The Science Journal of the American Association for Respiratory Care

2008 OPEN FORUM Abstracts

USE OF SMALLER TIDAL VOLUMES IN A PORCINE CARDIOVASCULAR RESEARCH MODEL MAY BE UNPHYSIOLOGICAL

George A. Steer1,2, David S. Broadway1, Tsai-ying Wang1, George C. Kramer2



Background: Lower tidal volumes during mechanical ventilation, 6-8 ml/kg range vs. 10-12 ml/kg, have been recommended as more physiological in the human population. In the patient with acute lung injury this is considered a protective ventilation strategy and at times elevated arterial carbon dioxide levels have been tolerated in order to minimize ventilator induced lung injury. However, lower levels of tidal ventilation may not be appropriate for animal models used during research protocols due to interspecies anatomic and physiological differences. VD/VT and static compliance (CS) measurements were obtained to determine optimal tidal ventilation in a porcine model.

Methods:
Propofol anesthetized swine (20-31 kg) were intubated and ventilated with a Siemens Servo 300 at 15 ml/kg at a rate of 8-12/min to maintain EtCO2 at 35 mmHg with 5 cm H2O PEEP. The tidal volume was then manipulated through values of 7.5, 10, 12.5, 15, and 17.5 ml/kg and respiratory rate was adjusted to maintain EtCO2 between 35-40 mmHg. Mixed exhaled CO2 (PeCO2) was measured from a mixing chamber. Arterial blood samples were drawn for gas analysis. Dynamic and static pulmonary compliance values were simultaneously recorded for each tidal volume.

Results:
The table shows mean (�1std) VD/VT and CS values for the range of tidal volumes tested. 15 ml/kg optimized compliance and dead space values. 17.5 ml/kg had the lowest VD/VT but resulted in a worsening compliance and abnormally low respiratory rates. Respiratory rates of 40-44 and 6-8/min were required to maintain EtCO2 at 35-40 mmHg for 7.5 and 17.5 ml/kg tidal ventilation respectively.

Conclusion:
Tidal ventilation with 12.5 to 15 ml/Kg required more normal respiratory rates to maintain an EtCO2 of 35 - 40 mmHg and had lower VD/VT values with better compliance versus 7.5, 10 and 17.5 ml/kg.