Diffusion of radiodense contrast medium after a mid‐pastern ring block

Abstract

Diffusion of local anaesthetic solution after a mid-pastern ring block has not previously been investigated. The aim of this study was to demonstrate potential distribution of local anaesthetic solution following injection of radiodense contrast medium as performed for a mid-pastern ring block. Twelve mature horses were used and 1.5 ml radiodense contrast medium injected over the medial or lateral palmar digital nerve at the level of the proximal aspect of the ungular cartilages. A dorsal ring block was performed on the ipsilateral side, 1.5 cm proximal to the palpable palmar aspect of the proximal eminence of the middle phalanx, using 2 or 5 ml contrast medium. Both forelimbs were injected on 2 days (48 injections). Four standard radiographic views of the pastern were obtained immediately, 10 and 20 min after injections. Images were analysed subjectively and objectively. After dorsal injections, the contrast medium was distributed in a diffuse patch over the ipsilateral half of the proximal phalanx (P1), extending proximally over the half of the length of P1 in all limbs (greatest proximal extension: 89.0% of the length of P1 [from distal] after 2 ml, 94.2% after 5 ml). There was significant proximal diffusion in the first 10 min after injection and significant dorsal diffusion between all time points (P<0.01). There was significant positive association between injected volume and the proximal extension of the dorsal contrast patch (P = 0.01). The median dorsal diffusion was to the dorsal midline of P1; 5 ml contrast medium resulted in significantly greater dorsal diffusion than 2 ml (P<0.01). The dorsal and palmar contrast patches did not merge. In conclusion, diffusion to the proximal aspect of P1 following a mid-pastern ring block may occur even if only 2 ml of local anaesthetic solution is used.

The effect of a compression bandage on the distribution of radiodense contrast medium after palmar digital nerve blocks

“…Spread of contrast medium along lymphatic vessels was observed in the nonbandaged limbs, and over the 30-min observation period, the contrast patch became more indistinctpossibly due to contrast medium being drained from the area. These findings have been described in previous studies [2,3], but ours is the first to report prevention of lymphatic spread with a compression bandage. Clinical effects, if any, of distribution of injected drugs along lymphatic vessels are not known.…”

“…The fact that contrast medium did not diffuse proximal to the distal end of the bandage may prove to be more clinically relevant than the diffusion distances, as these findings suggest that the compression bandage may be placed at any anatomical location where the clinician wants the proximal diffusion to stop. The compression bandage also prevented the distribution of the contrast medium to the lymphatic vessels demonstrated both by us after perineural injection without use of the bandage and in several previous studies [1][2][3][4][5][6]8]. The bandage may thus cause injected substances to remain localised at the injection site for a longer duration.…”

“…Whether contrast medium can be used as an analogue for local anaesthetics is currently not clear. Contrast medium and mepivacaine differ with regard to viscosity, molecular weight and osmolality, and others have suggested that this could cause dissimilar tissue diffusion [1,2]. Future studies investigating specificity (area of desensitisation) and duration of palmar digital nerve blocks after application of a compression bandage are thus warranted.…”

“…Multiple studies have shown that diffusion occurs in palmar digital nerve blocks [1], mid-pastern ring blocks [2], abaxial sesamoid nerve blocks [3], low 4-point blocks [4,5] and in subcarpal [6] and subtarsal blocks [7,8]. This diffusion has been shown with injection of contrast medium [3,4], contrast medium and dye [1][2][3][4][5], local analgesics [8] and local analgesics and contrast medium [3][4][5].…”

“…This diffusion causes the injected material to spread over a larger area than anticipated, which may reduce the specificity of the nerve block. In addition, radiographic studies have shown that contrast material is removed by lymphatic drainage from the injection site after perineural injection of radiodense contrast medium [1][2][3][4][5][6]8]. It is not clear how this affects specificity of the block and duration of effects of the injected drugs; it has been suggested to potentially cause insufficient analgesia, when small volumes of local analgesics are used [1].…”