Fig. 33.1
Pediatric spine model showing spinal column in the thoracic and lumbar regions
The distance of catheter threading will determine which technique to use for continuous blocks. The epidural stimulation test is useful when catheters are threaded at a distance into the epidural space but has only moderate application during direct placement of the catheter. In these cases, loss of resistance to saline will accurately determine epidural placement, and ultrasound guidance can be helpful. Nevertheless, the epidural stimulation test is useful in warning against unintentional intrathecal or intravascular local anesthetic injection.
Indications
Usually provides postoperative analgesia but occasionally provides surgical anesthesia for:
Lower limb surgery
Pelvic and urological surgery
Abdominal surgery
Thoracic surgery
33.1.2 Patient Positioning
Position an anesthetized patient in the lateral position; awake patients can be sitting up.
33.1.3 Surface Anatomy
Lumbar (Fig. 33.2) and thoracic (Fig. 33.3) spine:
Fig. 33.2
Surface anatomy of the lumbar spine
Fig. 33.3
Surface anatomy of the thoracic spine
Spinous Processes*
In the midline, palpate the spinous processes; a line drawn between the tips of the scapulae will run through the T7 vertebrae.
In the lumbar region, L4 is at level of the iliac crests, and S1 is on horizontal line drawn between the two posterior superior iliac spines.
Transverse processes
These lie lateral to the midline in varying distances, depending on the age of the child.
* The neural arches, forming the spinous processes, are not completely fused until the end of the first year (except at L5, where they fuse significantly later), so it is common to palpate two separate bony landmarks in infants.
33.1.4 Approaches
33.1.4.1 Direct Lumbar Approach
Perform the lumbar epidural puncture with the patient in the lateral position.
Locate iliac crests of pelvic and use this landmark to identify the spine of L4.
Resistance to air or saline can be used to identify the lumbar epidural space, although both have their own advantages and disadvantages.
Air embolism (if needle accidently punctures a vein) and pneumoencephalocele are rare but potential risks when using loss of resistance (LOR) to air.
Occasionally, the loss of resistance to saline may be equivocal.
When using LOR to saline, use a small amount of saline to avoid diluting the local anesthetic.
The hanging drop technique can be used to identify the epidural space although this method is more suited to patients in the sitting position (i.e., not well suited for sedated pediatric patients).
The skin to epidural space can be estimated as 1 mm/kg or [10 + (age in years ×2)] mm.
Midline approach is typically used (Fig. 33.4), and paramedian approach is generally not needed or recommended unless abnormal anatomical landmarks are encountered or suspected in the lumbar region.
Fig. 33.4
Loss-of-resistance (LOR) technique. (left) Midline approach. In the lumbar region, the needle is typically inserted with minimal angulation. (Right) Paramedian approach. In the thoracic region, cephalad angulation is often needed to reach the epidural space
When abnormal anatomical landmarks are encountered, a careful survey of the patient’s spinal anatomy using ultrasound is strongly recommended to allow:
Identification of the midline
Measurement of epidural (ligamentum flavum) and dural space depth
Prediction of the best path (projection and angulation) that the needle should take to reach the epidural space
33.1.4.2 Direct Thoracic Approach
In the pediatric population, continuous thoracic epidural anesthesia is used only for major surgery. Placement of a thoracic epidural requires a high skill level and can be a challenging procedure to perform. In children over the age of 6–12 months, the effectiveness of ultrasound guidance for thoracic epidural placement is limited since the bony anatomy of the region restricts and obstructs ultrasound visualization of the epidural space. Therefore, anesthesiologists continue to rely on a tactile approach with either a loss of resistance method or a hanging drop technique, both of which result in challenging needle insertion due to the angle required to access the thoracic epidural space. A median or paramedian approach can be used; however, each approach can be difficult and even dangerous if the patient is uncooperative. This procedure is therefore typically performed with the patient asleep. On rare occasions and for selective cases (i.e., mature teenager), it is occasionally possible to insert a thoracic epidural in an awake patient.
