BACKGROUND: During arterial line sampling, 2 to 3 times of deadspace volume (from sampling port to catheter tip) are discarded to obtain accurate arterial blood gas variables. In this study, minimal discard volume was determined to decrease unnecessary blood loss.

METHODS: Eighty patients scheduled for elective cardiac surgery were included. Two consecutive studies were conducted, in which the radial arterial line was placed with 20-gauge catheter. Once patients had stable hemodynamics, 5.5 times the deadspace of 1.5 ml was discarded before the sampling to obtain control value. In first study (n = 50), deadspace volume was fixed to 1.5 ml. After that, 1, 1.5, 2 and 2.5 times the deadspace were discarded in random before the sampling. In second study (n = 30), discard volume was fixed to 2 times the deadspace. The deadspace volumes were 1.5, 3.5 and 5.5 ml. Samples were analyzed for pH, PaCO₂, PaO₂, HCO₃^-, Na⁺, K⁺, Ca⁺⁺ and hematocrit.

RESULTS: In first study of fixed deadspace, there was no statistical difference in blood gas variables between discard volume of 2.5 ml and control value. In second study of fixed discard volume, the difference in blood gas variables was statistically significant only between deadspace volume of 1.5 ml and control value.

CONCLUSIONS: The optimal discard volume was 2.5 times the deadspace to obtain accurate blood gas variables while decreasing unnecessary blood loss during arterial line sampling. On the other hand, when deadspace was larger than 3.5 ml, discard volume of 2 times the deadspace was sufficient.