end-expiratory position of the chest wall and the relative contributions of the rib cage and abdomen-diaphragm to ventilation, as estimated by anteroposterior chest wall magnetometers, were not consistently altered by any load.(ABSTRACT TRUNCATED AT 250 WORDS)