33.1.4.3 General Approaches
Perform the thoracic epidural puncture with the patient in the lateral position; locate the prominent spine of the seventh cervical vertebra and use this landmark to calculate other vertebral levels. The lower extremity of the shoulder blade, which is at the level of the seventh thoracic vertebra, can also be used as a landmark.
The paramedian approach allows one to choose the angle of insertion into the epidural space regardless of the space available between adjacent vertebral spines. However, the paramedian approach requires skill and experience to ensure that the needle tip is in the midline when it enters the epidural space.
Compared to the lumbar region, the spinal canal at the level of T10–T12 is slightly superficial due to short spinous processes.
Choose a needle that is appropriate for the size of the child; for children ≤12 kg, a 5 cm, 19G Tuohy needle is suitable, while an 18G version can be used for larger children. Also available is a thin-walled, 18G needle that allows a 20G catheter to be inserted through; an advantage of this needle is that when the needle is inserted with cephalad angulation, the bevel is parallel to the epidural space, reducing the risk of dural puncture by the needle tip.
Similar to lumbar epidural, resistance to air or saline can be used to identify the thoracic epidural space. In children 1 year or older, the ligamentum flavum provides adequate resistance, and the negative pressure of the thoracic epidural space can be identified easily.
For infants and young children, small 3 or 5 mL syringes can be used; for older patients, use a 5–10 mL syringe.
33.1.4.4 Median Approaches
A successful thoracic block will rely on an acute angle of insertion relative to the skin due to the angle of the spines. This is particularly noticeable when using the median approach in the midthoracic region compared to lumbar region, since the needle typically needs to be angulated more cephalad to reach the epidural space (Fig. 33.4). This technique may facilitate easier catheter movement in a cephalad direction within the epidural space.
33.1.4.5 Paramedian Approach
Most anesthesiologists find the paramedian approach to be the most effective technique for thoracic epidural placement. This technique involves the following steps (Fig. 33.5):
Fig. 33.5
Paramedian approach for thoracic epidural placement in a spine model. (Top) Insert needle lateral to spinous process and perpendicular to the skin to contact the lamina. (Middle) Redirect the needle approximately 15° medially to “walk off” the lamina. (Bottom) Angulate the needle cephalad while maintaining needle 15° medially, and continue to “walk off” to reach the epidural space
1.
Insert the epidural needle 0.5–2 cm (e.g., one to two fingerbreadths distance) lateral to the spinous process of the vertebra. Note: Use the patient’s finger as measurement of the distance lateral to the spinous process.
2.
Advance the needle perpendicular to the skin until it contacts the lamina.
3.
Redirect the needle approximately 15° medially to “walk off” the lamina.
4.
Angle the needle cephalad, and continue the “walk off” technique to locate the epidural space.
Note: As shown in Fig. 33.6, steps 2 and 3 must be performed separately, as the epidural space may be missed if the needle is “walked off” diagonally (i.e., medial and cephalad angulation at the same time).
Fig. 33.6
Common mistakes: with simultaneous medial and cephalad angulation, the epidural space may be missed as the needle is “walked off” diagonally onto the spinous process or lamina of the spinal level above
This procedure can be difficult to master. Accurate placement of the needle requires fine motor skills, which are best learned through hands-on practice. Training may be aided by a phantom model that reproduces the anatomy necessary to learn and practice the paramedian approach to thoracic epidurals.
33.1.5 Nerve Stimulation Technique
Electrical stimulation has been used in epidural anesthesia and for peripheral nerve blocks for many years. The epidural stimulation test (Tsui test) was developed to confirm epidural catheter placement, applying similar principles to those of peripheral nerve blockade (i.e., using electrical pulses and the current versus nerve-distance relationship). This test has demonstrated 80–100 % positive prediction for epidural catheter placement. It has also been shown to be effective to guide catheters to within two segmental levels (with radiologic confirmation), which can assist smooth placement to appropriate levels and allow adjustments in the event of catheter migration, kinking, or coiling. In addition to confirming and guiding epidural catheter placement, this test has been shown to be useful for detecting intrathecal, subdural, or intravascular catheter placement.
