Transcutaneous electrical stimulation (TES), a 60-mA, 50-Hz continuous square wave, has been considered equivalent to surgical incision. We examined whether TES at a smaller current (10 mA) can be used to predict surgical anesthesia and compare the results with sensory block to cold, pinprick, and touch after the administration of spinal tetracaine. Two groups of 40 consecutive patients, 17–69 yr old and 70 yr old or older received a subarachnoid injection of 0.5% tetracaine in 10% glucose or saline according to the type of surgery. Patients undergoing abdominal surgery received glucose solution, and those scheduled for lower extremities surgery received saline solution, and thus, the resultant four groups of patients were studied. Neural block was assessed by the loss of sensation to cold, pinprick, touch, and TES at 10 mA (T10s), and tolerance (i.e., the loss of pain or discomfort) to TES at 10 (T10p) and 60 (T60) mA. Dermatomal levels of sensory block to cold, pinprick, and touch that were cephalad to T60 varied widely. In contrast, dermatomal levels of T10s and T10p cephalad to T60 were less variable, and the difference between T10s and T60 was the smallest among all the differences in any groups. Our results demonstrate that, regardless of patient age and baricity of a local anesthetic solution, T10s is a good predictor of T60 equivalent to the dermatomal level of surgical anesthesia.
Implications: Our results show that the loss of sensation to transcutaneous electrical stimulation at 10 mA, but not cold, pinprick, or touch, is a good predictor of the dermatomal level of block to transcutaneous electrical stimulation at 60 mA, which is considered equivalent to the dermatomal level of surgical anesthesia after the administration of spinal anesthesia.
*Department of Anesthesiology, Shimane Medical University, Izumo City; and †Department of Anesthesia, Miyoshi General Hospital, Miyoshi City, Japan
February 17, 2000.
Address correspondence and reprint requests to Shinichi Sakura, MD, Department of Anesthesiology, Shimane Medical University, 89–1 Enya-cho, Izumo City, 693-8501 Japan. Address e-mail to email@example.com.
Presented, in part, at the Annual Meeting of the American Society of Anesthesiologist, Dallas, TX, October 9–13, 1999.
Although cold, pinprick, and touch discrimination is conventionally used to determine the level of sensory block during spinal anesthesia, their ability to predict surgical anesthesia has been questioned, even in relatively young patients receiving hyperbaric bupivacaine (1). Transcutaneous electrical stimulation (TES), a 60-mA, 50-Hz continuous square wave, has been considered equivalent to surgical incision (2) and has been used in a small number of awake volunteers (1,3,4) or patients under inhaled anesthesia (5,6). However, because the electric current produces supramaximal stimulation (7), this method may not be well tolerated in routine clinical practice, and thus, alternatives that produce less intense stimulation are needed. Accordingly, we performed this study to examine whether TES at a smaller current (10 mA) can be a surrogate for surgical stimulation and compared the results with those of three conventional modalities after spinal anesthesia using isobaric and hyperbaric tetracaine in two different age groups.
After obtaining institutional review board approval and written, informed consent, we studied two groups of 40 consecutive patients, 17–69 yr old and 70 yr old or older. All the patients were classified as ASA physical status I or II and were scheduled for elective surgery on a lower limb or lower abdomen. Patients with a history of major back problems, coagulation abnormality, neurologic disease, or infections at the site of injection were excluded. In addition, patients who showed no demonstrable sensory block after spinal anesthetic were excluded from the study.
Patients received only atropine (0.05–0.1 mg/kg) for premedication, given IM approximately 1 h before the administration of anesthesia. Intraoperative monitoring included noninvasive blood pressure and heart rate monitoring, electrocardiography, and pulse oximetry. After an IV infusion of acetated Ringer’s solution was initiated at a rate of 10 mL · kg−1 · h−1, patients were placed in the lateral decubitus position, with either the operated side or right side nondependent for those undergoing surgery on a limb or for those undergoing other surgery, respectively. The lumbar puncture was performed at the L3-4 interspace with a 25-gauge Quincke needle (Top Corporation, Tokyo, Japan) by using the midline approach. After we obtained free flow of cerebrospinal fluid, 2–3 mL of 0.5% tetracaine in 10% glucose or saline was administered at a rate of 0.1–0.2 mL/s according to the type of surgery: patients undergoing abdominal surgery received glucose solution, and those scheduled for surgery in lower extremities received saline solution. Thus, each subject was eventually assigned to one of four groups: Groups 1-D and 1-S consisted of patients, between 17 and 69 yr old, given glucose solution and saline solution, respectively; groups 2-D and 2-S consisted of patients, 70 yr old or older, given glucose solution and saline solution, respectively. The local anesthetic solution was prepared immediately before the injection by dissolving 20 mg of crystalline tetracaine hydrochloride in 4 mL of 10% glucose or saline.
