In studies of the minimum alveolar concentration of inhaled anesthetics, transcutaneous electrical stimulation (TES) (60 mA, square wave pulse at 50 Hz) has been shown to be “equivalent” to a surgical incision (1). More recently, this same TES stimulus has been used in studies of spinal anesthesia (2–5). However, because this TES (60 mA, 50 Hz) is painful, Sakura et al. (5) have investigated a less painful low-current TES (10 mA, 50 Hz) and they have observed that during spinal anesthesia there is a high correlation between the level at which a patient just begins to feel this low-current TES (10 mA, 50 Hz) and the level at which the painful TES (60 mA, 50 Hz) can just be tolerated. Accordingly, they state that the loss of sensation to TES (10 mA, 50 Hz) can be used to predict the level of block to a surgical stimulus (i.e., the painful TES [60 mA, 50 Hz]). All previous investigations (2–5) have noted that neither cold, sharp pinprick, nor touch can predict either the levels of appreciation to TES at 10 mA or the level of tolerance to TES at 60 mA.
In obstetric practice, the use of touch sensation, as compared with cold and sharp pinprick sensations, is thought to be a very good predictor of the likely adequacy of a spinal block for cesarean delivery (6–9) and a block to touch to T5 is recommended (6–9). After an editorial on the topic of testing blocks for cesarean delivery (10), at least one author (11) strongly believed that the predictive abilities of touch sensation were no better than cold or sharp pinprick, because none of these sensations could reliably predict the level of block to a simulated surgical stimulus (TES). However, some authors, although using TES as equivalent to a surgical stimulus in their studies, have pointed out that TES has never been shown to be equivalent to a surgical stimulus during regional anesthesia (12,13). There are reasons to doubt the validity of TES as a surrogate for a surgical stimulus during regional anesthesia (14,15): electrical stimulation at the ankle can create cortical evoked potentials despite adequate spinal anesthesia for pain relief or surgery (14,15), thus it may be that the neuronal activity created by an electrical stimulus is very different from that induced by a true surgical stimulus, and that the level of block for appreciation of or tolerance to an electrical stimulus may have little relationship to pain-free surgery.
If patients are totally pain free but can feel TES in dermatomes involved with surgery, then TES may not be a suitable surrogate for a surgical stimulus during regional anesthesia. Accordingly, we investigated whether women could feel an easily tolerated nonpainful TES within the dermatomes blocked by our routine spinal anesthetic for cesarean delivery.
After approval from the local research ethics committee, informed consent was obtained from 20 ASA physical status I and II women scheduled for elective cesarean delivery under spinal anesthesia. Patients received our routine antacid prophylaxis, ranitidine 150 mg orally, the night before surgery and again 1–2 h before surgery. This was then followed with 30 mL of sodium citrate (0.3 M solution) immediately before walking to the operating room. After insertion of a 16-gauge IV cannula, an infusion of Ringer’s lactate solution was initiated and baseline readings of arterial blood pressure, heart rate, and oxygen saturation were recorded. Before induction of the spinal anesthetic, 5 pairs of electrocardiograph electrodes (Ambu Blue Sensor R; AMBU, Medicotest A/S, Olstykke, Denmark) were attached to the skin. Each pair of electrodes was placed as closely as possible with their adhesive edges touching at specific points: on the left mid-thigh (L3), to the left of the umbilicus (T10), to the left of the xiphisternum (T6), to the upper part of the left breast (T2), and on the left upper arm (C5). The electrodes were 48 mm in diameter and the diameter of the conducting gel area in the middle of the electrodes was 18 mm. Thus, the edges of the conducting gel in each electrode pair were 15 mm apart. A baseline assessment of the TES sensation was performed before inducing the spinal anesthetic. The electrode pair and connecting wires at T10 were covered with transparent dressing (Tagaderm™ [15 cm × 20 cm]; 3M Health Care, St. Paul, MN) to facilitate skin cleansing at the surgical site.
With the women in either the right lateral or sitting position, the spinal anesthetic was performed using 2.5–3 mL of 0.5% hyperbaric bupivacaine with diamorphine 100 μg/mL through a 25-gauge pencil-point needle at what was believed to be the L3/4 interspace. The patient was immediately placed supine and the table was tilted toward the left to avoid aortocaval compression. Hypotension was prevented or treated by titrating a rapid infusion of an ephedrine solution (60 mg in 1 L of Ringer’s lactate solution) with additional boluses of ephedrine 3–6 mg as required if systolic blood pressure decreased to <25% of the baseline value.
Maternal appreciation of TES was assessed using a constant-current peripheral nerve stimulator (NS252; Fisher & Pykel Healthcare Division, Auckland, New Zealand) programmed to deliver 5 1-s bursts of a 10-mA, 50-Hz tetanus with each 1-s tetanus separated by 4 s. The output of the nerve stimulator was connected to an electronic multiplexer box that automatically switched each succeeding 1-s burst to the next set of electrodes in the series. A series of light-emitting diodes on the multiplexer indicated to us to which pair of electrodes the current was being applied. The tetanic stimuli were applied sequentially from the thigh (L3), in a cranial direction, ending with the C5 electrodes on the arm. With normal sensation, the patient felt a tingling sensation with each 1-s tetanus. Touch and sharp pinprick sensations were assessed using a neurotip tester (Owen Mumford Ltd., Oxford, UK).
