1 Vascular Diseases Research Unit, The Institute of Cardiovascular Research and 2 University Department of Anaesthesia, Ninewells Hospital and Medical School, Dundee DD1 9SY, UK
* Corresponding author. E-mail: d.j.newton{at}dundee.ac.uk
Accepted for publication January 10, 2005.
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Abstract |
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Methods. Laser Doppler imaging was used to measure the forearm skin blood flow responses to intradermal injection of eight doses of bupivacaine and levobupivacaine in 10 healthy male volunteers. The doses tested ranged from 0.008% to 0.75%, and the five highest doses were administered both with and without adjuvant epinephrine 2.5 µg ml1.
Results. The cumulative responses to the lower subclinical concentrations (0.0080.0625%) of both anaesthetics were smaller than or similar to that produced by saline alone, indicating a net vasoconstrictive effect. Higher doses caused net vasodilatation, and the levobupivacaine responses were generally lower than the corresponding bupivacaine responses (P=0.022). Epinephrine 2.5 µg ml1 significantly reduced the responses to clinical doses of both drugs (P<0.001), producing net vasoconstriction.
Conclusions. Bupivacaine and levobupivacaine have a biphasic vascular effect when injected intradermally, with subclinical doses causing net vasoconstriction. The addition of epinephrine 2.5 µg ml1 decreases these responses markedly.
Keywords: anaesthetics local, bupivacaine ; anaesthetics local, levobupivacaine ; analgesic techniques, infiltration ; sympathetic nervous system, epinephrine
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Introduction |
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In common with other amide anaesthetics, bupivacaine causes vasodilatation at clinical doses when administered by intradermal injection.4 5 Lower doses appear to cause vasoconstriction, although to date this has only been observed in isolated smooth muscle6 and animal models,7 and by subjective assessment of skin colour.8 Similar findings for levobupivacaine indicate that this (S)-isomer of bupivacaine may also have a biphasic effect on the cutaneous microcirculation.5 8
The aim of the present study was to characterize fully the vasoactive effects of bupivacaine and levobupivacaine using a range of subclinical and clinical doses, with or without the addition of epinephrine, in the peripheral skin of healthy humans. Laser Doppler imaging was used to provide an objective measure of skin blood flow.
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Methods |
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Injection sites, approximately 3 cm apart, were marked on the volar surface of both forearms, and the skin in this area was cleaned with an alcohol swab. Using a 27 G needle, we injected 0.1 ml of 0.9% physiological saline and eight concentrations of bupivacaine (Astra Zeneca, Luton, UK) and levobupivacaine (Abbott Laboratories, Kent, UK) intradermally at these sites over the course of two visits. The sites of injection were randomized over both arms and both visits, and were unknown to the participant and investigator. The visits were at least a week apart. The concentrations tested were 0.008%, 0.016%, 0.031%, 0.0625%, 0.125%, 0.25%, 0.5% and 0.75% of both anaesthetics, and the injections were made up in saline. At each visit, the injections were given consecutively over a period of 1 min. Pilot experiments demonstrated that a 3 cm separation between injection sites was sufficient to prevent the responses overlapping.
These experiments were repeated with 2.5 µg ml1 of epinephrine added to the five highest doses (0.0625%, 0.125%, 0.25%, 0.5% and 0.75%) of both drugs. This concentration of epinephrine was chosen because we demonstrated recently that it is as effective a vasoconstrictor as a concentration of 5 µg ml1 when injected intradermally alone or in combination with bupivacaine or levobupivacaine.9
We measured the microvascular skin blood flow at the delivery sites using laser Doppler imaging (moorLDI, Moor Instruments Ltd, Axminster, UK). This instrument works by scanning a 2 mW heliumneon laser across the surface of the skin. Light backscattered from moving erythrocytes is shifted in frequency by an amount proportional to their velocity according to the Doppler principle. These Doppler shifts are collected and processed by the instrument, which produces a colour-coded image representing a relative measure of skin perfusion (laser Doppler flux) in two dimensions.10 We have used this technique in previous studies of local anaesthetic vasoactivity,5 11 12 and the imager was calibrated monthly using a flux standard to ensure signal stability and measurement reproducibility.
The laser head was positioned 50 cm above the measurement site, with a hood mirror deflecting the beam onto the skin surface. The scan region, encompassing all four or five injection sites on each arm, was 10.5x5 cm. This translated to an area of 100x100 pixels onscreen after the resolution had been reduced to reduce the scan time to 1 min. We recorded images before and at 2, 10, 20, 40 and 60 min after the injections, and each arm was scanned in turn.
We analysed the images using dedicated image-processing software (Moor Instruments Ltd, Axminster, UK). A measure of blood flow at each site was obtained by calculating the median laser Doppler flux within a circular region (diameter 1 cm) centred on the point of injection. For an index of the total cumulative response over the first 20 min, we calculated the area under the response curve (AUC), in arbitrary units, with respect to the response to saline alone. All AUC results reported are divided by 1000.
