The Science Journal of the American Association for Respiratory Care

2005 OPEN FORUM Abstracts

SLOW DRAWING ARTERIAL LINES AND PACO2

by Robert G. Cronin RRT Maine Medical Center, Portland, Maine

Therapists frequently are faced with obtaining arterial blood gas samples from slow drawing arterial lines. In some case of arterial puncture, therapists have to aspirate the sample due to narrowed arterial lumens. The question arises, "Does the pressure gradient created in the syringe by aspirating blood gas samples affect the PaCO2?"

We took 20 left blood samples that had already been processed. We simulated a slow drawing line with a partially closed three-way stopcock. We attached the sample syringe to the stopcock and partially closed the lumen. A 3 cc aspirating ("line draw") blood gas syringe was placed on the other side of the stopcock. The lumen was closed to the point where blood flowed slowly between the syringes through the stopcock in response to negative pressure generated by pulling back on the aspirating syringe. We limited to the negative pressure to pulling back to the 1 cc mark. This correlated with a measure negative pressure of 12 to 14 cm H2O. We then analyzed both samples and recorded the PCO2. We chose to perform the analysis in this sequence to eliminate the possibility that any noted effect would be attributable to any aspiration between syringes. By measuring the PCO2 after the inter syringe transfer, we isolated the effect of using negative pressure to accomplish the transfer.

Results: Over the range of 49 to 86 torr of PaCO2 the effect of using 12-14 cm H2O of negative pressure was a drop in PaCO2 of 0.6 to 12.2 torr with an average of an 7.4% drop. pH change was not significant (average .01).

Conclusions: Arterial blood gas samples aspirated from slow drawing arterial lines (and perhaps arterial punctures) are biased towards lower PaCO2 as a result to applying negative pressure of 12-14 cm H2O of negative pressure to the aspirating syringe.

Recommendations: Clinicians should avoid using negative pressure to aspirate arterial blood gas samples. In cases where there is no alternative, results should be viewed with the realization that the actual PaCO2 is probably 7.4% higher.

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