Epidural Multiorifice Catheters Function as Single-Orifice Catheters: An In Vitro Study

Abstract

In an in vitro study, we determined the flow rates required to use selective orifices of multiorifice catheters. Saline was infused at rates between 1 and 360 mL/h through Portex and Braun 20-gauge multiorifice catheters using Baxter, Abbott Laboratories, and Alaris infusion pumps. The numbers of orifices used via infusion and manual injection, and the pressure within the catheter during continuous infusion, were recorded. Infusion rates <80 mL/h used one orifice, between 100 and 280 mL/h used two orifices, and >300 mL/h used three orifices. Catheter pressures with Braun catheters were 40% greater than with Portex catheters. Manual injections by all 12 residents used all three orifices. Twenty-gauge multiorifice catheters function as single-orifice catheters at clinically relevant infusion rates, but function as multiorifice catheters during manual boluses.

In Brief

IMPLICATIONS: We determined the flow rates required to use all three orifices of multiorifice catheters. Continuous infusion rates up to 80 mL/h used a single orifice; rates between 100 and 280 mL/h used two orifices; rates in excess of 300 mL/h used all three orifices. We conclude that 20-gauge multiorifice catheters function as single-orifice catheters at clinically relevant epidural infusion rates.

Previous investigators observed that intermittent “bolus” injections of local anesthetics provide superior analgesia^1,2 and a significantly reduced dose of local anesthetic^3,4 when compared with continuous infusions. Although parturients are often comfortable immediately after the initial bolus of local anesthetic through an epidural catheter, a substantive number require manually administered boluses of local anesthetic to re-establish analgesia despite continuous infusions of local anesthetics. Possible reasons for the improved analgesia during manual injections are that manual boluses spread local anesthetic more uniformly along nerve sheaths,⁵ that not all orifices are completely within the epidural space, that the orifices are remote from the target dermatome level, and/or continuous infusions use only one orifice.^6,7 Published studies have used 16- and 18-gauge epidural multiorifice catheters and the CADD-Prism volumetric pump, none of which are commonly used in clinical practice. Therefore, we investigated which orifices in 20-gauge multiorifice catheters were used during continuous infusions and manual fluid boluses.

METHODS

Four 20-gauge nylon closed-tip multiorifice catheters from two manufacturers [Portex (Portex, Inc., Keene, NH, n = 2) and Braun Perifix (B. Braun Medical Inc., Bethlehem, PA, n = 2)] were studied. Both multiorifice catheters were designed with three orifices oriented in a helicoidal distribution along the long axis of the catheter near the closed tip. All catheters were unfiltered and secured horizontally during the in vitro experiments.

In Part 1 of the study, 0.9% saline was infused through the four epidural catheters using three different commercial pumps: Medley Pump Module 8100 (Alaris^TM Medical Systems, Inc., San Diego, CA, n = 3), Auto Syringe® Model AS50 (Baxter Healthcare Corporation, Deerfield, IL, n = 3) and Abbott APM II (Abbott Laboratories, Abbott Park, IL, n = 3). The pumps were all approved by the hospital for clinical use. Each experiment was repeated separately in triplicate with each of the orifices oriented superiorly. The infusion rates ranged from 1 mL/h to the maximum flow rate for the pump when combined with a 20-gauge epidural catheter: 300 mL/h for the Alaris pump, 360 mL/h for the Baxter pump and 25 mL/h for the Abbott APM II.

After each catheter was primed with saline and the patency of all three orifices confirmed, saline was infused at the designated rate, chosen in a random order, for 5 min to reach steady-state before each experimental condition. One investigator, who was blinded to the study conditions, recorded the orifice(s) that were used. All experiments were performed at 22°C.

In Part 2 of the study, the pressure in the epidural catheter was measured under identical conditions as in Part 1. Infusion rates were randomly assigned after reaching steady-state conditions for 5 min. Pressures were measured using pressure transducers (Edwards Lifesciences, Irvine, CA #PX272) and were displayed on an Agilent monitor (Boeblingen, Germany #M1046B). All conditions were repeated in triplicate. The maximum pressure measurable was 375 mm Hg.