Assessment of the levels of block to touch and sharp pinprick was performed as previously described (16) along with TES appreciation at 5, 10, 15, and 20 min and then every 10 min until the end of surgery. After assessing the levels of block to touch and sharp pinprick at each time interval, the level of block to TES on the left was determined as described above. The TES appreciation was recorded as present or absent at each electrode pair. Only at the end of surgery were women asked to score intraoperative sensations and/or pain on 10-cm visual analog pain scales.
Twenty women had elective cesarean delivery performed under spinal anesthesia. Patient characteristics, various surgical time intervals, and visual analog pain scores are shown in Table 1.
The onset of block to sharp pinprick and touch is shown in Figure 1 and the median difference between the levels of block to sharp pinprick and touch (and the range of these differences) at each assessment time are shown in Table 2.
The levels of TES appreciation and block to touch at skin incision, delivery, and uterine exteriorization are shown in Figures 2–4. Touch sensation was not appreciated below T6 in any patient during delivery and uterine exteriorization, but 30% of the women could appreciate the TES stimulus in dermatomes level with or caudal to the surgical field. Despite feeling TES in these low dermatomes, all women were totally pain free.
Because we did not know the dermatomes within which TES would be appreciated, we accessed a wide range of dermatomes. Applying TES to this range of dermatomes regularly during surgery would have been difficult so we chose to use fixed electrode placements which involved no disturbance to the patient or the surgical team. An advantage of this was that, by not disturbing the patient, we were able to apply the TES stimuli “blind”: although the patient was forewarned that the TES stimuli were about to be started, she did not know exactly when, and we were able to observe the light-emitting diodes and see to which particular set of electrodes current was flowing when the patient indicated that she could feel the electrical tingle. Unfortunately, the disadvantage of the fixed electrodes was that it prevented us from knowing the exact level of block to TES, and patients could have had a block anywhere between the electrode pair where they felt the stimulus and the electrode pair where the stimulus was blocked. Nevertheless, these unknowns add to, rather than detract from, our findings, because the TES block levels may actually be lower than illustrated.
With the nonpainful TES being felt over a variable and, at times, wide area within dermatomal levels blocked to touch and sharp pinprick sensations, we confirmed the findings of others (5): neither touch nor sharp pinprick sensation can predict the level of block to the nonpainful TES. However, we also observed that a patient’s ability to appreciate the TES stimulus, level with or caudal to the surgical field, was not reflected in the adequacy of the anesthetic for surgery. Uterine exteriorization is generally believed to be the most stimulating stage of surgery, yet women in our study were totally pain free and unaware that anything untoward was taking place. At the time of uterine exteriorization the block to touch was T7 or above but the block to the TES stimulus varied from L3 (or below) up to between T2 and C5. In 30% of women, the TES was felt at T10 or below, dermatomes completely blocked to touch sensation and normally expected to be involved in the transmission of surgical stimuli. Quite clearly, painful stimuli from the real surgical process were not being transmitted up the spinal cord but the nonpainful TES stimulus was. After an editorial on the testing of spinal blocks (10), the clinical value of touch was challenged (11) on the grounds that loss of touch sensation could not predict whether pain would be felt from a simulated surgical stimulus (TES 60 mA, 50 Hz). Because TES may not be equivalent to a surgical stimulus during regional anesthesia, the lack of ability of touch to predict the level of TES block may have no relationship with the ability of touch to predict an adequate block for a cesarean delivery in clinical practice.
We used a low-current intensity with electrodes reasonably close together and the dipole current density of this configuration decreases rapidly according to an inverse cube law. Within 5 cm from the center of the dipole, the current density has decreased by 99.4% and is below nerve stimulation threshold. Because the patients had no difficulty accurately identifying the site of the TES stimulus, it is very unlikely that nerves outside the blocked area were being stimulated. Thus, a block of somatic surgical pain by spinal anesthesia does not necessarily prevent TES being appreciated in the same dermatomes. Although our stimulus was much less intense than that used by Sakura et al. (5), this does not detract from the results because intuitively one would expect a more intense stimulus to be more readily appreciated. Thus, it is possible that an even larger proportion of women would have felt a more intense TES within the dermatomes associated with the surgical procedure.
The present study cannot assess the specificity or sensitivity of TES as a predictor of adequate anesthesia because it was designed only to investigate whether a very mild TES can be appreciated in dermatomes blocked to surgical stimulation. These data indicate that neuronal activity initiated by peripheral electrical stimulation can pass up the spinal cord, despite otherwise adequate sensory block for surgery. Such an observation is not new (14,15): TES bypasses receptors, stimulating all nerves directly. Many of the nerves stimulated by TES normally have little or no role in the transmission of conventional nociceptive information, thus the neurophysiological response to TES is different from that of a true surgical stimulus. Whether a painful or nonpainful TES applied through different types of electrodes would be more equivalent to the real surgical stimulus during spinal anesthesia requires further investigation.
In conclusion, all our patients were pain free and, although the level of appreciation of TES varied widely from L3 (or below) to C5, the level of touch appreciation was never below T6. These data indicate that, during a clinically effective spinal anesthetic for cesarean delivery, approximately 30% of women can feel a TES (10 mA, 50 Hz) caudal to the T9 dermatome.
We acknowledge Mr. Graham Naylor (Department of Medical Physics, Hull Royal Infirmary) for designing and constructing the electronic multiplexer box, and the obstetric anesthetic staff for their cooperation in this study.
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