Auto-positive end-expiratory pressure during tracheal gas insufflation: testing a hypothetical model.
OBJECTIVE
The major benefit of tracheal gas insufflation (TGI) is an increase in CO2 elimination efficiency by removal of CO2 from the anatomical deadspace. In conjunction with mechanical ventilation, TGI may also alter variables that affect CO2 elimination, such as minute ventilation and peak airway pressure (peak Paw) and cause the development of auto-positive end-expiratory pressure (auto-PEEP). We tested the hypothesis that TGI-induced auto-PEEP alters ventilatory variables. We predicted that TGI-induced auto-PEEP offsets the beneficial effects of TGI on CO2 elimination and that keeping total PEEP (ventilator PEEP + auto-PEEP) constant enhances the CO2 elimination efficiency afforded by TGI.


METHODS
Prospective study of two series of patients with acute respiratory distress syndrome receiving mechanical ventilation.


METHODS
Intensive care units at a university medical center.


METHODS
Each series consisted of eight sequential hypercapnic patients.


METHODS
In series 1, we examined the effect of continuous TGI at 0 and 10 L/min on PaCO2, without compensating for the development of auto-PEEP. In series 2, we examined this same effect of continuous TGI while reducing ventilator PEEP to keep total PEEP constant. TGI-induced auto-PEEP was calculated based on dynamic compliance measurements during zero TGI flow conditions (deltaV/deltaP) after averaging the two baseline values for peak Paw and tidal volume and assuming compliance did not change between the zero TGI and TGI flow conditions (deltaVTGI/deltaPTGI).


RESULTS
In series 1, total PEEP increased from 13.2 +/- 3.2 cm H2O to 17.8 +/- 3.5 cm H2O without compensation for auto-PEEP (p = .01). PaCO2 decreased (p = .03) from 56.2 +/- 10.6 mm Hg (zero TGI) to 52.9 +/- 9.3 mm Hg (TGI at 10 L/min), a 6% decrement. In series 2, total PEEP was unchanged (p = NS). PaCO2 decreased (p = .03) from 59.5 +/- 10.4 mm Hg (zero TGI) to 52.2 +/- 8.3 mm Hg (TGI at 10 L/min), a 12% decrement. There was no significant change in PaO2; there were no untoward hemodynamic effects in either series.


CONCLUSIONS
These data are consistent with the hypothesis that mechanical ventilation + TGI causes an increase in auto-PEEP that can blunt CO2 elimination. In addition to the ventilator modifications necessary to keep ventilatory variables constant when TGI is used, it is also necessary to reduce ventilator PEEP to keep total PEEP constant and further enhance CO2 elimination efficiency.
Miro
A M
AM
Department of Anesthesiology and Critical Care Medicine, School of Medicine, University of Pittsburgh, PA, USA.
Hoffman
L A
LA
Tasota
F J
FJ
Delgado
E
E
Lutz
J
J
Zullo
T G
TG
Pinsky
M R
MR
D000368:Aged; D000369:Aged, 80 and over; D000403:Airway Resistance; D002245:Carbon Dioxide / Q000378:metabolism; D015984:Causality; D003131:Combined Modality Therapy; D005260:Female; D006801:Humans; D007327:Insufflation / Q000009:adverse effects* / Q000379:methods*; D008170:Lung Compliance; D008297:Male; D008875:Middle Aged; D008954:Models, Biological; D018467:Positive-Pressure Respiration, Intrinsic / Q000209:etiology*; D011237:Predictive Value of Tests; D011446:Prospective Studies; D012123:Pulmonary Ventilation; D012121:Respiration, Artificial / Q000009:adverse effects* / Q000379:methods*; D012128:Respiratory Distress Syndrome / Q000209:etiology / Q000378:metabolism / Q000503:physiopathology / Q000628:therapy*; D012720:Severity of Illness Index; D013990:Tidal Volume
D002245:Carbon Dioxide