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Effect of catheter size and flow rate on epidural boluses spread in an experimental model
Abstract Number: S-82
Abstract Type: Original Research
Distribution of solutions in the epidural space in human cadavers is non -uniform and directed between structures according to pressures by which they were compressed. Boluses delivered at higher pressure could result in higher spread of solution associated with improved analgesia. Previous in vitro work has suggested that flow rate has little effect on the area of distribution. We hoped to demonstrate a difference in spread using three different flow rates and two different catheter sizes in an experimental epidural model.
Local research and ethics committee approval was waived. A multi-orifice unfiltered Portex© epidural catheter was secured between two perspex sheets and a plastic sheet containing air-filled hemispheres. Five millilitres of dye impregnated saline were delivered at three pre-set flow rates (250, 500 and 1000 mls/hr) via a CME Bodyguard 545© epidural pump through 18G and 20G catheters. A bolus was delivered thirteen separate times with each catheter at each flow rate with the primary outcome measured being the total height of spread of each bolus. We also measured the maximum flow rate achieved by the pump whilst delivering the bolus. The data were analysed using two-sample t-test. A p value of less than 0.05 was considered significant.
The height of spread of the solution with each set of parameters is shown in the table. The results show a reduction in spread through the 18 gauge catheters as the flow rate was increased. The reduction was statistically significant between 250mls/hr and 1000mls/hr (p=0.03). In the 20 gauge catheters, spread was significantly greater in the 500mls/hr group when compared to the two other flow rates (both p=0.03), although the preset flow rate was never actually achieved.
Our model is not a true representation of the epidural space, but is reproducible and allowed us to compare the effect of flow rate and catheter gauge on the spread of a bolus of solution. High flow rates were not achieved secondary to in -built flow rate compensation with pressures higher than 19 psi. The greatest spread seemed to be with lower flow rates and wider catheters. A possible reason for this finding may be differential flow from the three orifices in the multi orifice catheter with lower flow rates.
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