I. General principles

A. Regional anesthesia techniques can be performed with almost any syringe and needle. Success depends more on knowledge of anatomy and the operator’s skill set than the quality of the instruments. Nevertheless, selection of proper equipment can optimize the performance of regional anesthesia techniques.

II. Block trays

A. Equipment for regional blocks is usually stocked in prepared sterile trays. The latter usually contain skin-preparation swabs, drapes, needles, syringes, solution cups, and a sterility indicator. The choice of equipment will be dictated by the specific blocks attempted and by personal preference. However, some general comments are warranted. Concern about infectious diseases, especially newer ones that are resistant to conventional sterilization techniques, has created a greater reliance on disposable equipment. The quality of disposable trays has improved, and the willingness of the manufacturers to “customize” trays to the needs of individual institutions is widespread. They remove the burden of sterilization from the local department or hospital (but not the responsibility of checking for sterility).

III. Peripheral nerve stimulators

A. Peripheral nerve stimulators deliver a pulsed electric current to a needle. As the latter approaches a nerve, depolarization is produced. Efferent motor nerves (A-α fibers) are most easily depolarized, so these devices confer the distinct benefit of identifying mixed peripheral nerves by producing muscular contraction rather than eliciting uncomfortable sensory paresthesiae.

B. The degree of stimulation depends on the total current (amperage) and (presumably) the distance between the current source and the nerve. This principle led to the development of nerve stimulators with variable outputs. A high current (approximately 1 to 2 mA) can be used to confirm that the needle is approaching a nerve. A progressively lower current may document increasing proximity between the needle and a nerve. In practice, 2 mA will produce depolarization of a motor nerve at a distance. As the needle is moved closer to the nerve, a smaller current suggests adequate proximity to the latter. However, recent reports have challenged the relationship between the current and neural proximity, bringing into question whether any correlation can be assumed. Specifically, needles in direct contact with nerves (based on paresthesiae) may require currents from 0.1 mA to greater than 1 mA in order to produce an evoked motor response, so the relevance of the final stimulating current remains unclear (1). Current practice suggests that a current of 0.5 mA is ideal, but adequate anesthesia can be produced with greater and lesser stimulating currents.

C. The characteristics of the stimulating current can also be modified to produce a sensory response. The short-duration impulse commonly used (0.1 ms) is effective in stimulating motor fibers, but a longer duration pulse (0.3 ms) will also stimulate sensory fibers, a useful feature if a pure sensory nerve (e.g., lateral femoral cutaneous or saphenous nerve) is being sought.

D. The ideal nerve stimulator possesses a variable linear output with a clear display of current delivered. The positive (red, anode) lead of the stimulator is connected to a skin electrode (the anatomic location of the anode does not influence stimulation). The negative (black, cathode) lead is attached to the exploring needle. The connection can be secured with an “alligator”-type clamp, but commercial needles with electrical connectors incorporated into their design are more commonly used. Cathodal stimulation via the needle is approximately twice as efficient as anodal stimulation, so lead polarity is important.

E. Electrically insulated (sheathed, Teflon-coated) needles concentrate more current at the needle tip, thus causing neural depolarization to decrease after the needle tip passes the nerve. In contrast, noninsulated needles can continue to stimulate the target nerve with their shafts even when the needle tips have moved beyond the neural target. Therefore, although more expensive, insulated needles constitute the criterion standard for neurostimulation-guided nerve blocks. Injection of a small volume of local anesthetic (1 mL, the Raj test) will eliminate the evoked response when the needle tip is adjacent to the nerve target. This is because of dissipation of the current density at the needle tip and depends on the ionic strength of the injectate (2).

F. Nerve stimulators do not provide a substitute for knowledge of anatomy and proper initial needle placement. They will only help document the proximity of the needle to the nerve. Although it is speculated that their use may reduce the potential for nerve damage, no study has shown an increased safety margin with nerve stimulators, because nerve injuries can still occur despite their use. Thus neurostimulators do not eliminate the risk of nerve injury when blocks are performed on unconscious adults.

IV. Ultrasound machines (Fig. 2.1)

A. Ultrasound imaging allows direct visualization of peripheral nerves. Ultrasound machines have been critically evaluated for their ability to meet the needs of regional anesthesiologists (3). Several features have been identified as important to ergonomics and overall ease of use.

B. The start-up time (from power on to readiness for scanning) is important in a busy practice, particularly if the machine requires an uninterrupted power source rather than battery life. Screen size, positioning (swivel, articulating arm, etc.), and angle can influence image viewing. Basic image quality controls (adjustment of depth, receiver gain, and probe selection) are critical to assessment of a machine. Expectedly, novices prefer a relatively straightforward user interface (3).

C. Many commercial ultrasound machines are now marketed with nerve imaging presets so that fewer adjustments of imaging quality control are necessary in clinical practice. Most important is the identification of nerve fascicles. Machines and transducers can be evaluated on live models for their ability to resolve the fine layers of collagen that divide peripheral nerves into neural fascicles (i.e., the fascicle count). When evaluating ultrasound equipment, it is important to control for factors that influence image interpretation (e.g., the model, the anatomic region and peripheral nerve, room lighting, etc.). The portability of ultrasound machines is paramount for settings where blocks are performed in the induction room, operating room, and recovery room.

V. Transducers (Fig. 2.2A and B)

A. Ultrasound transducers have an array of piezoelectric crystals that emit and receive sound waves that travel through soft tissues. A wide variety of transducers are available for clinical imaging purposes. Transducer selection is critical for optimal regional anesthesia imaging.

B. Most practitioners use linear arrays with a large footprint for regional blocks. These transducers contain a large number of crystals (elements) for high-resolution imaging of peripheral nerves and a broad field of view. For deeper blocks or blocks that require a larger viewing field (such as neuraxial, lumbar plexus, and parasacral/subgluteal/anterior sciatic blocks), curved arrays are often used.

C. Higher insonation frequencies allow better imaging quality (smaller wavelength). However, attenuation of sound waves is frequency dependent. It is particularly important to select the highest frequency that will allow sound waves to travel to the target and back to the transducer. Increasing the receiver gain remains a poor remedy when the insonation frequency is too high, because it results in amplification of the background noise. There exists considerable variation in attenuation among patients and anatomic regions. As an approximate guide, the penetration depth (cm) is 60/center frequency (MHz) (4). For example, a transducer with a center frequency of 10 MHz has a penetration depth of 6 cm.

VI. Doppler (Fig. 2.3)

A. Most ultrasound machines possess a variety of Doppler imaging modalities that allow detection of blood flow. Power Doppler (integration of the power spectrum of the Doppler shift) is particularly useful for the detection of small arteries that accompany peripheral nerves. Power Doppler displays significant advantages over traditional color Doppler (5). It is more sensitive (by a factor of 3 to 5 in some cases), exhibits less angle dependence, and there has no aliasing of the signals. Aliasing arises when the signal is off scale because of undersampling from too low pulse repetition frequency. The potential disadvantages of power Doppler stem from the absence of directional information and the high sensitivity to motion (resulting in flash artifact). Power Doppler appears to be the best current Doppler modality to detect intraneural blood flow (6). Many practitioners prescan (scout imaging) with power Doppler to detect adjacent blood vessels prior to the performance of regional blocks.