Chapter Outline
Anatomical Classification Of Regional Anesthetic Techniques 197
General Considerations 197
Spinal Anesthesia 201
Continuous Spinal Anesthesia 201
Epidural Anesthesia 201
Combined Spinal/Epidural 202
Peripheral Nerve Blocks 203
Locating Neural Structures 204
Principles Of Nerve Stimulation 204
Sensitivity Of Nerves To Current 204
Energy Delivery To The Nerve 205
The Ideal Characteristics Of A Nerve Stimulator 205
Using A Nerve Stimulator 205
Ultrasound Guidance 205
Regional anesthetic techniques are used to render a portion of the body insensate for the purposes of operative anesthesia or postoperative analgesia. The basis of regional anesthesia is selective neural blockade with local anesthetics. A precise knowledge of anatomy is required to locate the neural structures, and an understanding of pharmacology is required to manipulate the density and duration of blockade. A range of equipment is available to aid in anatomic location and delivery of local anesthetic agents.
Anatomical Classification of Regional Anesthetic Techniques
Regional anesthetic techniques can be classified based on anatomical regions:
Neuraxial : Spinal and epidural
Truncal : Paravertebral, transversus abdominis plane
Peripheral : brachial plexus, lumbo-sacral plexus, distal nerves, intravenous regional anesthesia
Location of specific neural structures has traditionally relied on a solid understanding of surface anatomy and eliciting paresthesia. Use of nerve stimulators and ultrasound may play a role in improving safety and efficiency of block performance.
Delivery of local anesthetics during nerve blocks may be performed with a simple hypodermic needle; however, many modifications of needle design have been made to facilitate performance and potentially improve safety. This chapter will not discuss intravenous regional anesthesia, and we refer you to the following reference.
General Considerations
Treating Early Complications
Early complications of regional anesthesia, though rare, may be life-threatening and require immediate medical intervention. They include intravascular injection of local anesthetic leading to systemic toxicity and complete spinal anesthesia (which is most commonly seen in neuraxial techniques but may also be a complication of interscalene, paravertebral, or ophthalmic blockade). Patients undergoing regional anesthesia may be exposed to anaphylactic triggers, and vasovagal reactions are not uncommon. To perform regional anesthesia safely, all patients should be monitored as defined in the American Society of Anesthesiologists standards, and sufficient resuscitation equipment should be available to provide advanced cardiac life support.
Recent animal experiments and human case reports suggest that lipid emulsion (Intralipid 20%) may reverse the cardiac toxicity of local anesthetics. Although this use of intra-lipid is “off-label,” it is currently stocked as part of standard resuscitation supplies in many hospitals practicing regional techniques.
Preventing Infectious Complications
Strict aseptic technique should be followed when practicing regional anesthesia, to prevent infectious complications. Commercially available sterile solutions including alcohol, povidone-iodine, and chlorhexidine gluconate (with and without isopropyl alcohol) are available for skin disinfection, although none is specifically FDA approved for use in regional anesthesia (spinal, epidural, or peripheral nerve block) povidone-iodine has been used safely for many years, provided that it is allowed to dry on the skin before needle insertion. Recent research demonstrates that alcohol-based chlorhexidine solution has superior antimicrobial coverage and bactericidal activity compared to that of povidone-iodine solution. The use of chlorhexidine is now currently recommended for peripheral nerve and epidural blocks. Due to concerns about possible neurotoxicity of chlorhexidine, and given the paucity of clinical studies, the FDA has not formally approved its use for skin antisepsis before lumbar puncture. There is no current data to support the use of sterile gowns to reduce the incidence of infection associated with regional anesthesia, and their use should be adopted based on individual or institutional guidelines. However it would seem prudent to use full aseptic technique when performing catheter-based techniques. Careful hand hygiene and sterile surgical gloves must be used to protect patients from contamination and healthcare workers from blood-borne pathogen exposure.
Providing Equipment
Specific carts for peripheral or neuraxial blockade allow practitioners to bring all the required equipment to the patient in an efficient manner. By having carts that can be locked one can protect valuable equipment and allow medications to be securely colocated.
Sterile kits containing the equipment to perform specific blocks may be purchased. Typically these kits contain skin preparation, drapes, and the needles and syringes required to deliver local anesthetic superficially and perform the block itself. Preprepared kits may be an efficient and economical way of providing equipment to an anesthesia practitioner ( Figure 14–1 ).
Needles
A large variety of needles are available for regional anesthesia ( Figure 14–2 ). The features used to describe a needle are its hub type, length, diameter, and tip shape. Special features unique to regional anesthesia are insulation (to facilitate nerve stimulation) and echogenicity (to improve visualization under ultrasound).
