Figure: Connections and deadspaces in arterial needle and bloodline tubings.
The method of sampling is shown. The "vacutainer" is attached directly to point "A", the end of the needle tubing, which has a deadspace of about 2.6 ml. To clear this deadspace one must wait for 10 sec prior to sampling after having slowed blood flow to 50-100 ml/min. This method avoids use of a needle and the risk of needlestick injury.
An alternative sampling site is point "B", which requires use of a needle. One must clear the 9-10 ml of deadspace present up to this point prior to sampling. One does this by slowing the flow to 50-100 ml/min for about 15 sec, and then stopping the pump prior to obtaining the sample. Drawing blood from the sampling port (B) engenders the risk of needlestick injury, so drawing from point "A" is preferred.