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From the Department of Anesthesiology, Queen's University, Kingston, Ontario, Canada.
Address correspondence to: Dr. Joel L. Parlow, Department of Anesthesiology, Kingston General Hospital, 76 Stuart Street, Kingston, Ontario K7L 2V7, Canada. Phone: 613-548-7827; Fax: 613-548-1375; Email: parlowj{at}post.queensu.ca
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Abstract |
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Methods: Forty-four ASA I–III patients undergoing lower limb orthopedic procedures completed this double-blind, placebo-controlled trial. Patients were randomized into one of two groups, receiving intrathecal bupivacaine 15 mg and preservative-free morphine 200 µg without (Control group), or with the addition of fentanyl 0.02 mg (Fentanyl group). Patients were maintained at a slight head-up tilt. Variables studied over three hours included sensory level to cold and pinprick, motor blockade (Bromage scale), and circulatory data.
Results: No differences existed between the Fentanyl and Control groups with respect to highest level of block for cold: T4 (T2–T5) vs T3.5 (T3–T8) respectively (median, 95% confidence interval) or pinprick: T4 (T3–T6) vs T4.5 (T3–T8). Similarly, there was no difference in the time taken to reach maximum block height to cold (20 ± 9 vs 23 ± 13 min, mean ± SD) or pinprick (20 ± 9 vs 24 ± 13 min).
Conclusion: The addition of fentanyl 0.02 mg to 0.5% bupivacaine with morphine does not affect the maximal block height or time to maximal block in clinical practice.
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Introduction |
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The addition of opioids to local anesthetic solutions for spinal anesthesia enhances surgical anesthesia and provides postoperative analgesia. Intrathecal morphine provides prolonged, good quality postoperative analgesia, while fentanyl has the advantage of decreasing visceral sensation and may prolong the duration of anesthesia.3,4 In vitro studies examining the effect of adding opioids to local anesthetics have shown morphine to cause minimal change in baricity of the final solution, whereas fentanyl decreases the density of 0.5% bupivacaine by up to 0.0011 g•mL–1,5,6 where a difference of as little as 0.0006 g•mL–1 in density alters the movement of local anesthetics in a spinal model.2 In a more recent study, our group found that the addition of fentanyl to 0.5% bupivacaine made the resultant solution hypobaric within simulated CSF in a spinal model, leading to increased upward spread.7
The objective of the current study was to determine whether the addition of fentanyl to an isobaric spinal local anesthetic alters the level of sensory block in the clinical setting.
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Methods |
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Patients were randomly allocated by the hospital research pharmacist into one of two groups. The control group received intrathecal injections of 3.4 mL of solution containing: isobaric bupivacaine 15 mg (Marcaine 0.5%, Sanofi Canada, Markham, Ontario), and preservative-free morphine 0.2 mg (morphine LP, 0.5 mg•mL–1, Sabex, Boucherville, Quebec). The fentanyl group received 3.8 mL of solution containing the above solution with the addition of fentanyl 0.02 mg (fentanyl citrate 0.05 mg•mL–1, DBL, Laval, Quebec). Study injectates were prepared in identical 5-mL syringes, with the volume of solution blinded by a label wrapped around the syringe.
Monitoring consisted of noninvasive blood pressure and electrocardiogram (Model 90603A, Spacelabs, Redmond, WA) and pulse oximetry. An iv cannula was inserted and 500 mL of Ringers lactate solution was infused over ten minutes. Patients were positioned in the sitting position, and using standard sterile procedure, the intrathecal space was entered using a 24G Sprotte needle at the L2–3 or L3–4 interspace. The study solution was then injected over 60 sec, and confirmation of intrathecal placement was obtained at the beginning and end of the injections. In order to encourage upward spread of hypobaric solutions, patients were kept in the sitting position for 60 sec, then positioned supine on the operating table with a 6° head-up tilt, calibrated using a magnetic protractor.
