There is considerable evidence that lower-limb somatosensation contributes to the control of upright balance. In this study, we investigated the specific role of foot sole cutaneous afferents in the generation of balance corrections following lateral accelerations of the support surface. Participants were subjected to balance perturbations before and after targeted anesthesia of the cutaneous soles induced by intradermal injections of local anesthetic. Subject responses were quantified in terms of net joint torques at the ankles, hips and trunk. Contrary to the conclusions drawn in earlier studies, response torque impulses at the ankles and hips were clearly scaled with the perturbation impulse under both control and anesthetized conditions. Reduced plantar sensitivity produced a relative shift in compensatory torque production from the ankles and trunk to the hips. These findings demonstrate that plantar cutaneous afferents play an important role in the shaping of dynamic postural responses. Furthermore, the results suggest that loss of plantar sensation may be an important contributor to the dynamic balance deficits and increased risk of falls associated with peripheral neuropathies.
Considerable evidence shows that sensation from the feet and ankles is important for standing balance control. It remains unclear, however, to what extent specific foot and ankle sensory systems are involved. This study focused on the role of plantar cutaneous sensation in quasi-static balance control. Iontophoretic delivery of anesthesia was used to reduce the sensitivity of the forefoot soles. In a follow-up experiment, subjects received intradermal injections of local anesthetic into the entire weight-bearing surface of the foot soles. Properties of the center-of-foot-pressure (COP) trajectories and ground reaction shear forces were analyzed using stabilogram-diffusion analysis and summary statistics. Effects of foot-sole anesthesia were generally small and mostly manifested as increases in COP velocity. Magnitude of COP displacement was unaffected by foot-sole anesthesia. Forefoot anesthesia mainly influenced mediolateral posture control, whereas complete foot-sole anesthesia had an impact on anteroposterior control. During bipedal stance, statistically significant effects of foot-sole anesthesia on COP were present only under eyes-closed conditions and included increases in COP velocity (11-12%) and shear force root-mean-square (13%), the latter indicating increases in body center-of-mass accelerations due to the foot-sole anesthesia. Similar effects were seen for unipedal stance in addition to an increase in anteroposterior COP median frequency (36%). Changes in stabilogram-diffusion parameters were confined to the short-term region suggesting that sensory information from the foot soles is mainly used to set a relevant background muscle activity for a given posture and support surface characteristic, and consequently is of little importance for feedback control during unperturbed stance. In general, this study demonstrates that plantar sensation is of moderate importance for the maintenance of normal standing balance when the postural control system is challenged by unipedal stance or by closing of the eyes. The impact of reduced plantar sensitivity on postural control is expected to increase with the loss of additional sensory modalities such as the concomitant proprioceptive deficits commonly associated with peripheral neuropathies.
A new type of disposable external defibrillation electrode has been developed to reduce the skin irritation commonly associated with defibrillation and synchronised cardioversion. This design employs an impedance gradient to reduce the proportion of current delivered to the electrode periphery. The temperature distribution under the new electrode was compared with that of four other types of commercially available electrodes after repeated high-energy biphasic defibrillation discharges to domestic swine. Skin temperature distributions were acquired using non-invasive thermography. Measurements of the maximum temperature rise at each electrode site, taken 3.6s after the fifth defibrillation discharge, demonstrated that the new impedance-gradient electrode produced 50-60% less skin heating than two of the three uniform-impedance electrode designs. Histological examination of erythematous sites excised 24 h after defibrillation quantified the associated skin damage using a scoring protocol developed for this study. In contrast to previous studies, histological examinations demonstrated second-degree skin burns following defibrillation. The new electrode design, however, induced 44-46% less skin damage than two of the traditional uniform-impedance electrodes.
AMmct-Prolonged exterual cardiac pacing can produce electrochemical skin burns associated with the production of OH;and eventually 6, through hydrolysis. This study compares the performance of a new multifunclion deiibrillation/pacing/ECG electrode design to that of other commercially available electrodes during sustaiined pacing. Pacing voltage changes and pH measurements predict the eventual depletion of electrode mnstituenb and tbe onset of hydrolysls. The resdb indicate that tbe new electrode delays the rise of electrode ofiset voltage and resulting pB increases compared to several competing electrode designs.INTRODUCTION Sustained bradycardia may require external cardiac pacing for several minutes or h o w until other therapies can be implemented. While prolonged external pacing is known to produce localized skin necrosis, or pacing ' %bums", little published research bas addressed the origin and nature of such lesions. The propensity for various types of external pacing electrodes to produce pacing bums has not been established.Pacing bums are electrochemical in nature, arising ftom the iontophoretic driving of caustic ions @redominately OK) into the skin (Fig. I). These ions are produced through hydrolysis of water within the electrode gel when the metallic conductors are depleted. In Ag/AgCI electrodes, the reaction is normally spontaneous and requires no driving potential (i.e. E' =II positive):As the supply of AgCl within the cathode (apex electrode) is depleted, the voltage required to deliver the desired pacing current will increase. The reaction Anode: Ag(s) + Cr(aq) + AgCl(s) +e-Eo = -0.22V Cathode: 2H20(1) + Ze-+ H2(g) + ZOH(aq) E" = -0.83V becomes possible when the driving potential exceeds 0.61V. OK is iontophoretically driven into the skin under the apex electrode, eventually producing pacing bums. As the supply of Ag is depleted, hydrolysis can take place at the anode as well:t . .. t ; .Fig.1. Left: Elechochnnical bum due to prolonged external pacing. Right: pH indicator fluid demonstrates the smng &line nahlre of the bum site.Anode: 2Cl-(aq) + CIz(g) + 2e.E" = +l.36VCathode: 2H20(1) + Ze-3 H2(g) + 20H-(aq) E" = -0.83VSince the oxidation of water is kinetically slow, the preferential anodic reaction is the production of chlorine gas, requiring a total system driving voltage of 2.19V. Pacing bums under the sternum electrode, which would be acidic in nature. are therefore uncommon.In tin-based electrodes (e.g. Sn/SnC12 or Sn/S&r2), hydrolysis occurs at the cathode with driving potentials greater than 0.97V Anode: ~n ( s ) + sn2+ (aq) + 2e-Cathode: 2H20(1) + 2 6 + H2(g) + 2OHyaq) E"= -0.83V [I] ANSVAAMI standard DF21996 requires that all selfadhesive multifunction defibrillatiodpacingECG electrodes continue to meet defibrillation standards after 1 hour of continuous pacing at maximum pacing current and rate. There are no guidelines for establishing a maximum pacing duration recommendation greater than 1 hour, nor do any standards consider the potential for pacing bums. The result is a wide var...
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