Patients were immediately placed supine and remained level during the study period. The dermatomal levels of neural blockade were assessed by using the loss of sensation to cold, touch, pinprick, and TES at 10 mA (T10s), and tolerance to TES at 10 (T10p) and 60 (T60) mA in the order described. The loss of each sensory modality was determined by the patient’s verbal response to the stimulus applied in the midclavicular line starting caudad and moving cephalad on the dependent side. Assessments were performed every 5 min until 30 min after the intrathecal injection of tetracaine. An alcohol-soaked swab was used to examine temperature; 4.56 Von Frey hair for touch; the sharp tip of a safety pin for pinprick. TES was performed by using 5 s of 50-Hz tetanus with a nerve stimulator (NS252; Fisher & Paykel, Auckland, New Zealand). Subjects were initially asked to report as soon as they experienced any sensation when the current intensity was set at 10 mA (T10s). Then, the levels of tolerance to TES at 10 (T10p) and 60 (T60) mA were determined, where they did not feel pain or discomfort to the current. Hypotension was treated with 5 mg ephedrine IV if systolic arterial pressure decreased by >25% of preanesthetic value.
Results were expressed as mean ± sd or median (10th, 90th percentiles). Sensory block data were recorded as the number of dermatomes cephalad to S-4 that were blocked (e.g., a dermatomal level of T-12 was considered to be nine blocked dermatomes) and used for analysis at the time when the highest dermatomal level to T60 was first obtained. The peak levels of sensory block among the groups were compared by using the Kruskal-Wallis test and the Student-Newman-Keuls test. The dermatomal levels of T10p and T10s and those of sensory block to cold, pinprick, and touch that were cephalad to T60 in each group were compared by using the Friedman test and Wilcoxon’s signed rank test with Bonferroni correction. Continuous variables were compared by using one-way analysis of variance and the Student-Newman-Keuls test. Hemodynamic values were analyzed by using data that showed a maximum decrease after spinal injection. P < 0.05 was considered significant.
Of 80 patients enrolled in the study, 28, 12, 12, and 28 patients were assigned to groups 1-D, 1-S, 2-D, and 2-S, respectively (Table 1). Patients in groups 1-D and 1-S were significantly younger, taller, and heavier than those in groups 2-D and 2-S. Groups 1-D and 1-S or 2-D and 2-S did not differ in patient characteristics, except for patient age, in which patients in Group 2-S were significantly older than those in Group 2-D. The dose of tetracaine administered was significantly larger in Group 1-S than in the other groups.
Data regarding the dermatomal levels of neural blockade to six different stimuli were obtained from all the patients in Groups 1-D, 1-S, and 2-D. In contrast, two in Group 2-S failed to report the loss of sensation to cold and touch, or to cold, pinprick, and touch.
Solutions in 10% glucose had a tendency to produce more extensive block than those in saline, and the difference in maximal cephalad level of T60 between groups 1-D and 1-S was significant (Figure 1).
Dermatomal levels of sensory block to cold, pinprick, and touch that were cephalad to T60 varied widely: 3 (1,5), 2 (0,3), 0 (−1,2) in Group 1-D, respectively; 3 (2,8), 2 (1,5), 2 (0,3) in Group 1-S, respectively; 3 (1,4), 2 (1,3), 1 (0,3) in Group 2-D, respectively; 2 (−1,6), 1 (0,3), 2 (−1,4) in Group 2-S, respectively (Figures 2–5). In contrast, dermatomal levels of T10p and T10s cephalad to T60 were less variable: 1 (0,2), 0 (−1,0) in Group 1-D, respectively; 1 (0,2), 0 (−1,1) in Group 1-S, respectively; 1 (1,2), 0 (−1,1) in Group 2-D, respectively; 1 (0,1), 0 (0,1) in Group 2-S, respectively. The difference between T10s and T60 was the smallest among all the differences in any groups.
All the groups developed similar maximum reduction in both systolic blood pressure and heart rate.
All the patients tolerated subsequent surgery well.
Our results show that TES at 10 mA can be used to predict the dermatomal level of surgical anesthesia after spinal tetracaine administration better than cold, pinprick, or touch discrimination. The ranges (between 10th and 90th percentiles) of the extent of T10s and T10p cephalad to T60 were narrower as compared with those of any difference in the number of dermatomes blocked between cold, pinprick, or touch and T60. In addition, because the median difference between T10s and T60 was significantly smaller than any other differences in almost all the study groups, T10s can be a surrogate for T60 that is considered equivalent to surgical incision.
