To the Editor: 

In Charous et al. 's comparison of continuous versus  intermittent bolus techniques for continuous femoral nerve block,1the authors concluded that “the study did not find evidence to support the hypothesis that varying the method of local anesthetic administration – basal infusion versus  repeated bolus doses – influences continuous femoral nerve block to a clinically significant degree.” However, no mention was made of the study design with respect to the use of an end-hole perineural catheter rather than a multi-orifice design.

The demonstrated benefits of the intermittent bolus technique over the continuous technique (improved analgesia,2,,4reduced local anesthetic requirement,2and perhaps better differential sensory-motor block) are thought to be enhanced by multi-orifice flow;5,6and thus, to maximize these benefits, a multi-orifice catheter is required. Flow from a multi-orifice catheter depends on flow rate: below 80 ml/h, multi-orifice catheters function as single-orifice catheters; above 100 ml/h, they progressively function as multi-orifice catheters.7Therefore, a continuous-only regimen will likely only deliver single-orifice flow, whereas an intermittent bolus technique will likely deliver multi-orifice flow.7Multi-orifice flow results in better local anesthetic spread,5,6and it is this better spread that is thought to be responsible for the improved block characteristics with the intermittent bolus technique: improved analgesia and reduced local anesthetic consumption (for a given analgesic effect). Recent evidence also suggests that by enabling a local anesthetic dose reduction through the use of the intermittent bolus technique, a higher sensory-to-motor block ratio can be achieved (less motor block for a given analgesic effect).8Although some studies have demonstrated benefits using the intermittent bolus technique with end-hole catheters,9the majority have incorporated a multi-orifice design.2,,4,8 

We, therefore, do not believe that the conclusion “it is doubtful that, when using continuous femoral nerve block, varying local anesthetic administration will provide an increased sensory-to-motor block ratio” is yet warranted.

Charous MT, Madison SJ, Suresh PJ, Sandhu NS, Loland VJ, Mariano ER, Donohue MC, Dutton PH, Ferguson EJ, Ilfeld BM: Continuous femoral nerve blocks: Varying local anesthetic delivery method (bolus versus  basal) to minimize quadriceps motor block while maintaining sensory block. ANESTHESIOLOGY 2011; 115:774–81
Chua SM, Sia AT: Automated intermittent epidural boluses improve analgesia induced by intrathecal fentanyl during labour. Can J Anaesth 2004; 51:581–5
Fettes PD, Moore CS, Whiteside JB, McLeod GA, Wildsmith JA: Intermittent versus  continuous administration of epidural ropivacaine with fentanyl for analgesia during labour. Br J Anaesth 2006; 97:359–64
Ueda K, Ueda W, Manabe M: A comparative study of sequential epidural bolus technique and continuous epidural infusion. ANESTHESIOLOGY 2005; 103:126–9
D'Angelo R, Foss ML, Livesay CH: A comparison of multiport and uniport epidural catheters in laboring patients. Anesth Analg 1997; 84:1276–9
Michael S, Richmond MN, Birks RJ: A comparison between open-end (single hole) and closed-end (three lateral holes) epidural catheters. Complications and quality of sensory blockade. Anaesthesia 1989; 44:578–80
Fegley AJ, Lerman J, Wissler R: Epidural multiorifice catheters function as single-orifice catheters: An in vitro  study. Anesth Analg 2008; 107:1079–81
Fredrickson MJ, Abeysekera A, Price DJ, Wong AC: Patient-initiated mandatory boluses for ambulatory continuous interscalene analgesia: An effective strategy for optimizing analgesia and minimizing side-effects. Br J Anaesth 2011; 106:239–45
Taboada M, Rodríguez J, Bermudez M, Amor M, Ulloa B, Aneiros F, Sebate S, Cortés J, Alvarez J, Atanassoff PG: Comparison of continuous infusion versus  automated bolus for postoperative patient-controlled analgesia with popliteal sciatic nerve catheters. ANESTHESIOLOGY 2009; 110:150–4