The flow rate of saline through a 16-G intravenous cannula was most rapid with the pressure infusor, followed by the piston pump, and gravity-fed infusion. The approximate mean flow rate (in ml min− 1) in the pressure-infusor group and piston-pump group was 130 and 100, respectively. Use of the pressure infusor doubled the flow compared with that using gravity-fed infusion. However, a commercially available rapid-infusion system, such as the Level 1® H-1000 (Smiths Medical, Dublin, OH), can infuse more than 400 ml min− 1 of fluids rapidly when used in conjunction with a 16-G intravenous cannula [10]. Anesthesiologists should change to this system if it can be prepared.
Smart et al. [1] and Stoneham [9] reported that the flow rate of saline through a16-G intravenous cannula was 240 ml min− 1 and 340 ml min− 1 using a pressure infusor at 300 mmHg, respectively, which are not in accordance with our results. They used an infusion circuit that was similar to the one that we employed, but they did not use a substitute vessel. The substitute-vessel pressure was remarkably lower than the circuit pressure because it was downstream of the resistance of a thin intravenous cannula. We considered that the combination of lower substitute-vessel pressure and the resistance of a substitute vessel could have affected considerably the flow rate. In the Stoneham’s study, the flow rate by gravity from a height of 100 cm (which was the same height used in our study) was 190 ml min− 1, compared with 70 ml min− 1 in our study. A difference in the flow rate by gravity may support our opinion. We believe that the result in the present study is close to the flow rate seen in the clinical setting.
Regarding the piston-pump method, previous investigators have demonstrated that a modification of the standard method by placing a one-way valve both upstream and downstream of the three-way tap can increase a flow rate of fluids because of no necessity for turning the three-way tap during syringing of fluid [1, 2]. O’Callaghan et al. [2] have clarified that the modified method allows administration of 500 ml saline an average of one minute more quickly, compared with the standard method. As for rapid infusion of fluids, this method may be a useful alternative if the pressure infusor is not available.
Among the piston-pump groups, the infusion time at the third attempt of infusion was significantly longer than that at the first attempt. A possible reason for the variation in infusion time is the fatigue elicited by infusing 500 mL of saline three times successively because this task was hard work. Also, increased friction due to repeat pumping of the piston may cause the variation because of use of the same syringe when infusing 500 mL of saline thrice. Further studies are needed. The effort devoted to pumping by the anesthesiologist when unexpected massive bleeding, which is a life-threatening emergency situation, occurs is another cause for concern in this method. Conversely, in the pressure-infusor method, the variation in the infusion time was not observed when infusing 1500 mL of saline, which was an obvious advantage of the pressure-infusor method.
The circuit pressure in the pressure-infusor group was significantly lower than that in the piston-pump group. The maximum pressure was ~ 150 mmHg in the pressure-infusor group, which was approximately identical to the systolic arterial pressure. The pressure of the substitute-vessel, which was a polyvinyl-chloride extension tube, was as low as 5 mmHg in the pressure-infusor group. Venous pressure will be lower than it because the vein wall is much more flexible than the polyvinyl-chloride substitute-vessel. In the piston-pump group, the maximum circuit pressure was too high to measure in this study. Smart et al. [1] have demonstrated that the piston-pump method generates more than 600 mmHg when attempting rapid push of the piston, which can cause barotrauma. In contrast, the substitute-vessel pressure was less than 20 mmHg. Thus, neither the piston-pump method nor the pressure-infusor method will cause barotrauma. However, there have been some case reports of extravasation and compartment syndrome resulting from pressurized infusion [3–6]. Moreover, even if the cannula is placed appropriately in a vein and the proximal run-off from the vein is occluded, the venous pressure can increase markedly [11]. Thus, intravenous sites should be checked closely if using both the piston-pump and pressure-infusor methods.
A unique problem in the piston-pump method was excessive negative pressure (< -200 mmHg) when withdrawing the syringe plunger before refilling. Use of the pressure infusor can avoid this problem. Negative pressure can hemolyze red blood cells. Studies have shown that hemolysis is not caused by negative pressure alone [1, 7]. However, Pohlmann et al. [7] demonstrated a combination of negative pressure and an air-blood interface to be associated with hemolysis. Thus, anesthesiologists should be careful not to mix air in blood when using the piston-pump method.
Although anesthesiologists undertook repeat pumping of the piston in a 50-ml syringe more than 10 times to infuse 499 ml of saline, bacterial contamination of the infused saline was not observed in this study. Huey et al. [12] showed that bacteria were not detected in the drainage after five reciprocations by grasping the protruding part of a disposable-syringe plunger with dry hands that were not disinfected; their data are consistent with our results. Conversely, Blogg et al. [8] demonstrated bacterial contamination of syringe contents after repeated refilling. Bacterial contamination of a syringe may not always result in bacterial contamination of fluids. In addition, we used a Terumo 50-ml syringe but the risk of bacterial contamination may differ if other types of syringes are employed.