Hub
In 1925 B&D introduced the Luer slip and Luer lock system for attaching needles to syringes. This system is now the standard in the United States and is described by the International Standards Organization. The system involves a tapered (6% taper) male attachment, and matching female component made of metal or plastic. The taper allows the components to be held together by friction in the Luer slip system, whereas the Luer lock system has a built-in screw mechanism ( Figure 14–3 ).
Length
The length of needle chosen is determined by the anatomic location in which it is to be used. Needle length is typically described in either centimeter or inch long measurements. Longer needles are more difficult to control as a small movement at the hub is amplified at the tip. They are also more likely to flex. This may contribute to uncertainty about the position of the tip and increased likelihood of needle fracture. Needles may also be designed with length markings (typically in cm along the shaft), which are especially useful in determining the distance a needle or catheter has been inserted.
Diameter
The outer diameter of medical needles is typically represented using the Stubbs Iron Wire Gauge system (which is also known as the Birmingham Wire Gauge System). In this system of measurement, gauge is inversely proportional to the outside diameter of the needle. The diameter of the needles lumen is determined by the manufacturing process. Inner needle diameter determines resistance to flow on injection ( Table 14–1 ).
| Gauge | Outside Diameter (mm) | Outside Diameter (in) |
|---|---|---|
| 14 | 2.11 | 0.083 |
| 16 | 1.65 | 0.065 |
| 18 | 1.25 | 0.049 |
| 20 | 0.889 | 0.035 |
| 22 | 0.711 | 0.028 |
| 24 | 0.559 | 0.022 |
The French gauge system, denoted by Fr, FR, or F, is also used to measure the outer diameter of needles and catheters. In this system, dividing the French gauge measurement by 3 gives the outer diameter in millimeters. For example, a 12-Fr catheter would have a 4-mm outer diameter.
Tip Shape
The key feature of most needles is the shape of the tip, a wide variety of tip shapes are used during regional anesthetic techniques. A typical hypodermic needle is a metal tube cut obliquely at its distal end; this cut gives the needle its primary bevel, which determines the length of its end orifice ( Figure 14–4 ). In turn the bevel is angled to make the needle sharp. Sharp-tipped, or “A” bevel (12- to 15-degree angle), needles pass through skin and underlying tissues with minimal disruption, decreasing pain associated with insertion and injection. Short bevel needles (18- to 30-degree angle) disrupt tissue layers as they pass through, thus allowing the user to feel “clicks” and “pops” as the needle passes through fascial planes.
Needle Tips for Spinal Anesthesia
Spinal needles come with a variety of tip shapes ( Figure 14–5 ). The most simple tip is a cutting edge that resembles the “A” bevel hypodermic needle, this is also known as a “Quincke” tip. Pencil point spinal needles were introduced in an attempt to decrease the incidence of postdural puncture headache (PDPH). These needles have a conical tip with a lateral distal aperture. Common examples are the “Sprotte” and “Whitaker” needles. These needle tips may also spread nerve fibers rather than cut them or may produce a traumatic hole in the dura, which acts as a focus for fibroblast activity.
Needle Tips for Epidural Anesthesia
The most popular styles of epidural needle have noncoring tips ( Figure 14–6 ). Noncoring needles have an end that is curved so that the distal orifice is on the lateral aspect. The leading edge of the needle may be sharpened or rounded.
Needle Tips for Peripheral Nerve Blocks
There is controversy about the appropriate needle tip for performing peripheral nerve blocks. Sharp needles have the benefit of causing less discomfort to the patient; however, some authors feel that sharp needles are more likely to pierce nerves, rather than laterally displace them. As mentioned previously, blunt needles provide better “feel” and may be more likely to deflect nerves rather than pierce them. However, if a blunt needle does penetrate a nerve, it may be more traumatic.
Echogenicity
The introduction of ultrasound-guided regional anesthesia has led to the development of a new group of needles that are more visible under ultrasound. These needles may have echogenic markers or etching at the tip (e.g., Echostim, Havel’s Inc., Cincinnati) or may have a special coating, (e.g., Nanoline, Pajunk, Geisingen, Germany). The markers may be on the surface of the needle or may be on a removable stylet contained within the needle. The markers allow the user to confirm visualization of the needle tip under ultrasound.
Catheters
Needles allow for single injection of local anesthetic drugs; however, in some instances neural blockade of longer duration may be required. Under these circumstances, catheters are placed in proximity to neural structures to allow infusion or episodic bolus dosing of local anesthetics and adjuncts.
Catheters for continuous epidural, spinal, or perineural infusions are made of small diameter, hollow polyvinyl, nylon, or silicon tubing that is placed using a through needle or over needle technique.
Typically these catheters are radiopaque and have either a single or multiorifice tip ( Figure 14–7 ). Markings along the length denote distance from the tip. Catheters reinforced with coiled wire may be less prone to kinking. Still others may have a stylet inserted, either to stiffen the catheter to aid insertion, or to act as a conduit for current if nerve stimulation is to be used to aid placement.
