EQUINE VETERINARY JOURNAL Equine vet. J. (2004) 36 (8) 723-726 723 Summary Reasons for performing study: Perineural analgesics are used for lameness diagnosis but the duration of effect, knowledge of which would provide valuable information when performing subsequent blocks, is unknown. Objective: To evaluate the duration of a palmar digital nerve block using force plate measurements. Methods: Ten horses diagnosed with unilateral navicular syndrome were trotted at range of 3 ± 0.15 m/sec over a force plate to record ground reaction forces for 5 trials of each forelimb. Data were recorded before nerve block, and then at 15 mins, 1, 2 and 24 h post nerve block. Results: Before nerve block, peak vertical force (mean ± s.e.) was significantly higher in the contralateral forelimb (CL = 5345 ± 188 N) than in the lame forelimb (L = 4256 ± 204 N; P<0.05). At 15 mins post nerve block there was no significant difference between the 2 forelimbs (CL = 5140 ± 184 N; L = 5126 ± 129 N), and this remained the case for 1 h. By 2 h, the mean score for the lame leg had decreased (L = 4642 ± 182 N) but was still greater than preblock. By 24 h, vertical forces had returned to preblock values. Conclusions: The palmar digital nerve block was fully effective between 15 mins and 1 h. The analgesic effect began to subside between 1 and 2 h but sufficient analgesia persisted to affect gait characteristics beyond 2 h. Potential relevance: When using a palmar digital nerve block, it is important to perform lameness evaluations between 15 mins and 1 h to be sure of effective nerve blockade. Introduction Mepivacaine hydrochloride produces conduction blockade in sensory neurons by slowing sodium influx into cells (Altman et al. 1985). The resulting local anaesthetic effect has a rapid onset of action, which is advantageous when perineural analgesia is used as an adjunct in lameness diagnosis (Harkins et al. 1999). A palmar digital nerve block is a useful tool to assess and localise lameness to the palmar aspect of the foot. Decrease in lameness after a block at this location may be used to support a diagnosis of navicular syndrome (Derksen 1980; Stashak 2002). When it is difficult to determine the origin of lameness, serial blocks may be performed, often beginning with a palmar digital nerve block and moving up the limb until resolution of the lameness is achieved (Wright 1993). Structures desensitised by a palmar digital nerve block can include the navicular bone, podotrochlear bursa, distal sesamoidean ligaments, distal parts of the deep and superficial flexor tendons and their sheaths, digital cushion, corium of the frog, sole and palmar aspects of the distal phalanx and proximal and distal interphalangeal joints. There is variability in the desensitisation of some of these structures related to dose and placement of anaesthetic (Anderson and Turner 1993). Ideally, the block should be placed at the proximal border of the lateral cartilages of the distal phalanx to reduce the chance of blocking the dorsal and intermediate branches of the palmar digital nerves (Wheat and Jones 1981). Local anaesthetic duration of procaine, tetracaine, lidocaine, mepivacaine and bupivacaine have been compared in studies involving rat sciatic nerve blocking procedures (Leblanc 1990; Dyhre et al. 1997) and in human studies (Moore et al. 1970). Previous reports have examined the effects and duration of mepivacaine in equine intra-articular spaces (Dyson and Kidd 1993; Andreen et al. 1994; Gough et al. 2002), the epidural space (Skarda et al. 1984) and systemically (Harkins et al. 1999). Knowledge of the duration of effect would provide valuable information when performing subsequent, additional blocks, especially if performed on different limbs. To evaluate the effects and duration of the anaesthetic, previous studies have used observation (Pleasant et al. 1997), motion analysis (Keegan et al. 1997), superficial and deep muscular pinprick stimulations (Skarda and Muir 1982), or serum (Skarda et al. 1984) and urine (Harkins et al. 1999) mepivacaine concentrations. Lameness and the effectiveness of a nerve block are difficult to evaluate objectively by observation (Keegan et al. 1998). A force plate offers an objective means of measuring forces induced during weightbearing and is an effective method of detecting biomechanical asymmetry that is known to exist with lameness between contralateral limb pairs (Khumsap et al. 2003). The objective of this study was to determine the duration of a mepivacaine nerve block over the palmar digital nerves in horses with navicular syndrome. Additionally, the usefulness of a cutaneous pinprick test to evaluate the effectiveness of a perineural anaesthetic block was evaluated. Materials and methods The study included 10 client-owned horses, age range 5–12 years, weight 473–558 kg and of American Quarter Horse, Paint Horse Mepivacaine local anaesthetic duration in equine palmar digital nerve blocks L. A. BIDWELL* † , K. E. BROWN † , A. CORDIER ‡ , D. R. MULLINEAUX ‡ and H. M. CLAYTON ‡ † Large Animal Clinical Sciences and ‡ McPhail Equine Performance Center, Michigan State University, East Lansing, Michigan 48824, USA. Keywords: horse; local anaesthetic; mepivacaine; navicular syndrome; palmar digital nerve *Author to whom correspondence should be addressed. [Paper received for publication 10.05.04; Accepted 01.11.04]