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REGIONAL ANESTHESIA

What Constitutes an Effective but Safe Initial Dose of Lidocaine to Test a Thoracic Epidural Catheter?

Stephen J. Holman, MD^*, Richard R. Bosco, MD^*, Tzu-Cheg Kao, PhD, Michael A. Mazzilli, MD, Keith J. Dietrich, MD, Rick A. Rolain, MD, and Rom A. Stevens, MD^*

Departments of *Anesthesiology and Preventive Medicine & Biometrics, Uniformed Services University of the Health Sciences, Bethesda, Maryland; Department of Anesthesiology, National Naval Medical Center, Bethesda, Maryland; and Department of Anesthesiology, Portsmouth Naval Medical Center, Portsmouth, Virginia

Address correspondence to Dr. Stephen J. Holman, Department of Anesthesiology, USUHS, 4301 Jones Bridge Rd., Bethesda, MD 20814-4799. Address e-mail to sholman{at}usuhs.mil

	Abstract

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To investigate the effects of age and dose on the spread of thoracic epidural anesthesia, we placed thoracic epidural catheters in 50 surgical patients divided into groups by age (Group I [young], 18–51 yr; Group II [old], 56–80 yr) and randomly assigned patients to receive either 5 mL (A) or 9 mL (B) of 2% lidocaine (plain) injected via the epidural catheter. Hemodynamic variables were measured (heart rate, mean arterial blood pressure, noninvasive impedance cardiac index) at baseline and every 5 min for 30 min. Detectable blockade occurred within 8 min after injection of 3 + 2 mL or 3 + 6 mL in 48 of 50 patients. Maximum spread of analgesia to pinprick occurred 15–23 min after completion of local anesthetic injection and was significantly different between age and volume groups by two-way analysis of variance (Group IA [young 5], 10.9 ± 4.0 dermatomes; Group IIB [young 9], 13.9 ± 4.5 dermatomes; Group IIA [old 5], 14.1 ± 5.6 dermatomes; and Group IIB [old 9], 17.4 ± 5.1dermatomes). Minor decreases in mean arterial blood pressure (8%–17%) and heart rate (4%–11%) were noted. Two patients in the Old 9 group required IV ephedrine or ephedrine/atropine to treat hypotension and bradycardia. We conclude that given the rapid onset (3–8 min), extensive spread (11–14 dermatomal segments), and consistent hemodynamic stability, thoracic epidural anesthesia should be initiated with lidocaine 100 mg (5 mL 2% lidocaine) to establish proper location of the catheter in the epidural space in both younger and older patients.

IMPLICATIONS: In young and old patients, we evaluated the cardiovascular effects and spread of numbness achieved from injection of local anesthetic (lidocaine 100–180 mg) into the thoracic epidural space and concluded that the smaller dose was quite effective and possibly safer, particularly in older patients.

	Introduction

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Patients undergoing thoracic and abdominal surgical procedures may benefit from preoperative placement of thoracic epidural catheters to provide intraoperative and postoperative analgesia. To ensure that these catheters are correctly located in the epidural space, it is essential that adequate local anesthetic be injected via the catheter and that bilaterally symmetrical sensory analgesia be demonstrated before the induction of general anesthesia. The goal of this study was to determine an optimal initial dose of local anesthetic to ensure adequate perioperative thoracic analgesia.

Segmental spread of local anesthetics has been well studied for lumbar anesthesia, but not for thoracic epidural anesthesia (TEA) (1–4). Bromage (5) and Veering and Cousins (6) state that approximately 70% of a calculated lumbar epidural dose is adequate to achieve a similar dermatomal spread in the thoracic epidural space. This recommendation undoubtedly stems from the extensive clinical experience of the authors, yet no references are cited.

Although the effect of age on lumbar epidural anesthesia has been well studied (1–4), there is a paucity of literature concerning the effect of age on TEA. If more extensive spread of local anesthetic occurs in older patients during thoracic epidural injection, as occurs during lumbar epidural injection, then older patients might be at increased risk for developing hypotension, even after a small dose of local anesthetic is used for confirming the position of thoracic epidural catheters. Hypotension is reported more frequently with the use of TEA than with lumbar epidural anesthesia (7).

The aims of this study were to determine and compare the spread of blockade achieved with two different thoracic epidural doses of lidocaine (100 mg and 180 mg) administered to younger and older patients, to delineate the extent of hemodynamic changes resulting from these doses, and to evaluate the effect of age on thoracic epidural injection.

