Background
Automated intermittent or patient-controlled analgesia bolus doses are an increasingly popular method to deliver local anesthetic via continuous peripheral nerve catheters (CPNCs). Many experts believe that bolus injection improves perineural fluid spread compared to slower conventional continuous infusion. Perineural injection pressure generated from a bolus (typically administered at 10 mL/min) is approximately 8 to 10 psi (or 400 to 500 mmHg) via a short needle. Additional pressure is expected to be introduced into the system when using a long, small-bore CPNC. Such high overall pressure can potentially exceed the delivery pressure limit and activate occlusion alarms of modern ambulatory infusion pumps. This can result in unwanted cessation of medication administration. We performed an in vitro study of the relationship between pressure and flow rate assessed using a range of catheter lengths.
Methods
For this in vitro study, IRB review was not applicable. Five 19G (1.1mm) x 60 cm catheters (FlexBlock; Teleflex, Research Triangle Park, NC, USA) were used for testing. Injections with normal saline were administered via catheter with an automated syringe pump (Asena; Alaris Medical Systems, Basingstoke, UK) with built-in pressure monitoring. An injection rate starting at 1 mL/min was used for each catheter, and increased in 1mL/min increments until generated pressure reached the maximum 1000 mmHg pump pressure limit (or up to the maximum 20 mL/min pump delivery rate). The length of each catheter was shortened by 10 cm and retested through the range of injection rates. This was repeated until pressures for five samples were measured at each flow rate for each length (20, 30, 40, 50, 60 cm). All tests were conducted at room temperature.
For analysis purposes, an occlusion pressure limit of 900mmHg (common among ambulatory infusion pumps e.g. Curlin Medical 4000 Series Pumps, Huntington Beach, CA, USA) and perineural bolus injection pressure of 400 mmHg were used as reference pressures.
Results
Average pressures computed from five samples of each catheter length are plotted against flow rate in Figure 1. Table 1 summarizes the average pressures at a flow rate of 10 mL/min (typical bolus injection rate) for each catheter length.
Conclusions
To our knowledge, this is the first in vitro study to evaluate the relationship between injection rate and pressure generated from CPNCs. Subtracting the 400 mmHg perineural bolus injection pressure results in a theoretical allowable pressure of 500 mmHg from catheter contribution alone before alarm activation (if the pump’s occlusion pressure limit is 900 mmHg). The implications of our results suggest that commercially available 19G x 60 cm CPNCs generate high pressures that can potentially exceed occlusion alarm limits when attempting to bolus at 10 mL/min or greater. Clinicians should be familiar with occlusion pressure limits of their infusion pumps and management techniques to address alarm triggers. Options include programming infusions at slower rates for longer catheters or possibly shortening catheter length to improve flow characteristics.
