Removal of deadspace volume from arterial catheter: How muchis enough?

OBJECTIVE: To evaluate the amount of volume needed to be removed from arterial catheter systems to compensate for "deadspace" and to allow an accurate measurement of pH and hemoglobin (Hb). DESIGN: Twenty patients undergoing heart surgery were evaluated in a steady state after the induction of anesthesia before surgery. Six blood samples were removed from the arterial catheter, the total volume of which was 1.5 mL at 30-sec intervals and measured for pH and Hb. The first sample was then taken after removing 1.5 mL from the tubing. In subsequent samples, the volume removed before sampling increased by 0.5-mL intervals. All other samples were compared with sample number 6, in which 4 mL of volume were removed before measurements. RESULTS: The first three samples with volumes of 1.5, 2.0, and 2.5 mL before measurement were inaccurate compared with sample number 6 (p <.000), giving artificially low values for both pH and Hb. There was no significant difference between the values measured in sample numbers 4, 5, and 6 (3.0, 3.5, and 4.0 mL, respectively). CONCLUSION: The amount of volume needed to be removed before measurement from an arterial catheter system, the volume of which is 1.5 mL, is 3 mL to achieve accurate measurements of pH and Hb. Removal of less volume results in an artificially low measurement.