	Methods

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After obtaining IRB approval (from the Naval Medical Center, Portsmouth, VA) and written, informed consent, 50 ASA physical status I, II, or III patients scheduled for thoracic or upper abdominal surgical procedures were enrolled in the study. Exclusion criteria were coagulopathy, skin infection at the site of intended epidural puncture, uncontrolled hypertension, significant cardiac valvular obstructive lesions, history of congestive heart failure or unstable angina pectoris, ß blockade, and age younger than 18 or older than 80 yr. Sample size was determined by using a two-sided two-sample Student’s t-test, expecting a difference in extension of blockade between the young and old patients of 6 ± 4 segments and considering an increase in extension of blockade of 50% to be clinically relevant. We set the type I error = 0.05 () and the type II error = 0.1 (ß), and we then calculated a sample size of 11 patients per group. We obtained IRB approval for 14 patients per group to allow for failed epidural anesthesia and other unexpected clinical problems.

Patients were assigned to four groups according to age and total dose of 2% lidocaine: 18–51 yr of age, Groups 1A (young, 5 mL) and 1B (young, 9 mL); 56–80 yr of age, Groups 2A (old, 5 mL) and 2B (old, 9 mL). No sedation was used. After placement of a peripheral IV line and a 20-gauge radial arterial catheter, patients were given an IV 500-mL bolus of lactated Ringer’s solution. An 18-gauge Hustead needle was inserted by using a midline or paramedian approach at the T6 through T10 level. Loss of resistance to saline (3 mL) was used to identify the epidural space. An epidural catheter (Perifix® 20-gauge closed-tip multipore; B. Braun Medical Inc., Bethlehem, PA) was placed 3 cm into the epidural space. A total of 5–10 mL 1% lidocaine was used for local anesthesia for these catheter placements.

A continuous transthoracic impedance cardiac output monitor, CIC-1000 (Sorba, Milwaukee, WI), was used to monitor cardiac index (CI). Transthoracic impedance measurement of CI was described by our group in a previous publication (8). The accuracy of changes in cardiac output detected by the CIC-1000 in normal healthy subjects correlates well with the changes measured in the same patients by using thermodilution (9,10).

After epidural catheter placement and attachment of noninvasive transthoracic impedance patch electrodes, the patient lay supine and undisturbed for 10 min. Before the epidural test dose, baseline hemodynamic values (heart rate [HR] and mean arterial blood pressure [MAP], measured via a radial arterial catheter) were recorded and continuously measured for the duration of the study. Transthoracic impedance CI was measured at baseline (before initial epidural test dose) and attempted every 5 min between T = 0 (begun at completion of epidural injection of the final aliquot of lidocaine) and T = 30.

A test dose of 3 mL 2% lidocaine (plain) (60 mg) was injected via the epidural catheter after a negative aspiration test. Three minutes later, if no signs of a motor block of the lower extremities were evident, an additional dose of 2 or 6 mL, according to a computer-generated table of random numbers, of the same local anesthetic solution was injected. Thus, the total volume of epidural local anesthetic injected was either 5 mL (Groups 1A [young 5] and 2A [old 5]) or 9 mL (Groups 1B [young 9] and 2B [old 9]).

Beginning at T = 0, the extent of dermatomal analgesia by pinprick and cold insensitivity was assessed every minute until the first detectable blockade, then every 5 min until 30 min after T = 0. Anesthesia was defined as absence of any sensation over a tested dermatome and was also recorded every 5 min. Analgesia was defined as absence of a sharp sensation when compared with an unblocked dermatome (e.g., shoulder). Cold insensitivity was noted when the patient could not detect a cold sensation when touched by an ice-filled test tube (previously filled with water, frozen, and stored on ice between uses), with an unblocked shoulder dermatome for reference. Both cephalad and caudad extent of analgesia, anesthesia, and cold insensitivity were tested bilaterally in the midclavicular line, starting at the dermatome of epidural catheter insertion. If bilateral blockade was not detected within 15 min, the study was terminated and the patient’s data excluded. An examiner blinded to the volume of local anesthetic injected performed these tests.

Time to first detectable block, time to maximum spread of analgesia, cold insensitivity and anesthesia, initial spread (number of dermatomes) of analgesia at the first testing (T = 0), and maximum number of dermatomes of analgesia, cold insensitivity, and anesthesia were recorded for each patient.

Mean ± SD or frequency with percentage (%) was used for reporting descriptive statistics for continuous or discrete variables, respectively. P < 0.05 was considered significant. Segmental dose requirements (milliliters of 2% lidocaine/dermatome) were calculated. For a discrete variable, the Mantel-Haenszel test was used to compare differences in proportions between the two age groups and between the two volumes of epidural local anesthetics. For a continuous variable, two-way analysis of variance (ANOVA) was used to compare the differences in means between two age groups and between two volumes. Repeated-measure analysis (three-way ANOVA) by PROC MIXED was used to compare differences in means (mean maximum segments anesthetized, for example) among three response levels (analgesia, loss of cold sensation, and anesthesia). Dermatomes S5 to S1, L5 to L1, T12 to T1, and C8 to C1 were coded as 1 to 5, 6 to 10, 11 to 22, and 23 to 30, respectively. The statistical software SAS (SAS Corporation, Cary, NC) was used for analysis.

