The Science Journal of the American Association for Respiratory Care

2008 OPEN FORUM Abstracts

CO2 REBREATHING DURING SIMULATED NON INVASIVE VENTILATION: COMPARISON OF DEVICE, TIDAL VOLUME SIZE, RESPIRATORY RATE, AND EXHALATION VALVE TYPE

Mark Siobal1, Julin Tang1, Hannah Ong2, Wesley Toy2



Background: CO2 rebreathing (RCO2) during NIV is influenced by the mechanical dead space added by the mask and the exhalation valve type. Use of a single limb circuit that lacks a true exhalation valve may cause RCO2 because a portion of the exhaled volume moves down the circuit limb during exhalation. If flow through the exhalation port and expiratory time are less than required to flush exhaled CO2 from the circuit, RCO2 occurs. RCO2 is also affected by RR, and Vt size. We tested the effects of RR, Vt, and exhalation valve type on RCO2 using the Pulmonectic Systems LTV 1000 (LT), Drager Savina (DS), Drager XL (DX), and Respironics Vision using two exhalation port types, Plateau Exhalation Valve (VV) and Exhalation Port Adapter (VP).

Methods:
A Michigan Instruments Test Lung with the two chambers locked together was powered on one side (muscle chamber) by a Drager XL ventilator. 100% CO2 was bled into the other side of the test lung (lung chamber) to simulate VCO2. End tidal CO2 (ETCO2) at the lung chamber was measured by the CO2 sensor from the XL ventilator. The muscle chamber was powered at Vt of 400 and 600 mL, RR of 20, 25, 30, and 35, I:E ratio of 1:1, and flow rate of 100 L/min. CO2 bleed-in to the lung chamber was adjusted until an ETCO2 of 35-40 mm Hg was achieved at each setting. Following stabilization at each setting the LT, DS, DX, VV, VP set to deliver a pressure support PIP of 15 and PEEP of 5 was attached to the lung chamber with a 24 inch section of aerosol tubing placed between the circuit and lung chamber to simulate the approximate mechanical dead space volume of a face mask (100mL). The maximum change in ETCO2 from baseline at the lung chamber at each setting combination using each device was recorded.

Results:
The mean % increase and range of ETCO2 changes at all settings was lowest for the LT (16 ± 2%, 14% to 21%), similar for the DS (23 ± 6%, 14 to 28%), DX (23 ± 5%, 14 to 30%), and VV (21 ± 12%, 11 to 46%), and highest for the VP (53 ± 32%, 11 to 108%). ETCO2 with the VV were lower or comparable to the DS and DX at RR of 25 to 35 and Vt of 400 to 600. RCO2 and ETCO2 worsened with the VP at RR > 25 and with both the VV and VP at Vt of 600.

Conclusion:
RCO2 and an increase in ETCO2 occurred with all devices tested. Bias flow through the LT reduces RCO2. RCO2 with the VV and VP can be lower than the DS and DX but is dependent on RR and Vt size. The impact of RCO2 during the application and study of NIV needs to be recognized.