The ShapiroWilk test revealed that all data were distributed normally. Differences between the two anaesthetics, and between the doses administered, were tested by univariate analysis of variance.
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Results |
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Adding epinephrine 2.5 µg ml1 to the five highest doses of the local anaesthetic preparations reduced the responses to both bupivacaine (Fig. 4) and levobupivacaine (Fig. 5). These responses were all below the response to saline, indicating a net vasoconstrictive effect (Fig. 6), and the cumulative responses with respect to saline were significantly lower than those without epinephrine (P<0.001 for both anaesthetics).
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Discussion |
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These biphasic characteristics have been noted before, although only in vitro or by subjective assessment. Using an isolated smooth muscle preparation from the umbilical artery, Tuvemo and Willdeck-Lund6 found that lower concentrations (525 µg ml1) of bupivacaine caused dose-dependent constriction, while higher concentrations (125250 µg ml1) caused relaxation. Similarly, Johns and colleagues7 noted constriction in the rat cremaster muscle arteriole with 0.1100 µg ml1 bupivacaine, with higher doses showing the tendency to dilate.7 However, the in vitro nature of these experiments means these findings cannot be compared directly with ours, although the trend is the same.
In vivo, Aps and Reynolds4 showed by visual inspection of skin colour that 0.250.5% bupivacaine produced vasodilatation, while there was either no change or else skin pallor in response to 0.125% bupivacaine. Studies using a more objective measurement technique (such as laser Doppler flowmetry or imaging) have reported dose-dependent vasodilatation to intradermal bupivacaine,5 13 14 but they have not looked at subclinical concentrations. Similar results have also been obtained with levobupivacaine,5 8 although this isomer tends to cause less dilatation and more constriction.15
Biphasic vasoactivity has also been reported after intradermal injection of other amide-type local anaesthetics. At concentrations <1%, lidocaine appears to cause cutaneous vasoconstriction on visual inspection of skin colour,4 16 while laser Doppler studies have revealed dose-dependent constriction in response to concentrations of ropivacaine <1%.13 17 These effects may be associated with tissue calcium. Åberg and colleagues18 19 suggested that the vasoconstrictor effect at low concentrations is caused by mobilization of Ca2+ in smooth muscle, while at high concentrations smooth muscle is relaxed by stabilization of the membrane surrounding Ca2+ stores, thereby inhibiting its release.
As reported previously,5 we found that levobupivacaine had greater vasoconstrictive properties than bupivacaine, even though levobupivacaine contains 11% more molecules, because it is expressed as the base rather than the hydrochloride.20
We looked at the vascular effects of these two drugs when used for infiltration anaesthesia in peripheral skin, and therefore we cannot extrapolate to other vascular beds such as the peripheral nerves or epidural space. In addition, we have not looked at the effects of anaesthetic or epinephrine dose on the duration of block. Pinprick tests to assess the depth and duration of analgesia were not practical here because they would have disturbed the blood flow that we were measuring. We also did not measure whether there was any effect on the blood levels of the anaesthetics. Kopacz and colleagues21 have shown that levels of levobupivacaine are reduced when epinephrine 2.5 or 5 µg ml1 is added to the anaesthetic mixture for spinal surgery. Studies of infiltration anaesthesia in this setting are still required.
We made up the local anaesthetic solutions in physiological saline, and we also used saline to control for the mechanical effects of injection (i.e. skin penetration and tissue compression). We have shown previously, using non-invasive iontophoretic delivery, that 0.5% sodium chloride has no vasodilatory effects in human skin.22
In summary, we have used the objective method of laser Doppler imaging to show that intradermal injection of subclinical concentrations of bupivacaine and levobupivacaine cause less vasodilatation than saline alone, suggesting a dose-dependent vasoconstrictive effect of both drugs. These findings confirm those of previous studies using in vitro models or more subjective, less accurate measurement techniques. Levobupivacaine was the more vasoconstrictive of the two drugs.
We have also shown that adding epinephrine 2.5 µg ml1 has a marked vasoconstrictive effect on the responses to both bupivacaine and levobupivacaine (in a previous study, we found that a larger dose of epinephrine had no additional effect9). Despite the introduction of new safer local anaesthetics such as levobupivacaine and ropivacaine, toxicity is still a problem,23 with reports of convulsions with levobupivacaine24 and ropivacaine,25 and cardiac arrest caused by frank overdose with ropivacaine.26 Our findings suggest that, in a clinical setting, adding epinephrine to these local anaesthetics has the potential to decrease the risk of systemic toxicity and to increase the margin of safety when giving large doses for infiltration anaesthesia.
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Acknowledgments |
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References |
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