Continuous spinal analgesia (CSA) is a relatively long established technique that has potential advantages over both single-shot spinal anaesthesia and continuous epidural analgesia particularly in elderly or high-risk patients [1,2]. In CSA, a catheter is placed in the subarachnoid space which is then either fixed to the skin or subcutaneously tunnelled with an external outlet for supply of drugs. Infusion devices are used for epidural or intrathecal administration of drugs . In Japan, portable and disposable infusion kits are increasingly being employed for epidurals and CSA. The flow rates of such pumps are determined according to the Hagen-Poissuille law:
where r, is the radius of catheter; P, the infusion pressure; η, the viscosity and L is the catheter length.
Microcatheters have a limited flow rate and this may limit distribution of the local anaesthetic into the subarachnoid space. We have often experienced inaccurate flow rates in CSA when using a disposable infusion pump because the microcatheter needed a high infusion pressure. It is reported that the flow rate of balloon-type infusion kits is reduced to 71% when used in conjunction with a microcatheter (30-G) compared to an epidural catheter (17-G) . There are two types of disposable infusion pump; balloon types employing contraction pressure made by injecting fluid into the balloon, and syringe types employing negative pressure made by injecting fluid into the pump. In this study, we investigate the influence of the CSA catheter length on the flow rate of the two different devices.
Materials and methods
We evaluated flow rates with a 28-G, 91 cm microcatheter composed of clear nylon (MicroCatheter System for CSA, Portex, USA). We used a Surefuser-A (Nipro, Osaka, Japan) as the balloon-type infusion pump and a Coopdech Syrinjector (Daiken Medical, Osaka, Japan) as the syringe-type infusion pump. The volume used in the infusion pumps was 50 mL, and the flow rate setting was 1 mL h−1. We established four groups according to catheter length and type of pump:
- Group A: balloon-type infuser, 91 cm, n = 10.
- Group B: balloon-type infuser, 20 cm, n = 10.
- Group C: syringe-type infuser, 91 cm, n = 10.
- Group D: syringe-type infuser, 20 cm, n = 10.
Infusion pumps were not reused. The 20 cm catheters used in the study were made by cutting standard 91-cm 28-G microcatheters to size. Saline solutions were injected into the infusion pumps, which were connected with the microcatheters. The infusion device was neither above nor below the exit point of the catheter. The flow rate was calculated from the weight loss of the equipment over the observation period during which the infusion pumps were continuously weighed on an electronic scale (Precision balance PF300E, capacity 300 g, readability 0.001 g, Sinko Denshi Co., Ltd., Tokyo, Japan). The values were recorded on a computer system connected to the electronic scale. The temperature of the room was maintained at approximately 25°C.
Between-group comparisons of each system were conducted using one-way analysis of variance (ANOVA) (one between factor), followed by Fisher's least significant difference test for comparisons. The effects of length and time on the flow rate were analysed with general linear regression model procedures for two-way ANOVA with repeated measures (one between factor, one within factor), followed by Fisher's least significant difference test. Statistical significance was set at the P < 0.05 level. All values are reported as mean (SEM).
Figure 1 shows the time courses of the flow rates under the different conditions. Flow rates in all groups tended to gradually decrease over time. Flow rates in the syringe-type decreased rapidly at around 50 h from the start, while those in the balloon-type decreased rapidly at around 70 h from the start. The remaining reservoir volumes, when the flow rate dramatically decreased, were 4.0 ± 1.1 mL in Group A, 3.5 ± 1.1 mL in Group B, 5.0 ± 1.4 mL in Group C, 5.7 ± 1.0 mL in Group D; these differences were not significant. Mean flow rates were 66.4% of the expected value in Group A, 81.0% in Group B, 77.0% in Group C, and 88.6% in Group D. These differences were significant (P < 0.05). The flow rate in Group A was significantly less than that in Group B until 60 h from the start and that in Group C was significantly less than that in Group D until 45 h from the start (P < 0.05). The flow rate in Group C was significantly less than that in the Group A until 50 h from the start and that in Group B was significantly less than that in Group D until 45 h from the start (P < 0.05).
Our study suggested that the flow rates of disposable infusion kits with shortened microcatheters were more accurate than with an unaltered full-length microcatheter because the shortened microcatheter had less resistance than the unaltered one. When we use CSA with a 28- or 32-G microcatheter in a clinical setting, the shortened microcatheter is effective in reducing the high resistance caused by the fine diameter of the microcatheter. Flow rates are determined by Hagen-Poissuille law, and in out tests we compared flow rates under the same conditions except for catheter length. From the Hagen-Poissuille law, the expected flow rates of the microcatheter that were cut short to 20 cm should be 4.5 times the flow rate for an unaltered microcatheter (91 cm). However, in our study, the flow rates of the 20 cm microcatheter were approximately 1.2 times that of the 91 cm microcatheter in both devices. The reason for this difference could be that the fine diameter of the microcatheter was a rate-limiting factor, which in practice has the greatest effect on the flow rates.
Disadvantages of continuous spinal anaesthesia include increased incidence of headache with large-diameter catheters and the potential for infection or nerve trauma [5,6]. The Food and Drug Administration in 1992 banned the used of spinal catheters smaller than 24-G due to a 20% incidence of technical complications resulting in 15% failed block and 3.4% incidence of broken catheters and 12 cases of cauda equina syndrome .
As the viscosity of the saline solution we used was nearly equal to that of local anaesthetics and narcotics, we examined the flow rate with saline solution rather than local anaesthetics and narcotics. The setting of flow rates was also based on data with saline solution in both devices.
The expected time for completion of an infusion is 50 h from the start (volume: 50 mL, the setting of flow rate: 1 mL h−1). In this study, the completion times for the infusion ranged from 70 h to 90 h with the flow rates decreasing rapidly at around 50 h in the syringe-type and 70 h in the balloon-type. This indicated that in clinical use, syringe-type disposable infusion kits should be changed at 50 h and at 70 h for the balloon-type, even if there are drugs remaining. Intrathecal opioids can lead to fatal complications if devices are not appropriately used.
The infusion pressure of the balloon-type (approximately 450 mmHg) is greater than that of the syringe-type (approximately 300 mmHg). The flow rates of the balloon-type were significantly less than those of the syringe-type, however, until 50 h from the start with the long microcatheter and 45 h with the shortened microcatheter. As we maintained the temperature of the room at approximately 25°C, the change of the saline solution's viscosity from the lower room temperature compared to the pump at 32°C could also have had some effect on the reduction of flow rates.
In conclusion, the flow rate is more accurate when shorter catheters are used compared to the standard length catheters. Attention should also be paid to instability of the flow rate with the clinical use of CSA. The flow rate of the infusion does not correlate with the rate set towards the end of the volume infused.
We thank Dr Takashi Matsukawa, Department of Anaesthesia, University of Yamanashi, for his helpful suggestions in the writing of this manuscript.
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