1997 OPEN FORUM Abstracts
NEGATIVE EXPIRATORY AIRWAY PRESSURE (NEAP) AND TRACHEAL GAS INSUFFLATION (TGI) IN COMBINATION AUGMENTS CO2 ELIMINATION MORE THAN TGI ALONE.
Takahashi T, Adams A, Bliss P, Logid R, Marini J. University of Minnesota-St. Paul Ramsey Medical Center, St. Paul, MN.
Background: A fresh gas flowing from a catheter positioned in the trachea has been shown to flush CO2 from anatomical and mechanical deadspace during the expiratory phase. The subsequent inspiratory phase then causes less CO2 rebreathing. As reported in clinical studies, TGI can cause ~ 15% reduction in PaCO2. NEAP was used in the 1950's to counter the adverse effects of positive pressure ventilation on the cardiovascular system. NEAP was abandoned due to its potential for airway closure. With currently available microprocessing and solenoid technology, it may be possible to phase NEAP to support the cardiovascular system, reduce mean lung volume or accelerate CO2 elimination without causing airway closure. We developed a microprocessor driven, double-solenoid system to deliver TGI, NEAP and TGI-NEAP combined to evaluate their respective effects on CO2 elimination during selected portions of expiration. Methods: In 5 paralyzed, sedated mongrel dogs we applied the TGI-NEAP system via 2 rigid catheters positioned one centimeter above the carina. Catheter flows were +9 L/min for TGI and -9 L/min for NEAP during phasing combinations of TGI and NEAP. We monitored arterial pressure, PAP, cardiac output, Pao, Ptr, Pes, inductance plethysmography (IP) and ABGs via an indwelling catheter (Paratrend 7). End expiratory lung volume was maintained constant by adjusting PEEP while guided by IP. Results: PaCO2 reduction was greatest in three phasing combinations: TGI-NEAP combined during exhalation (-12%), TGI in the second half of exhalation (-25%) and NEAP first half-TGI second half of exhalation (- 46%). (Figure - PaCO2 vs. Time in 10 min increments) NEAP had a lesser effect on CO2 elimination than TGI. Overall, NEAP was effective in early exhalation while TGI was more effective in late exhalation. Lung volume excursion, as measured by IP, was observed to increase during early NEAP/late TGI. In this setting, no evidence of airway closure was observed. Conclusions: NEAP had a role in CO2 elimination that was phase specific and less than TGI. We speculate that a combination of early NEAP-late TGI is additive due to their different mechanisms of action and that minute ventilation is increased by this combination. Supported by SCOR HL50512.
(See original for figure)