MODES OF TRACHEAL GAS INSUFFLATION - COMPARISON OF CONTINUOUS AND PHASE-SPECIFIC GAS INJECTION IN NORMAL DOGS

Tracheal gas insufflation (TGI) improves the efficiency of CO2 elimina tion accomplished by conventional mechanical ventilation, primarily by reducing the anatomic (series) dead space volume. Dead space proximal to the catheter tip can be reduced by two methods. Fresh gas introduc ed at the carinal level during inspiration may effectively ''bypass' t he upper airway. Alternatively, proximal dead space can be ''washed ou t'' with fresh gas during expiration to reduce CO2 rebreathing. We exa mined these two modes of TGI-aided dead space reduction in nine paraly zed normal dogs receiving conventional mechanical ventilation and comp ared these results to those obtained with a catheter that delivered fr esh gas continuously at the same flow rate, thereby accomplishing both bypass and washout. Total inspired tidal volume and cycling frequency were held constant. Differences in CO2 elimination efficiency among t he TGI modes were flow dependent. Continuous catheter flow at 5 or 10 L/min reduced Pa(CO2) and physiologic dead space fraction (VD/VT) more than either proximal bypass or end-expiratory washout (p < 0.001). At the same catheter flow settings expiratory washout tended to improve VD/VT more than did inspiratory bypass. Under the conditions tested, c onstant tracheal insufflation of fresh gas improves alveolar ventilati on by mechanisms that include, but are not limited to, a functional re duction in the dead space proximal to the catheter tip.