Sensory block level was assessed at five, ten, 15, 20, 30, 45, 60, 90, and 120 min after injection, using thermal (cold alcohol swab), and pinprick stimuli. Muscle power was assessed using a modified Bromage scale (0=no block, 1=hip flexion blocked, 2=hip and knee flexion blocked, 3=complete block of hip, knee and ankle ). Heart rate and blood pressure were recorded simultaneously. Hypotension, defined as greater than a 30% drop in resting systolic pressure or less than 90 mmHg, was treated with fluid resuscitation and ephedrine 5 mg iv. Bradycardia, defined as heart rate <50 beats•min–1 was treated with atropine 0.6 mg iv.
Baseline demographic data included age, sex, weight, height, concurrent medications, and past medical and surgical history. The type and duration of surgery, the interspace used for intrathecal injection, and neurological assessments were recorded. Episodes of hypotension or bradycardia and the need to convert to a general anesthetic were documented.
Using mean and variability data from previous work,2 sample size analysis indicated a need for a minimum of 17 subjects per group to demonstrate a difference in two dermatomes with a power of 0.80 and alpha error of 0.05. Data were analyzed using unpaired t tests for parametric data (change in blood pressure and heart rate), and Wilcoxon and Mann Whitney rank sums for between-group differences at each time point and maximal dermatomal block level. Incidence of adverse effects was compared using Chi-square test, and relationship of block level to patient demographic parameters using Spearman's rank order correlation.
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Results |
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). The figure illustrates the block height for both groups at each time period (cold stimulus). There was no significant difference found between groups at any time period. Similarly, there was no difference in the maximal level of block to either pinprick or cold stimulus, or in the time to reach the highest dermatome (Table II). There was fairly wide distribution about the median in both groups for maximum block height (Table II). No significant relationship existed between maximum block level and either patient height or weight. Motor block was equal between groups (Bromage score 3 in all patients). Similarly, there was no difference between groups in incidence of hypotension or bradycardia after institution of spinal anesthesia (Table II), with eight patients being treated for hypotension (six control, two fentanyl).
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Discussion |
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As opposed to studies utilizing a rigid and non-dynamic mechanical spinal model,7,9 a number of factors unique to the clinical situation are relevant to the wide variability in block level found. Although some patient factors such as height and weight showed no correlation with block level, other factors such as the volume of CSF in the lumbar space vary among individuals, and could be important in determining the degree of movement of solution within the CSF. However this was impossible to quantify or control in the current protocol.
A number of technical or procedure related factors may be relevant. Although some local anesthetics such as isobaric amethocaine have been shown to give more predictable spread with slower injection,10 no effect of injection speed was seen with 0.5% bupivacaine.11 In the current study, injection was performed slowly and steadily over one minute to minimize this source of variability. Patient position during and following intrathecal injection is also important in determining block level, although theoretically less so with isobaric agents. All patients were sitting for placement of the spinal anesthetic, and remained so for 60 sec after intrathecal injection. Throughout the duration of surgery the patient was maintained in a 6 head-up tilt. These manoeuvres were used to enhance cephalad spread of the local anesthetic and emphasize any difference in baricity between groups, hypothesizing that fentanyl reduces the baricity of the local anesthetic solution.5 However even with this approach, no difference in height of block was found between the two groups. Some anatomical factors related to positioning may exist which are not possible to control, such as the degree of thoracic kyphosis. This could conceivably influence the movement of CSF within the spinal canal, particularly with the use of head-up tilt, and increase the interindividual variability seen.
