Needles, catheters, and syringes
Effective regional anesthesia requires comprehensive knowledge of equipment—that is, the needles, syringes, and catheters that allow the anesthetic to be injected into the desired area. In early years, regional anesthesia found many variations in the method of joining needle to syringe. Around the turn of the century, Schneider developed the first all-glass syringe for Hermann Wülfing-Luer. Luer is credited with the innovation of a simple conical tip for easy exchange of needle to syringe, but the “Luer-Lok” found in use on most syringes today is thought to have been designed by Dickenson in the mid-1920s. The Luer fitting became virtually universal, and both the Luer slip tip and the Luer-Lok were standardized in 1955.
In almost all disposable and reusable needles used in regional anesthesia, the bevel is cut on three planes. The design theoretically creates less tissue laceration and discomfort than the earlier styles did, and it limits tissue coring. Many needles that are to be used for deep injection during regional block incorporate a security bead in the shaft so that the needle can be easily retrieved on the rare occasions when the needle hub separates from the needle shaft. Fig. 3.1 contrasts a blunt-beveled, 25-gauge needle with a 25-gauge “hypodermic” needle. Traditional teaching holds that the short-beveled needle is less traumatic to neural structures. There is little clinical evidence that this is so, and experimental data about whether sharp or blunt needle tips minimize nerve injury are equivocal.
Fig. 3.2 shows various spinal needles. The key to their successful use is to find the size and bevel tip that allow one to cannulate the subarachnoid space easily without causing repeated unrecognized puncture. For equivalent needle size, rounded needle tips that spread the dural fibers are associated with a lesser incidence of headache than are those that cut fibers. The past interest in very-small-gauge spinal catheters to reduce the incidence of spinal headache, with controllability of a continuous technique, faded during the controversy over lidocaine neurotoxicity.
Fig. 3.3 depicts epidural needles. Needle tip design is often mandated by the decision to use a catheter with the epidural technique. Fig. 3.4 shows two catheters available for either subarachnoid or epidural use. Although each has advantages and disadvantages, a single–end-hole catheter appears to provide the highest level of certainty of catheter tip location at the time of injection, whereas a multiple–side-hole catheter may be preferred for continuous analgesia techniques.
Continuous Infusion Dosage With the advent of ultrasound and better training, more and more continuous nerve block catheters are performed to help patients. Current practice is to limit continuous infusion at 0.4/mg/kg/h (bupivacaine/ropivacaine). See Table 3.1 for specific block recommendations.
|Block type||Local anesthetic *||Continuous rate (mL/hr)||Bolus dose (mL)||Lock-out interval (min)||Number of bolus per hour|
|Interscalene||0.25 % Bupivacaine or 0.2 % ropivacaine||8-10||8-12||60||1|
|Supraclavicular||0.25 % Bupivacaine or 0.2 % ropivacaine||8-10||8-12||60||1|
|Popliteal||0.25 % Bupivacaine or 0.2 % ropivacaine||8-10||8-12||60||1|
|Femoral or Adductor canal ^||0.12 % Bupivacaine or 0.1 % ropivacaine||6-8||0||—||—|
In recent years, use of nerve stimulators has increased from occasional use to common use and is often of critical importance. The growing emphasis on techniques that use either multiple injections near individual nerves or placement of stimulating catheters has provided impetus for this change. The primary impediment to successful use of a nerve stimulator in a clinical practice is that it is at least a three-handed or two-individual technique ( Fig. 3.5 ), although there are devices allowing control of the stimulator current using a foot control, eliminating the need for a third hand or a second individual. In those situations, requiring a second set of hands, correct operation of contemporary peripheral nerve stimulators is straightforward and easily taught during the course of the block. There are a variety of circumstances in which a nerve stimulator is helpful, such as in children and adults who are already anesthetized, when a decision is made that regional block is an appropriate technique, in individuals who are unable to report paresthesias accurately, in performing local anesthetic administration on specific nerves, and in placement of stimulating catheters for anesthesia or postoperative analgesia. Another group that may benefit from the use of a nerve stimulator is patients with chronic pain, in whom accurate needle placement and reproduction of pain with electrical stimulation or elimination of pain with accurate administration of small volumes of local anesthetic may improve diagnosis and treatment.
When nerve stimulation is used during regional block, insulated needles are most appropriate because the current from such needles results in a current sphere around the needle tip, whereas uninsulated needles emit current at the tip as well as along the shaft, potentially resulting in less precise needle location. A peripheral nerve stimulator should allow between 0.1 and 10 milliamperes (mA) of current in pulses lasting approximately 200 ms at a frequency of 1 or 2 pulses per second. The peripheral nerve stimulator should have a readily apparent readout of when a complete circuit is present, a consistent and accurate current output over its entire range, and a digital display of the current delivered with each pulse. This facilitates generalized location of the nerve while stimulating at 2 mA and allows refinement of needle positioning as the current pulse is reduced to 0.5–0.1 mA. The nerve stimulator should have the polarity of the terminals clearly identified because peripheral nerves are most effectively stimulated by using the needle as the cathode (negative terminal). Alternatively, if the circuit is established with the needle as anode (positive terminal), approximately four times as much current is necessary to produce equivalent stimulation. The positive lead of the stimulator should be placed in a site remote from the site of stimulation by connecting the lead to a common electrocardiographic electrode (see Fig. 3.5 ).
The use of a nerve stimulator is not a substitute for a complete knowledge of anatomy and careful site selection for needle insertion; in fact, as much attention should be paid to the anatomy and technique when using a nerve stimulator as when not using it. Large myelinated motor fibers are stimulated by less current than are smaller unmyelinated fibers, and muscle contraction is most often produced before patient discomfort. The needle should be carefully positioned to a point where muscle contraction can be elicited with 0.5–0.1 mA. If a pure sensory nerve is to be blocked, a similar procedure is followed; however, correct needle localization will require the patient to report a sense of pulsed “tingling or burning” over the cutaneous distribution of the sensory nerve. Once the needle is in the final position and stimulation is achieved with 0.5–0.1 mA, 1 mL of local anesthetic should be injected through the needle. If the needle is accurately positioned, this amount of solution should rapidly abolish the muscle contraction or the sensation with pulsed current.
In the last decade, image-guided peripheral nerve blocks have become the norm for anesthesiologists at the forefront of regional anesthesia innovation. The dominant method of imaging is ultrasonography. Ultrasonographic imaging devices are noninvasive, portable, and moderately priced. Most work has been done using scanning probes with frequencies in the range of 5–10 megahertz (MHz). These devices are capable of identifying vascular and bony structures but not nerves. Contemporary devices using high-resolution probes (12–15 MHz) and compound imaging allow clear visualization of nerves, vessels, catheters, and local anesthetic injection and can potentially improve the techniques of ultrasonography-assisted peripheral nerve block. Use of these devices is limited by their cost, the need for training in their use and familiarity with ultrasonographic image anatomy, and the extra set of hands required. They work best with superficial nerve plexuses and can be limited by excessive obesity or anatomically distant structures. One of the keys to using this technology effectively is a sound understanding of the physics behind ultrasonography. A corollary to understanding the physics is the need for study and appreciation of the relevant human anatomy.