	Results

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Demographic data are shown in Table 1. Although we had IRB approval for 56 patients, because of rotation of surgeons and anesthesiologists, only 50 patients were enrolled initially, with 48 completing the study. One patient from Group IA (young 5) developed no detectable blockade within 15 min of epidural lidocaine injection. Another patient from Group IB (young 9) developed a unilateral block. Both patients were excluded from the study.

View larger version (29K): [in this window] [in a new window]   Figure 1. Segmental blockade after 2% lidocaine injection via thoracic epidural catheter, young versus old patients (A = 100 mg, B = 180 mg). Group IA = young 5; Group IB = young 9; Group IIA = old 5; Group IIB = old 9. Bars denote SE. *Significance between volume and age groups, $significance between age groups only. There was a significant difference in total dermatomal segments blocked, younger versus older group, at time intervals 5, 15, 20, 25, and 30 min (P = 0.0136, 0.0251, 0.0090, 0.0184, and 0.0306, respectively) and between volumes at times 15, 20, 25, and 30 min (P = 0.0203, 0.0154, 0.0136, and 0.0318, respectively). There was a significant difference noted between blocked dermatomal segments of analgesia and anesthesia (P = 0.0001) at every 5-min interval from time 0 to 30 min. No significant interaction effects were noted among levels, age groups, and volume groups for any time interval. Repeated-measures three-way analysis of variance was used.

View larger version (30K): [in this window] [in a new window]   Figure 2. Mean analgesic spread (cephalad/caudad) after 2% lidocaine injection via thoracic epidural catheter. Group IA = young 5; Group IB = young 9; Group IIA = old 5; Group IIB = old 9. At time point 0 (T= 0) only, the mean dermatomal level was calculated excluding patients without detectable blockade. *Significant difference in upper (cephalad) levels between volume groups and between age groups at T = 20, 25, and 30 (volume P = 0.0449, 0.0256, and 0.0297, respectively) (age P = 0.0088, 0.0191, and 0.0263, respectively). #Significant difference in upper (cephalad) levels between volumes only at T = 15 (P = 0.0099). No significant interaction effects were noted between age groups and volume groups for any time interval. Two-way analysis of variance was used.

View larger version (34K): [in this window] [in a new window]   Figure 3. Pulse and mean arterial pressure after 2% lidocaine injection via thoracic epidural catheter. Group IA = young 5; Group IB = young 9; Group IIA = old 5; Group IIB = old 9. B = baseline, MAP = mean arterial pressure. *Significant difference between age groups at T = 20 min for pulse (P = 0.0334) and at T = 25 and 30 min for MAP (P = 0.0379 and 0.0430, respectively). No significant interaction effects were noted among age groups and volume groups for any time interval. Two-way analysis of variance was used.

	Discussion

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In both age groups, maximum spread occurred later with the larger doses at about the same time, i.e., at approximately 20 minutes after injection. The upper level of analgesia and cold insensitivity in both age groups and with both lidocaine doses reached at least to the T3 dermatomes within 10 minutes after injection. In our study, anesthesia was not detectable until 10 minutes after injection and reached a maximum of two dermatomes 20 minutes after lidocaine injection. Therefore, anesthesia was not a useful early indicator of proper epidural catheter location after injection of lidocaine 100–180 mg.

The maximum decreases in HR and MAP were seen from 17 to 26 minutes after lidocaine injection and were significantly more in the older patients at T = 20 for HR and at T = 25 and 30 for MAP. CI and total peripheral resistance data were averaged for the period T = 15–25 minutes and were relatively unchanged with respect to baseline in all groups. All of the younger patients in this study were hemodynamically stable with both doses of lidocaine. Only two older patients, both of whom received lidocaine 180 mg, became hemodynamically unstable: in both patients the MAP had decreased by 20% (gradually over 20 and 15 minutes) when an abrupt decrease in HR of 25% and 50% resulted in MAP’s decreasing to 50% of baseline. Immediate intervention, as described, promptly restored stability, but these precipitous decreases probably would not respond as well if compounded with an induction dose of propofol or thiopental. Our hemodynamic data are consistent with the findings of Sjögren and Wright (11), who also found little change in HR, MAP, or total peripheral resistance after injection of lidocaine 160 mg (8 mL of 2% lidocaine) at T5-T6 in patients ranging from 20 to 50 years old. Therefore, in younger patients, few hemodynamic changes are observed.