With regard to the solutions injected, the dose of bupivacaine, morphine and fentanyl used reflects common clinical practice within our institution. Using formulas devised previously,5 the relative densities of the two solutions can be calculated. The change in density due to fentanyl may thus be calculated to be as low as 0.00040 or as high as 0.00067 g•mL–1, depending on the reference values used.5,9 Since a difference of 0.0006 g•mL–1 has been shown to influence the spread of local anesthetic in CSF,2 it is possible that the dose of fentanyl used in this study did not affect the density sufficiently to consistently alter the movement of the solution in CSF. In addition, CSF density may vary, leading to variability in baricity of the injected solution among individuals. We did not analyze CSF density in the current study, although our subjects were nonpregnant and mostly healthy patients, whose CSF density would be expected to fall within a relatively narrow range.13 Morphine, in the dose used in the current study, does not substantially alter the density of a solution with bupivacaine 0.5%, or affect its characteristics in a CSF model.5,7 Thus the findings of this study would not likely be different had fentanyl been mixed with bupivacaine 0.5% alone. Finally, temperature of the injectate affects its spread, with solutions warmed to 37° showing different characteristics in CSF than room temperature solutions.2,14,15 Although we did not control the temperature of the injectate, the ambient temperature of the operating suite used during the study was kept within a narrow range.
In conclusion, unlike the results found in the laboratory setting5 and artificial spine model,7 the addition of fentanyl to isobaric 0.5% bupivacaine with morphine does not result in a predictable difference in the height of anesthetic block in patients undergoing lower limb procedures. This may be due to wide variability in block level, related to patient or operator characteristics, dynamics of CSF flow, or the relative densities of solutions used.
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References |
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2 Stienstra R, Gielen M, Kroon JW, van Poorten F. The influence of temperature and speed of injection on the distribution of a solution containing bupivacaine and methylene blue in a spinal canal model. Reg Anesth 1990; 15: 6–11.[Medline]
3 Choi DH, Ahn HJ, Kim MH. Bupivacaine-sparing effect of fentanyl in spinal anesthesia for cesarean delivery. Reg Anesth Pain Med 2000; 25: 240–5.[Medline]
4 Shende D, Cooper GM, Bowden MI. The influence of intrathecal fentanyl on the characteristics of subarachnoid block for caesarean section. Anaesthesia 1998; 53: 706–10.[Medline]
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Hare GMT, Ngan JCS. Density determination of local anaesthetic opioid mixtures for spinal anaesthesia. Can J Anaesth 1998; 45: 341–6.
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Lui ACP, Polis TZ, Cicutti NJ. Densities of cerebrospinal fluid and spinal anaesthetic solutions in surgical patients at body temperature. Can J Anaesth 1998; 45: 297–303.
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Parlow JL, Money P, Chan PSL, Raymond J, Milne B. Addition of opioids alters the density and spread of intrathecal local anesthetics? An in vitro study. Can J Anesth 1999; 46: 66–70.
8 Cameron AE, Arnold RW, Ghons MW, Jamieson V. Spinal analgesia using bupivacaine 0.5% plain. Variation in the extent of the block with patient age. Anaesthesia 1981; 36: 318–22.[Medline]
9 Lui ACP, Munhall RJ, Winnie AP, Selander D. Baricity and the distribution of lidocaine in a spinal canal model. Can J Anaesth 1991; 38: 522–6.
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McClure JH, Brown DT, Wildsmith JAW. Effect of injected volume and speed of injection on the spread of spinal anaesthesia with isobaric amethocaine. Br J Anaesth 1982; 54: 917–20.
11 Stienstra R, van Poorten F. Speed of injection does not affect subarachnoid distribution of plain bupivacaine 0.5%. Reg Anesth 1990; 15: 208–10.[Medline]
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Nicol ME, Holdcroft A. Density of intrathecal agents. Br J Anaesth 1992; 68: 60–3.
13 Richardson MG, Wissler RN. Density of lumbar cerebrospinal fluid in pregnant and nonpregnant humans. Anesthesiology 1996; 85: 326–30.[Medline]
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Kristofferson E, Sloth E, Husted JC, Bach AB, Husegaard HC, Zülow I. Spinal anaesthesia with plain 0.5% bupivacaine at 19°C and 37°C. Br J Anaesth 1990; 65: 504–7.
15 Beardsworth D, Lambert DH. Warming 0.5% bupivacaine to 37°C increases duration of spinal anesthesia. Reg Anesth 1989; 14: 199–202.[Medline]
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