Surgical incision is usually a single event and cannot be repeated during an operation in an individual. Electrical stimuli via a peripheral nerve stimulator were, therefore, introduced and have been used to simulate the stimulus of a skin incision in clinical studies investigating the depth of general anesthesia (2,8–11). A current of 50 mA has proved sufficient to cause supramaximal stimulation (7). Petersen-Felix et al. (2) used the same tetanic stimulation as in the present study and found that minimum alveolar concentration tetanus was close to minimum alveolar concentration skin incision, indicating that a five-second, 60-mA, 50-Hz, 0.25-millisecond square-wave electrical impulse is comparable to skin incision.
Similarly, electrical stimulation has been used to monitor the area of anesthesia achieved by epidural or spinal anesthesia (6,12–16). Andrade and Wikinski (12) developed a device that was composed of a neuromuscular stimulator connected via a cross-over box to wires that ended in two groups of bipolar electrodes, and they examined the sensory level on both sides of the body before and during surgery. Initially, this technique was used in patients who were under general anesthesia or heavy sedation. For example, Larsen et al. (5) determined the level of sensory block during combined epidural/general anesthesia by observing a change in pupillary size in response to electrical stimulation. More recent studies used tetanic stimulation in awake subjects, mostly volunteers (1,3,4). In addition, the significance of electrical stimulation has been increased by a study by Liu et al. (1), who found that loss of any sensation to cold, pinprick, and touch was a poor predictor of surgical anesthesia. In young volunteers, there was wide interindividual variability of differential sensory block to cold, pinprick, or touch and TES at 60 mA after the administration of spinal anesthesia with hyperbaric bupivacaine.
Whereas two elderly patients failed to report sensory block to some of the conventional modalities, the examination using electrical stimulation was successfully conducted in all the patients, suggesting that even a smaller current, 10 mA, seems to elicit more intense and specific sensations than conventional testing by cold, pinprick, or touch. Laitinen and Eriksson (17) reported that the perception and pain threshold for electrical stimulation ranges from 1.0 to 2.0 mA and 2.5 to 4.3 mA, respectively. Thus, the 10 mA tetanic electrostimulus can produce both perception and pain. No patient had difficulty with verbal communication in the present study. It is possible that T10s and/or T10p cannot be interpreted by a patient with severe mental status changes or dementia, in whom successful use of TES at 60 mA was reported (14).
The extent of dermatomal levels of sensory blockade to cold, pinprick, and touch cephalad to T60 showed a very wide range (between 10th and 90th percentiles), confirming the previous findings (1) that sensory testing using conventional modalities was a poor predictor of dermatomal level of tolerance to surgical incision. In contrast, T10s and T10p had narrow and similar range (between 10th and 90th percentiles) of the extent cephalad to T60. However, T10s seems preferable to T10p because of the following reasons. First, T10s and T60 were almost identical: the median difference between the two was zero and significantly smaller than that of T10p and T60 in every group. Second, T10s causes less unpleasant experience than T10p, especially in an awake patient. A patient may object to repeated testing when pain thresholds are examined.
The group assignment was not conducted in a randomized fashion in the present study. However, we did not attempt to compare the data of four groups of patients, but to demonstrate how TES at 10 mA would work in patients of a wide range of age and with isobaric and hyperbaric anesthetic.
The type and the repetition of stimuli may have affected the sensory level. Because repeated noxious stimuli to the unblocked skin may cause adaptation and desensitization (17–19), the examination of the loss of sensation to pinprick and tolerance to TES may result in hysteresis. However, in our study, the stimulation was transferred from the desensitized level to another and discontinued when there was a response. In addition, the sensory testing was conducted in ascending order of intensity, i.e., cold followed by touch followed by pinprick followed by TES, which was performed in the order of T10s, T10p, and T60. Thus, habituation and desensitization were minimal.
Conversely, lack of randomization of order of sensory tests may have produced bias and affected the results. Because the examiner naturally knew which stimulation was being applied, assessment was not performed in a blinded fashion. However, lack of blindness does not appear to be the weakness of the study, because measurement of sensory block is, by definition, a subjective process.
In conclusion, regardless of patient age and the baricity of a local anesthetic solution, 10-mA, 50-Hz TES for five seconds is a better stimulus than cold, pinprick, or touch to assess the dermatomal level of surgical anesthesia after the administration of spinal tetracaine. The loss of sensation to TES at 10 mA can be a surrogate for surgical stimulation.
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