A model of ischemic pain in the calf and tens modification of lower limb pain: preliminary results.

Abstract

PURPOSE: To develop a model of laboratory induced ischemic calf pain; to test the analgesic efficacy of TENS for calf pain, and to describe induced calf pain, in healthy volunteers. RELEVANCE: We aimed to modify the sub maximum effort tourniquet technique (SMETT), a standardised method of inducing experimental ischemic pain into the upper limb of volunteers, to create a laboratory model of lower limb ischemia (LLI). With exercise, LLI could serve as a model of intermittent claudication (IC) - the exercise induced ischemic pain that is the main symptom of Peripheral Arterial Disease (PAD). Testing TENS for LLI pain could indicate its potential efficacy in IC related to PAD. PARTICIPANTS: 14 healthy males volunteered for Experiment 1 (E1) and 20 healthy females for Experiment 2 (E2). Subjects met inclusion and exclusion criteria for SMETT. METHODS: E1 was a within subject, crossover, single-blind study. A pilot study was undertaken with 4 independent subjects. The SMETT technique (with broad pressure cuff) was applied to the thigh. Subjects then performed 20 resisted plantar-flexion / dorsi-flexion exercises, using a pulley and weight apparatus developed for the study, in order to induce ischemia over a period of 5 minutes. Measurment of the Post Occlusive Reactive Hyperaemic response (PORHmax), demonstrated that ischemia had occurred in the calf of each subject, via the adapted SMETT technique. The main study in E1 applied (A) the standard SMETT in the upper limb and 20 minutes later (B) the modified SMETT (minus PORHmax) in the lower limb. Each application was for a maximum of 20 minutes. A linear pain scale rated subjective reports of pain intensity, at intervals between self-reported 'pain threshold' and 'pain tolerance'. The cuff was deflated at 'pain tolerance', or at 20 minutes, whichever came first. Subjects retrospectively described the ischemic pain in (i) the forearm and (ii) the calf, five minutes after cuff deflation. E2 utilised the E1 procedure for inducing lower limb ischemia and recording pain, to conduct an independent subject study of continuous TENS (100Hz, 200us) for induced LLI. Subjects were randomly allocated to a Control or TENS group. In the TENS group, electrodes were placed proximal and distal to the area of pain in the calf. ANALYSIS: Parametric or nonparametric tests examined differences between conditions; linear pain scales and weighted-rank MPQ - Pain Rating Index (PRI) scores For all statistical tests p-<-0.05. RESULTS: In E1, pain ratings at 300, 600, 900 12000 seconds showed similar incremental increases in pain intensity that are typical of SMETT induced ischemic pain, over time. In E2, TENS significantly raised the mean pain threshold and tolerance levels, and reduced MPQ-PRI scores, compared to controls (P-<-0.01 all comparisons). Qualitative MPQ descriptions reliably identified 4 sensory and 1 affective subclass of words to describe induced LLI pain. CONCLUSIONS: Lower limb ischemic pain induced with a modified SMETT can be modified by continuous TENS. IMPLICATIONS: A study into the efficacy of TENS during exercise in patients with PAD is indicated. Ref. 1. ROCHE PA et al.Physiotherapy Theory and Practice 2001, 18: 129-138 KEYWORDS: Pain; laboratory model, electrical stimulation. FUNDING ACKNOWLEDGEMENTS: None. CONTACT: proche@qmuc.ac.uk