33.1.5.1 Needle Confirmation
The procedure is performed along with standard mechanical tests (e.g., loss of resistance to saline or air) for determining epidural needle placement:
After sterile preparation, the nerve stimulator is attached to an 18G insulated Tuohy needle (see Fig. 1.3).
The current is applied after skin puncture and during advancement; the epidural stimulation test is used together with detection of a “pop” or loss of resistance to saline.
The threshold current range for determining correct needle placement is similar for the lumbar or caudal routes, but the higher limit of 10 mA may be extended for thoracic placement (mean of 11.1 mA and up to 17 mA) [1, 2].
The test may provide further confirmation during direct epidurals, with the added benefit of warning that a needle is entering lumbar epidural space if the current is set above 6 mA [3]. However, this technique has not been applied widely in clinical practice except in the study environment.
33.1.5.2 Catheter Confirmation
The same procedure as described above for an insulated needle applies, with the exception that the nerve stimulator is connected to an epidural stimulating catheter or a metal-containing epidural catheter using an electrode adaptor such as the Johans ECG adaptor (Fig. 2.9a).
Prime the catheter and adaptor with sterile normal saline (0.2–1 mL).
Attach the cathode lead of the nerve stimulator directly to the cable of the stimulating catheter or metal hub of the adaptor and the grounding anode lead to an electrode on the patient’s body surface.
Set the nerve stimulator to a low frequency and pulse width (2 Hz; 0.2 ms).
Carefully and slowly increase the current intensity until motor activity begins.
The characteristics of the various responses are compared in Table 33.1. Catheter location is identified, and the required adjustments are made depending to location indicated.
Table 33.1
Catheter location determined by motor response and current threshold during the epidural stimulation test
Catheter location
Motor response
Currenta
Subcutaneous
None
N/A
Subdural
Bilateral (many segments)
<1 mA
Subarachnoid
Unilateral or bilateral
<1 mA
Epidural space
Against nerve root
Unilateral
<1 mA
Non-intravascular
Unilateral or bilateral
1–15 mA (threshold current increases after local anesthetic injection)
Intravascular
Unilateral or bilateral
1–15 mA (no change in threshold current after local anesthetic injected)
33.1.5.3 Mechanism of the Test
Needle or catheter placement is confirmed by stimulating the spinal nerve roots (not spinal cord) with an electrically conducting catheter that conducts a low-amplitude electrical current through normal saline.
An appropriate motor response (1–10 mA; Table 33.1) confirms accurate placement of the epidural catheter tip, defined as 1–2 cm from the nerve roots (see Fig. 2.8).
Responses observed with a significantly lower threshold current (<1 mA), especially if substantially diffuse or bilateral, may warn of catheter placement in the intrathecal or subdural space (i.e., contacting highly conductive CSF) or in close proximity to a nerve root.
The segmental level of the catheter tip may be predicted based on the progressive nature of the motor twitches as the catheter is advanced (i.e., lower limbs followed by intercostals and then upper limbs).
A local anesthetic test dose helps confirm epidural versus intravascular location. If the catheter is localized in the epidural space, the motor threshold current should increase after injection of local anesthetic. Unintentional injection into the systemic circulation (i.e., removing local anesthetic from the vicinity of the nerve) will allow subsequent motor stimulation at the same current.
When Used for Needle Placement
Insulated needles seem to provide lower electrical threshold current required for this test.
The upper limit of the threshold current designating a positive test using needles is different depending on the level of needle placement (caudal = mean 3.7 mA; [4] lumbar = mean 3.84 mA; thoracic = mean 11.1 mA; intrathecal = mean 0.77 mA).
