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HMC Respiratory Care

The Effects of Mechanical Deadspace on PaCO2 in Patients Receiving Lung Protective Ventilation

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C Hinkson RRT, M Benson BS RRT, L Stephens RRT, S Deem MD Department of Respiratory Care, Anesthesiology, and Medicine, Harborview Medical Center, University of Washington 

Background: Lung Protective Ventilation (LPV) using tidal volumes (Vt) of 4-6 cc’s/kg/ (predicted body weight) has been shown to reduce mortality in patients with acute respiratory distress syndrome (ARDS) and acute lung injury (ALI). Standardized use of the lower tidal volume has raised new concerns about the appropriate configuration of patient ventilator circuits, especially with regards to mechanical deadspace. 

Hypothesis:  We hypothesized that the removal of all mechanical deadspace from a ventilator circuit on a patient receiving LPV would result in a reduction in PaCO2 and allow a reduction in minute ventilation (VE).  

Methods:  We prospectively tested 3 different ventilator circuit configurations: Standard hygroscopic moisture exchange filter (HME) with 6” flex tubing (1), 6” flex tubing only (2), no HME or flex tubing, wye adapter to inline suction catheter elbow (3). Patient selection: All patients met the American-European conference criteria for ARDS/ALI, were receiving LPV strategy, > 18 years of age, and hemo-dynamically stable.  The study was approved by the Investigational Review Board (IRB) and informed consent was obtained prior to any data collection. The order of the configuration was randomized and patients were not undergoing any procedures and were reasonably calm during study.  After 15 minutes with each circuit configuration an ABG was drawn from an indwelling catheter and ETCO2, FeCO2, VCO2, RR was measured using Novametrix CO2SMO Plus! Monitor, Model 8100 (Novametrix Medical Systems, Inc. Wallingford, CT). Physiologic dead space (Vd/Vt) was calculated using the Bohr equation.  Vt, respiratory rate, PEEP, and FIO2 were constant throughout study. 

Results: Seven patients were studied (n = 7). Removal of the HME from the circuit configuration resulted in a fall in Vd/Vt of approximately 6% (p = 0.004), and PaCO2 fell by approximately 5 Torr (p = 0.01) (Figure). Removal of the HME and flex tubing from the circuit configuration resulted in an additional 5% reduction in Vd/Vt (p = 0.002) and a 6 Torr reduction in PaCO2 ( p = 0.009) (Figure).  Across both interventions, VE fell from a mean of 11.51 L/min to 10.35 L/min (p = .008), and pH increased from 7.30 to 7.38 (p = 0.002). VCO2 did not change significantly.

Conclusion: In patients who are receiving smaller tidal volumes (4-6 cc’s/kg/PBW) removing all the mechanical deadspace results in a reduction in PaCO2 and an increase in pH at a lower VE. This information will help guide ventilator circuit configuration for patients receiving LPV.

 

graph of PaCO2 and Vd/Vt in relation to HME