In Part 3 of the study, 12 anesthesiology residents were recruited to manually infuse 4 mL of 0.9% saline through unfiltered multiorifice catheters from both manufacturers as they would during a labor epidural “top-up.” The rate of manual bolus injection was the ratio of the 4 mL volume infused and the time taken to inject the saline (hours), the latter measured using a stopwatch.

Statistical Analysis

The catheter pressure versus infusion rates were fitted using a nonlinear bestfit curve (Prism® 4, Version 4.0a for MAC, GraphPad Software, Inc., San Diego, CA) and R² values reported. The rates of manual injection of saline are summarized as means ± sd.

RESULTS

In Part 1, the number and order of the orifices used with both multiorifice catheters and the Alaris and Baxter infusion pumps were similar at flow rates up to 360 mL/h (Table 1). The Abbott APM II pump used one orifice at its maximum flow rate, 25 mL/h. The number of orifices that were used depended primarily on the infusion rate and secondarily on the orientation of the orifices (Table 1). At rates of ≤80 mL/h, only one orifice was used under all conditions. With the proximal orifice oriented superiorly, the distal orifice was used at rates up to 30 mL/h and the proximal orifice at rates between 30 and ≤100 mL/h. In contrast, with the middle or distal orifices oriented superiorly, the proximal orifice was used at rates between 100 and ≤180 mL/h. With the proximal orifice oriented superiorly, both the proximal and distal orifices were used at rates between 100 and ≤280 mL/h. With the middle or distal orifices oriented superiorly, both the proximal and middle orifices were used at rates between 180 and ≤280 mL/h. Three orifices were used under all conditions at infusion rates ≥280 mL/h for both catheters.

Table 1: Effect of Flow Rate on the Recruitment of Orifices with a Multiorifice Catheter

In Part 2, the maximum pressure recorded using the Braun catheter was 344–365 mm Hg at 180 mL/h and using the Portex catheter was 324–327 mm Hg at 250 mL/h. At 100 mL/h, the pressure with the Braun catheter was 200 mm Hg and with the Portex, 145 mm Hg. The best fit curves for flow rate versus pressure were second-order polynomials (r² > 0.99).

In Part 3, manual boluses delivered 0.3 ± 0.1 mL/s (1080 ± 360 mL/h) and used all three orifices.

DISCUSSION

We observed flow through a single orifice at infusion rates <80 mL/h and through three orifices at rates ≥280 mL/h with both the Portex and Braun 20-gauge multiorifice catheters. The latter was consistent with published data.⁸ In most experiments, flow was demonstrated only through the proximal orifice. Clinically, the proximal orifice may not always lie within the epidural space and this may explain, in part, instances of incomplete analgesia with multiorifice catheters. In contrast, the single-orifice catheter has one orifice at the distal tip of the catheter that is less likely to lie outside the epidural space during maintenance infusions. Depending on the infusion velocity, patient-controlled epidural analgesia may improve the performance of multiorifice epidural catheters compared with continuous infusions alone.

This study may be limited methodologically. We conducted this in vitro study at atmospheric conditions. However, the lumbar epidural pressure in the parturient varies with time, increasing during uterine contractions and Valsalva maneuvers. It is unlikely that the dynamic pressure changes within the epidural space can cause a differential flow through multiorifice epidural catheters, since these pressures apply uniformly along the entire surface of the catheter. However, if one of the orifices was physically occluded by tissue, then perfusion might be shifted to another orifice. Additionally, we did not calibrate each pump and catheter combination at all flow rates. Since none of the pumps alarmed at large flow rates (indicating the flow was not delivered), we interpreted this to mean that the designated flow rates were delivered. Finally, these studies were all conducted at room temperature whereas in vivo, the catheters and the fluid infused are close to body temperature given the prolonged transit time within the catheter at typical epidural infusion rates. Potential effects of temperature on this system are unknown.

The pressure generated with the Braun catheter was consistently 40% more than with the Portex catheter. The reduced pressure with the Portex catheter suggests that this catheter may be advantageous over other catheters in using more orifices at clinical infusion rates.

If multiorifice catheters and pumps are unable to use all three orifices at clinically relevant maintenance infusion rates, then some of the theoretical advantages of multiorifice catheters for continuous epidural infusions may not be recognized.