The higher threshold currents that may be seen with direct thoracic level needle insertion may be related to the minimal depth of needle penetration that is inherently used for caution at this level.
33.1.5.4 Considerations for Test Performance and Interpretation
Since local muscle contraction in the trunk region can be confused with epidural stimulation, it is recommended to place the anode lead on the upper extremity for lumbar epidurals and the lower extremity for thoracic epidurals.
The 1–10 mA test criteria should be used as a guideline only. Epidural stimulation may occur outside this range. The reason we recommend this range is that it is easy to remember. In addition:
It is important to begin using the lowest current to allow detection of intrathecal placement or nerve root proximity; the lower limit of 1 mA applies to all situations.
Some cases will require stimulation upwards of 17 mA, particularly when using insulated needles in the thoracic region.
The best predictor of epidural placement is a combination of the threshold current level and the distribution of elicited motor responses (i.e., correlation with approximate segmental location) since subcutaneous placement would not elicit such predictive and segmental responses.
33.1.5.5 Detecting Intrathecal and Intravascular Catheter and Needle Placement (Table 33.1)
Aspiration should always be performed prior to local anesthetic injection. However, the inability to aspirate blood or CSF does not indicate that an epidural needle/catheter is not in the intrathecal or intravascular space. In pediatric patients, aspiration alone may fail to detect up to 86 % of intravascular entry of epidural needles or catheters [5]. The threshold current for a motor response during catheter and needle placement with the electrical stimulation test may help predict intrathecal placement. Intravascular placement may be detected with the electrical stimulation test in conjunction with a local anesthetic test dose.
Intrathecal
When a needle/catheter is situated properly within the epidural space, a current greater than 1 mA should be required to elicit muscle twitches.
Using insulated needles, currents to elicit a motor response in the epidural space (3.84 ± 0.99 mA and 5.2 ± 2.4 mA) are much higher than that for the intrathecal space (0.77 ± 0.32 mA and 0.6 ± 0.3 mA).
If any motor response is detected with a current less than 1 mA, intrathecal catheter or needle placement should be suspected.
Intravascular
Repeated injections of local anesthetic into a properly placed epidural catheter result in impairment of nerve conduction and require a gradual increase in the amplitude of electrical current to produce a positive motor response.
If the local anesthetic is inadvertently injected into the intravascular space (i.e., systemic circulation, thereby reducing the concentration in the vicinity of the nerves), the threshold will remain the same instead of incrementally increasing with repeated application.
Caution is required when extrapolating the intravascular information because this application has only been tested in a few adult obstetric patients and never in the pediatric population.
An epinephrine test dose (0.5 μg/kg) should still be administered to identify inadvertent intravascular placement by observing specific ECG changes (i.e., >25 % increase in T-wave or ST segment changes irrespective of chosen lead).
When using a the test dose in combination with the epidural stimulation test, one can confidently rule out the risk of accidental intravascular or intrathecal injection.
33.1.6 Electrocardiograph (ECG) Monitoring Technique (Fig. 33.7)
Fig. 33.7
ECG monitoring technique for thoracic epidural catheter placement via advancement from lumbar or caudal levels
One limitation of the epidural stimulation technique is that it cannot be performed reliably if any significant clinical neuromuscular blockade is present or if local anesthetics have been administered in the epidural space. To overcome this limitation, an alternative monitoring technique using electrocardiograph (ECG) monitoring was developed [6]. Using epidural ECG, the anatomical position of the epidural catheter is determined by comparing the ECG signal from the tip of the catheter to a signal from a surface electrode positioned at the “target” segmental level. Thus, advancement of an epidural catheter from the lumbar or sacral region into the thoracic region can easily be monitored and placed within two vertebral spaces of the targeted level under ECG guidance.
Procedure
1.
Place left-leg (red in figure) and left-arm (black in figure) electrodes at their standard positions.
