Comparison of Sampler Filling Times Among Selected Arterial Blood Samplers

Discussion

Our study further supports previous studies conducted by Johnson et al,¹ Bender et al,² and Cortes et al,³ who concluded that arterial sampler filling time can distinguish between arterial and venous punctures. Johnson et al¹ used an extracorporeal laboratory model and tested various needle sizes, whereas Bender et al² studied subjects with normal mean arterial pressures, and Cortes et al³ studied subjects with abnormal blood pressures. These studies used only a single brand of sampler; our study extends their work to all currently available samplers with and without plungers in the United States. Because Johnson et al,¹ Bender et al,² and Cortes et al³ already showed that there is a significant difference between arterial and venous filling times with the Portex Pro-Vent, which is a sampler with a plunger, and because this sampler was included in our study, we chose to compare all samplers with a plunger at only mean arterial pressure. Although we found significant differences in sampler filling rates at a simulated mean arterial pressure between sampler brands, the difference of ∼1 s should not prevent distinguishing between arterial and venous punctures because the minimum difference between arterial and venous filling rates in all 3 previous studies was reported by Johnson et al¹ as 51.4 s. Currently, clinical practice guidelines suggest that blood color and pulsation may be used to identify arterial samples.⁵ Based on all 4 studies, arterial sampler filling rate may be a more objective indicator of obtaining an arterial sample and should be considered when guidelines are updated.

We used an extracorporeal circuit similar to that used by Johnson et al¹, but our filling rate for the Portex Pro-Vent sampler with a 23-gauge needle at a normal mean arterial pressure was 6.1 ± 0.5 s, almost 10 s less compared with the rate obtained by Johnson et al.¹ The reason for this difference may be that Johnson et al¹ collected 2-mL volumes and used more viscous reconstituted human blood, whereas we collected 1 mL of a less viscous synthetic blood. Also contributing to the difference may be our lower blood flow. Johnson et al¹ used a normal cardiac output of 5.0 L/min, whereas ours was 1.16 L/min. We reduced the flow to better mimic peripheral arterial flow.⁶

We used the 23-gauge needle for the samplers with a plunger and a 26-gauge needle for the Comfort-Sampler without a plunger. The ComfortSampler was originally available as the Roche Microsampler (Roche Diagnostics, Indianapolis, Indiana), and it was described and evaluated by d'Ortho et al⁴ in 2001. Because the volume of the sampler was 240 μL, it took significantly less time to fill to capacity. The difference in filling times would have more likely been longer if blood volumes were equal. Because smaller needles may provide enhanced patient safety and comfort, sampler filling times when using vented samplers and needles smaller than 23 gauge should be studied further.

Vented arterial samplers were initially described in 1986 by Hutchison et al,⁷ who compared blood gas values between plastic vented Monovette samplers (Sarstedt, Newton, North Carolina) and glass syringes and found a significant increase in P_O2 in the samplers. This difference was attributed to the blood being exposed to air during sampling and not removed before analysis. Hutchison et al⁷ iced all blood samples and did not report the pre-analytical time period; these variables have also been attributed to erroneously high P_O2.^8–10 Sampler filling time is also the time arterial blood is exposed to atmospheric air. If filling time is excessive, time exposed to air might have an effect on P_O2 values.⁴ The impact of this time on current vented samplers has not been reported and deserves further study.

In addition, we tested one sampler without a plunger at both arterial and venous pressures to determine whether the plunger had an effect on filling time. We found that there was a statistically significant difference in the filling times between arterial and venous punctures with the sampler without a plunger. Therefore, distinguishing between arterial and venous punctures does not depend on the brand or design of the sampler used.

The limitations of this study include the use of an extracorporeal circuit in place of human subjects. Although we simulated mean arterial pressure in our circuit, it is not the same as natural pulsatile arterial blood flow because it is a constant pressure. Results in humans or with a pulsatile pump may differ. We were also limited by our use of normal saline-based synthetic blood solution instead of human blood. Further research could include the direct comparison of filling times for venous and arterial pressures among all sampler brands and the effect of smaller gauge needles. P_O2 could also be measured to determine whether the different venting systems and filling times of the samplers used affect the results of blood gas analysis.