1 Department of Anaesthesia and Intensive Care, Tartu University Clinics, Tartu, Estonia. 2 Department of Anaesthesiology and Intensive Care, Turku University Hospital, Turku, Finland
* Corresponding author: Department of Anaesthesia and Intensive Care, Tartu University Clinics, 8 L. Puusepp Street, 51014 Tartu, Estonia. E-mail: andres.sell{at}kliinikum.ee
Accepted for publication August 28, 2004.
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Abstract |
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Methods. Forty-one patients undergoing hip replacement surgery were randomly allocated to one of the two local anaesthetic groups in a double-blind manner. The initial dose of local anaesthetic was determined by the response of the previous patient: the effective dose resulted in a 1 mg decrease in the dose of levobupivacaine or ropivacaine, and an ineffective dose resulted in a 1 mg increase. The MLAD was calculated by the Dixon up-and-down method.
Results. The MLAD of levobupivacaine was 11.7 mg (95% CI, 11.112.4) and that of ropivacaine 12.8 mg (95% CI, 12.213.4).
Conclusions. These doses are significantly smaller than doses reported before for single-shot spinal anaesthesia. Continuous spinal anaesthesia allows the use of relatively small doses of local anaesthetic.
Keywords: anaesthetic techniques, spinal ; pharmacology, levobupivacaine ; pharmacology, ropivacaine ; surgery, orthopaedic
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Introduction |
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Patients and methods |
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A spinal catheter with a tip hole and an additional side hole 7 mm from the tip (Spinocath®; B. Braun, Melsungen, Germany) was introduced 22.5 cm into the subarachnoid space at the L34 interspace in the midline with the patient in the lateral position and the side to be operated up. The study drug (3.28.5 ml) was administered manually via the intraspinal catheter at an injection speed of 2 ml in 30 s. The dose of the study drug was determined by the up-and-down method of Dixon.8 Based on our previous clinical experience, the intraspinal levobupivacaine dose for the first patient allocated to receive levobupivacaine was 12 mg. Similarly, the ropivacaine dose was 14 mg for the first patient. Twenty minutes after administration of the study drug, the following criteria were assessed: (i) response to pinprick at T12 dermatome level on the side of surgery; (ii) response to transcutaneous tetanic electric stimulation (50 Hz at 60 mA) for 5 s at T12 level on the side of surgery; and (iii) motor function on the side of surgery assessed with the modified Bromage scale (0=no motor block, 1=inability to raise extended legs, 2=inability to flex knees, 3=inability to flex ankle joints).9 Anaesthesia was considered successful when there was loss of sensation to pinprick and to tetanic electric stimulation, and complete motor block. If successful anaesthesia according to the above criteria was achieved within 20 min from study drug injection, the dose of the study drug for the next patient was decreased by 1 mg in that group (levobupivacaine or ropivacaine). Conversely, if successful anaesthesia was not observed, the dose of the study drug for the next patient was increased by 1 mg in that group. The study ended at the time when clinical anaesthesia was assessed. However, if successful clinical anaesthesia was not achieved within 20 min, additional doses of 26 mg of the study drug were administered for surgical anaesthesia. Patient monitoring included ECG, non-invasive measurement of blood pressure at 3-min intervals and recording of peripheral oxygen saturation. In addition, all drugs administered throughout surgery were recorded. Ephedrine was used for hypotension at the discretion of the attending anaesthetist.
Statistics
The sample size was based on previous literature, which has demonstrated that at least six independent pairs of patients with sufficient anaesthesia/insufficient anaesthesia (response/no response pairs) should provide reliable estimates of MLAD using the up-and-down method of Dixon.8 10 Dixon's method was used to calculate the MLAD with 95% confidence intervals.8 The data are presented as mean (SD) unless mentioned otherwise.
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Results |
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Discussion |
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An index that would be more relevant clinically than the MLAD would be the local anaesthetic dose that provides sufficient anaesthesia for 95% of the patients. As we used Dixon's up-and-down method to determine the MLAD, our data concentrate on the MLAD range. Thus, our study design does not allow us to evaluate the end of the doseresponse curve.10
There was a difference in the mean MLAD volumes of levobupivacaine (4.68 ml) and ropivacaine (6.4 ml), because of both the dose requirement in milligrams and also the slightly different concentrations of the available commercial formulations. However, previous studies indicate that the total dose of local anaesthetic in milligrams rather than the volume seems to be the more important in determining the extent of spinal anaesthesia.11 12 The total dose also determines the duration of the block.13 Thus, the difference in the volume of the study drugs in the present study probably has little or no effect on the extent of the block and the MLADs.
We defined sufficient anaesthesia for hip replacement surgery as loss of pinprick at the T12 dermatome. This definition, though often used clinically, might have been too low as one in six of the patients in whom anaesthesia was initially sufficient required additional local anaesthetic to complete surgery. Nevertheless, supplementary dosing with local anaesthetic was feasible using the intraspinal catheter. The previously reported dose of isobaric levobupivacaine for single-shot spinal anaesthesia for hip replacement surgery is 17.5 mg,14 and for isobaric ropivacaine doses of 17.525 mg have been used.15 16 The mean total doses of local anaesthetic required to complete surgery in our study are somewhat lower, though in the range of those reported earlier and higher than the calculated MLAD. Unfortunately, the clinical time frame did not allow us to assess the local anaesthetic requirements after the first 20 min post drug injection more closely (i.e. in small supplementary local anaesthetic increments). Another factor that may have influenced our result is the direction which the catheter tip takes upon insertion. It has been shown that in patients with cranially running catheters or catheters with the tip at the level of the puncture site, the onset of analgesia was faster and the required doses of local anaesthetics were smaller than in patients with caudally running catheters.17 Unfortunately, we were not able to assess whether the catheter tip slid cranially or caudally. As the catheter was inserted for 2.02.5 cm into the intrathecal space, this might potentially result in a difference of 5 cm (i.e. approximately one interspace) in the catheter tip position from one patient to another. Further studies assessing the effect of catheter tip position on the local anaesthetic dose requirements are warranted.
Potential neurotoxicity related to the use of intrathecal catheters has recently become a concern because of reports of cauda equina syndrome. Most published cases have been associated with the use of 5% lidocaine in hyperbaric (7.5%) dextrose.18 Unfortunately, only a few prospective studies have formally investigated the real incidence of neurological complications.19 At present we do not have sufficient proof that levobupivacaine or ropivacaine would be safer than lidocaine in this respect. Thus, neurological symptoms have to be surveyed carefully in all patients in whom the CSA technique is used until further evidence is available.
In conclusion, continuous spinal anaesthesia using an intraspinal catheter allowed the use of small doses of local anaesthetic without compromising patient comfort.
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References |
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