Quantifying cutaneous vasodilation reflexes in lower limb amputees

Introduction:: Diabetic peripheral neuropathy (DPN) contributes significantly to negative clinical outcomes and a high risk of amputation revision in diabetic amputees [1]. This includes difficulties with mobility and poor tissue health in the residuum [2]. Damage to autonomic fibers can cause a loss of sympathetically mediated vascular regulation, contributing to poor circulation. Several sympathetic reflex pathways have been implicated in limb ischemia and perfusion abnormalities in people with DPN, such as loss of the axon reflex which causes local vasodilation in response to heat or pain [3]. This phenomenon has not been well studied in either dysvascular or traumatic amputees, and its effect on local microcirculation has not been robustly quantified in real time. As a tool to probe the autonomic nervous system, measuring this reflex has important implications for the development of therapeutic technologies for diabetic peripheral neuropathy.


Materials and Methods:: Cutaneous red blood cell flux and skin temperature were measured in the intact and residual limbs of three traumatic transtibial amputees during activation of the thermal axon reflex. Flux measurements were performed using a laser Doppler fluximeter (Moor Instruments Inc), which quantifies the movement of red blood cells through the microvasculature based on the backscattering of light passed through the skin. The thermal axon reflex was activated by skin surface heating to 42°C with a heating pad. Flux and temperature were recorded for at least 10 minutes via small adhesive probes placed on the lateral aspects of the intact and residual limbs. Heart rate and mean arterial pressure were measured continuously via afinger cuff using the volume clamp technique (Finapres Medical Systems). Conductance of the cutaneous microvasculature was calculated by dividing the measured flux by the mean arterial pressure (MAP). A smoothing spline was applied to conductance and temperature data. The vasodilation reflex was modeled as a step response. The rise time, fold increase from lower to upper crossing, and overall change from lower to upper state was calculated for each trial (Figure 1). The temperature threshold for reflex action was defined as the temperature at the lower crossing. The unheated intact limb was used as a control.


Results, Conclusions, and Discussions:: A robust reflex response was observed in all trials (n = 6), with an average fold increase in conductance of 2.01 (± 0.44). The mean rise time was 3.86 (± 1.89) minutes. The average temperature threshold for activation of the reflex was 37.6°C (± 2.75). The difference between upper and lower states of the response was significantly greater in the heated limb than in the unheated limb (p = 0.0039, Kruskal-Wallis rank sum) as shown in Figure 2. These results demonstrate that a thermal vasodilation reflex can be reliably evoked and quantified in traumatic lower limb amputees at non-nociceptive temperatures and short time scales. Extensive work has been done characterizing the effects of peripheral nerve stimulation on sensory and motor nerves in amputees for a variety of applications, but the impact of these interventions on autonomic fibers is still not as well understood. This reflex could serve as a valuable tool for probing the autonomic nervous system, particularly in the context of peripheral neuropathy. This work represents part of a larger study that will investigate the effects of peripheral nerve stimulation on tissue health in the residual limb of amputees with diabetic peripheral neuropathy, with a focus on sensory restoration and autonomic function.




Acknowledgements (Optional): : This work was supported in part by the Department of Defense under Awards No. W81XWH-18-1-0321 and W81XWH-20-1-0802. This work was also supported in part by the Merit Review Award # 5 I01 RX003566-02 from the United States (U.S.) Department of Veterans Affairs, Rehabilitation Research and Development Service

References (Optional): : 1. Norvell DC, et al. (2020). EJVES. 60(4), 614-621; 2. Veves A, et al. (1992) Diabetologia, 35(7); 3. Nouri et al. (2012). PLoS One, 7(4)