Previous studies of lumbar epidural anesthesia have documented a significantly larger spread for a given dose in older patients (2,3,12,13). When, exactly, changes in spread of epidural analgesia become clinically significant are a matter of some interpretation. Park et al. (2) noted an increased spread of three dermatomal levels when they compared an "older" group (40–59 years) with a "younger" group (20–39 years) of patients receiving lidocaine doses of 150, 225, and 300 mg (10, 15, and 20 mL of 1.5% lidocaine with 1:200,000 epinephrine). Neither Park et al. (2) nor Sharrock (3) was able to demonstrate a linear relationship between age and segmental dose requirements during lumbar epidural anesthesia. Bromage (12) calculated that dose requirements increase until age 18.5 years and decrease thereafter in a linear fashion. Our data confirm the effect of age (increased spread) for TEA, with patients 56 years old demonstrating more total and cephalad spread than patients 51 years old for a given lidocaine dose.

The mechanism of increased spread of epidural analgesia in older patients is unknown. Some investigators have demonstrated a significant increase in pressure in a group of patients with previous laminectomy and "diseased" epidural space (14). They note that age-related changes may similarly compromise the epidural space in the elderly. Bromage and others have described a decreased egress of injected fluids via neural foramina in older patients, resulting in an increase in the spread for any given volume of injectate (5). Facet hypertrophy and calcification of tissue seen with aging may indeed cause the neural foramina to act as a Starling resistor, limiting egress of local anesthetic from the epidural space (14). In addition, a decrease in the number of myelinated nerve fibers in the nerve, as well as a general deterioration of the mucopolysaccharides of the ground substance, allows local anesthetics to more easily penetrate nerve roots in older patients (15). These factors may act in concert to increase susceptibility to neural blockade by epidural local anesthetic solutions and thus explain an increased spread of analgesia in older patients.

We used pinprick to detect anesthesia and analgesia. Some authors have advocated using a more noxious stimulus, such as a nerve stimulator at 50–100 Hz, to mimic surgical stimulation. We choose to use pinprick because this technique is widely used and easily available to all anesthesiologists. Our groups were small (n = 10–14), so the power of our study to predict adverse hemodynamic events is therefore low. In a larger cohort of patients, particularly in older patients, the incidence of hemodynamic instability could be more or less frequent than what we observed. The doses of lidocaine we used (100 and 180 mg) were clearly inadequate to produce surgical anesthesia. If an anesthesiologist plans to use a combined TEA/general anesthetic technique to provide surgical anesthesia, then injection of additional doses of local anesthetic would be necessary. In clinical practice, the onset and spread of TEA may be different than what we observed, depending on the choice of local anesthetic and dose.

We inserted thoracic epidural catheters at several levels (T6 to T10). Visser et al. (16) studied the extent of spread after thoracic injection of lidocaine 60 mg (3 mL of 2% lidocaine and 6 mL of 1% lidocaine) in 90 patients aged 18–80 years. No stratification on the basis of age was made. They found that the choice of the interspace among high (C7 to T2), mid (T3 to T5), and low (T7 to T9) thoracic injection does not influence the number of dermatomes of analgesia, although the site of puncture was highly correlated with the cephalad extent of blockade. Higher puncture sites resulted in less cephalad spread and more caudad spread. Lower thoracic puncture sites resulted in less caudad spread and more cephalad spread. Therefore, the number of dermatomes of analgesia does not seem to be greatly influenced by the level of thoracic epidural injection.

Small initial lidocaine doses (100 mg) result in easily detectable blockade without hemodynamic changes. Limiting initial blockade may reduce the hypotension often clinically observed after the induction of general anesthesia in combination with TEA (6). In clinical practice, a rapid onset of epidural blockade allows for early detection of failed epidural catheter placement and will give the anesthesiologist time to replace the catheter before the start of surgery. This factor is important for efficient operating room turnover when combined epidural/general anesthesia is planned. We conclude that after thoracic epidural injection of 100 mg lidocaine, bilateral and symmetrical analgesia of at least five to eight dermatomes with an upper dermatomal level of at least T3 should develop within five minutes, or the epidural catheter should be replaced.

	Footnotes

 
Presented in part at the annual meeting of the American Society of Regional Anesthesia, Seattle, WA, May 14–17, 1998.

The opinions expressed in this article represent the personal opinions of the authors and do not represent official policy of the Department of Defense, the Department of the Navy, or the Uniformed Services University.

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