At the proximal end of the catheter, a Luer lock attachment is either snapped or screwed into place to allow for easy connection to syringes or infusion pumps. Many commercial catheter kits contain a filter to limit injection of microscopic debris and also devices for securing catheters to the patient.
Careful attention to placement technique is necessary to prevent shearing the catheter with the sharp needle end, resulting in a portion of the catheter being retained in the patient. In general, catheters should never be drawn back through a needle. The catheter and needle should always be withdrawn as a unit, even if this means recommencing the technique.
Pumps
Traditionally, catheter-based regional anesthetic techniques have relied on bolus dosing or use of standard hospital infusion devices (syringe drivers, peristaltic pumps). These pumps are often programmable allowing for constant infusion and intermittent boluses, which may be triggered by a medical practitioner or by the patient ( Figure 14–8 ). An example would be patient-controlled epidural analgesia. Safety features of these infusion devices include locks that prevent alterations to the programming of the device, locked boxes to contain the infusate, and infusion tubing that does not contain injection ports.
The move toward same-day surgery has sparked a new wave of infusion devices for regional anesthesia. These devices are simple mechanical or elastomeric pumps that deliver constant infusions of drug through a catheter ( Figure 14–9 ). Variations include the ability to adjust the infusion rate and give either physician or patient-controlled boluses.
Implantable pump devices are frequently used in the management of chronic pain to deliver local anesthetic, opioids, or antispasmodics, and are beyond the scope of this chapter.
Spinal Anesthesia
Spinal needles are available in 18- to 27-gauge diameters, and lengths of 5 to 15 cm; 10-cm needles are most commonly used for adults. Shorter needles are used for children and longer needles for obese patients and combined spinal epidural techniques. Tight-fitting internal stylets are necessary to prevent coring of tissue as the needle is advanced and to make needles less flexible. Once the dura is penetrated, the stylet is removed, and cerebrospinal fluid (CSF) flow confirmed before injection of local anesthetic.
Continuous Spinal Anesthesia
Continuous spinal anesthesia technique involves the placement of a catheter into the CSF and is useful for precise titration of local anesthetic to develop a spinal block of desired level and duration. Microcatheters in diameters of 28 to 30 gauge and variable lengths are available, and may be passed through spinal needles as small as 23 gauge. Epidural catheters may be used, but require larger gauge needles increasing the chance of post-dural puncture headache (PDPH).
Early use of continuous spinal techniques with microcatheters was associated with cauda equina syndrome. However, it now appears that exposure of the nerve roots to high concentrations of local anesthetic (5% lidocaine) was the cause and not the microcatheters themselves. Multi-orifice rather than end-orifice catheters are probably better suited for continuous spinal infusion use.
Epidural Anesthesia
Epidural needles come in a variety of gauges, tip types, and lengths and may have “wings” (Weiss needle) at the hub to assist grip and stabilize the needle as it is inserted. The smallest gauge epidural needle of adequate tensile strength that will allow passage of appropriate catheters should be used. Typical needles for adult epidurals are available in 16- to 22-gauge diameters, with a smaller 25-gauge diameter available for pediatric patients. Needle lengths range from 0.9 to 15.5 cm, with the 10.5-cm length used most commonly for adults. In the 10.5-cm needle, the needle shaft is 8-cm long and the hub accounts for the other 2.5 cm.
The classic Tuohy needle has a sharp curved Huber tip, allowing the catheter to exit at a 20-degree angle to the shaft, and includes a stylet. The curved tip facilitates passage of the catheter into the epidural space, and the stylet prevents clogging of the needle with tissue as it is advanced. Hustead improved on the Tuohy design by changing the bevel angle (12 to 15 degrees), dulling the tip to reduce the risk of dural puncture, and rounding the heel to reduce the chance of trapping the catheter should it have to be withdrawn. Although the most common epidural needle used is referred to as the “Tuohy,” it more closely approximates the Hustead design.
Other tip designs include the Sprotte needle (“olive-shaped, rounded tip”), which has an internal ramp to deflect the catheter laterally and has been proposed to cause less tissue damage because it tends to spread rather than cut tissue fibers, and the short bevel (60 degree), straight-tip Crawford needle. The Crawford needle is often favored by practitioners of the “hanging-drop” technique, but unless positioned properly, the straight tip could direct the catheter toward the dura rather than the epidural space.
Entry of the needle tip into the epidural space is identified by “loss of resistance to injection,” or the “hanging-drop technique” ( Figure 14–10 ). These techniques are grounded in the premise of a negative pressure existing in the epidural space. In reality, the negative pressure in the epidural space results from impingement or “tenting” of the dura by the tip of the epidural needle.
