Abstract 225

Aim: To study dilution effects of the draw-up volume on measured arterial sodium concentration.

In vitro study: The tip of a UAC, infused with 0.45% saline, was placed in 1 litre of 0.9% saline. The dead space was 0.6 ml. Draw-up volumes from 0.1 to 2 ml were discarded. Fluid was then drawn for analysis of the sodium concentration.

In vivo study: A 22 g catheter was inserted into an adult volunteer's vein and infused with 0.45% saline. The dead space was 0.6 ml. Draw-up volumes were 0.6, 0.9, 1.3, 1.6 or 10 ml. 0.5 ml of blood was then analysed for the sodium concentration.

Results: In vitro study: A draw-up volume of 1.3 ml was needed before there was no difference from the control. In vivo study: A draw-up volume of 1.6 ml was needed before there was no significant difference from the control.

Conclusion: A least 1.6 ml draw-up volume should be removed (dead space 0.6 ml) before analysing blood for sodium concentration.