PNEUMATIC TRANSPORT EXACERBATES INTERFERENCE OF ROOM AIR CONTAMINATION IN BLOOD-GAS SAMPLES

Objective.--To characterize and control the potential interference to PO2 determinations when blood contaminated with air is sent via a pneu matic tube system (PTS). Design.--Both tonometered blood at PO(2)s of 65, 75, 142, and 339 mm Hg and arterial blood gas samples from patient s with baseline PO(2)s from 70 to 400 mm Hg were analyzed for PO2 to d etermine possible effects of air contamination from PTS transport. Set ting.--A large teaching hospital in which a variety of personnel routi nely send samples to the laboratory by PTS transport. Patients.--Twent y patients under anesthesia for elective surgery and 21 patients in an intensive care unit who had a wide range of PO(2)s. Several additiona l patients with a preexisting lung pathology likely to cause hypoxemia were selected to provide samples with low PO(2)s. Main Outcome Measur es.-Measurement of bias in PO2 between samples sent via PTS and sample s walked to the laboratory. Results.-Interference from air contaminati on was worse after PTS transport compared with manual transport of the specimen. Over a wide range, the PO2 in specimens after PTS transport tended toward 160 mm Hg. Samples from hypoxemic patients were prone t o errors in PO2 that could have resulted in clinical misinterpretation ; 5 of 10 samples with a baseline PO2 less than 85 mm Hg had increases of 10 mm Hg or more when contaminated with air. Cooling samples with high PO(2)s minimized changes to PO2, probably by increasing the solub ility of oxygen. Mechanical buffering by various liners used in the ca rriers did little to alleviate the interference. Decreasing the speed of pneumatic transport by 50% lessened the effect on PO2. Conclusion.- Interference can be minimized by carefully purging samples of all air bubbles using the following protocol: invert syringe to check for air bubbles, then retap and reexpel bubbles if necessary. Personnel that c ollect and send blood gas samples via PTS should be educated about the problem of interference. Modifications both to pneumatic sample trans port systems and to blood gas syringes should be investigated to minim ize the effect.