1995 OPEN FORUM Abstracts
Tracheal Gas Insufflation in a Mechanically Ventilated Canine Model with Raised Intracranial Pressure
Thomas Malinowski, Thomas O'Callahan, Clifford Douglas, Charles Kean, Mark Olfert, Departments of Respiratory Care, Surgery, and Animal Research, Loma Linda University Medical Center, Loma Linda, CA.
Introduction Continuous Tracheal Gas Insufflation (TGI) of fresh gas into the central airways has been proposed as an adjunctive CO_2 clearance technique which helps minimize airway pressures during mechanical ventilation. One of the mechanisms by which TGI is reported to work is by washing out central airways deadspace. We hypothesize that TGI would be beneficial in reducing arterial CO_2 in a raised intracranial pressure (ICP) canine model, a population in which it is undesirable to elevate ventilatory pressures or allow permissive hypercapnea, and that arterial-to-end -tidal carbon dioxide comparisons would substantiate the reduction in deadspace. Methods Six (6) dogs (10.5-12 kg) were sedated with non-barbiturate anesthesia and managed with central and arterial lines. Endotracheal intubation and volume ventilation (tidal volume 10-15 ml/kg, FIO_2=.3, PEEP=0) with a Servo 900B ventilator maintained normocarbia. A 7 Fr. pediatric feeding tube served as the TGI catheter, and was placed in the endotracheal tube via bronchoscopic adapter. TGI flow during insufflation was 5 lpm. The TGI catheter tip was located approximately 1 cm above the carina. Mean airway pressure (MAP) and end-tidal CO_2 (PetCO_2) values were monitored via a catheter tip distal to the TGI catheter. Catheter and airway pressure line placement was verified via bronchoscopy. An intracranial pressure bolt monitored ICP during the trial. A saline-filled balloon catheter placed in the epidural space simulated the cranial space occupying lesion. Results Statistical analysis by t test identified significant differences in ICP, PaCO_2, and cerebral perfusion pressure (CPP) (p < .01) between injury, TGI nadir, and post injury. There was no significant difference in peak (Ppk) or MAP at the three levels (p < .01). The arterial-to-end-tidal carbon dioxide gradient showed a statistically significant increase during TGI when compared to injury pre TGI(p < .05).
PaCO_2 PetCO_2MAP Ppk ICPCPP
mmHg mmHg cm H_2O cm H_2O mmHg mmHg
Injury 43±3 40±4.54±1.3 14±3 41±12.6 33±10.5
nadir27±1.4 20±4 4±1.6 14±2.427±9.553±7.6
Post 38±4.4 31±8 4±1.3 14±3 37±13 43±7
Conclusions 1) Preliminary experience with continuous TGI at 5 lpm has demonstrated it to be effective in reducing ICP in the mechanically ventilated head injury model by augmenting CO_2 clearance, as evidenced by the reduction in arterial CO_2. Furthermore, the improvement in ventilation was accomplished without an increase in peak or mean airway pressures. TGI may be particularly beneficial in treating patients with raised ICP, a group whom may not tolerate permissive hypercapnic ventilatory strategies. 2) The arterial - end tidal CO_2 gradient increased during TGI. This increase would be consistent with a traditional interpretation of increased deadspace, but is most likely attributable to increased CO_2 clearance from fresh, TGI gas.