2.
Record a standard reference ECG (lead II) by connecting the right-arm electrode (white in figure) to a skin electrode on the patient’s back at the target spinal level.
3.
Connect the right-arm electrode (white in figure) to the metal hub of an electrode adaptor to record a tracing from the epidural catheter.
Interpreting the ECG Method
When the epidural catheter tip is positioned in the lumbar and sacral regions, the amplitude of the QRS complex is relatively small since the recording electrode (epidural tip) is a considerable distance from the heart, and the vector of the cardiac electrical impulse is at approximately a 90° angle.
As the epidural tip advances toward the thoracic region, the amplitude of the QRS complex increases as the recording electrode comes closer to the heart and becomes more parallel to the cardiac electrical impulse.
As the catheter tip passes the target level, the amplitude of QRS should match the amplitude of the reference surface electrode.
Limitations
The ECG technique cannot warn of a catheter placed in the intrathecal or intravascular space. In addition, this technique may not be suitable when threading catheters a short distance, as ECG changes may be too subtle to observe.
33.1.7 Ultrasound-Guided Technique
Generally, two approaches can be used for applying ultrasound imaging for epidural procedures. First, an “ultrasound-supported” approach employs a pre-procedural scan to identify the puncture site, depth of epidural space, and ideal needle trajectory. These procedures require multiple “still images” in different planes to capture accurate measurement with the ultrasound device. Second, real-time or “online” imaging can be used during epidural procedures to observe needle puncture, catheter placement, and local anesthetic application (each of these actions is improved through direct visualization of movement or by indirect means). There are some challenges with using ultrasound in neuraxial procedures, but it has demonstrable benefits for infants and neonates and may have some efficacy (e.g., reduced needle attempts and visibility of the sacral hiatus for caudal blocks) with older pediatric patients.
Generally, the lower lumbar region has the highest visibility (i.e., ultrasound “window”), as it is less compact than the more cephalad high-lumbar and thoracic levels. In pediatric patients, the ratio of visible to nonvisible segments to be 2:1 at the sacral, 1:1 at the lumbar, and 1:2 at the thoracic levels. Visibility of neuraxial structures in the lumbar and thoracic regions is best in patients 3 months of age and under, with age-dependent decreases in quality thereafter. As shown in Fig. 33.8, there will be variation in the structures that are visible in different planes of ultrasound scanning as illustrated in the skeleton model (i.e., paramedian longitudinal scanning will visualize the dura mater better than median longitudinal), and different techniques may require separate planes, although many approaches will require two or more planes for comprehensive assessment.
Fig. 33.8
Possible ultrasound views using a pediatric skeleton model. (Top) For the lumbar region, an ultrasound epidural window (i.e., no bone obstruction) can be obtained with both transverse and paramedian views. (Bottom) For the thoracic region, an epidural window cannot be seen easily and only with a paramedian view
33.1.7.1 Preparing the Needle Insertion Site
Sterile technique is critical during performance of neuraxial blocks due to the disastrous consequences of infection in the epidural space.
Full aseptic technique should be undertaken, with the proceduralist gowned, gloved, and wearing a face mask and hat.
The area should be prepped with a chlorhexidine and alcohol solution, which should be allowed to dry.
Sterile draping of the area should be undertaken.
The needle, catheter, and other equipment required should be assembled on a sterile tray and be easily accessible.
33.1.7.2 Scanning Technique
Use a low-frequency (5–2 MHz) probe to scan the lumbar and thoracic spine in transverse and longitudinal planes.
Figures 33.9 and 33.10 show the MRI anatomical structure and ultrasound appearance in the transverse plane at the lumbar and thoracic spine, respectively. Positioning the patient with a flexed spine will be advantageous for viewing in this plane since it will improve location of an ultrasound window between the vertebrae. If the spinous processes obstruct the view of dura, then the “expected position” of the dura can be estimated in most cases (see below for explanation).
