Strength-duration Curves Research Articles

Surface electrical stimulation to evoke referred sensation.

Surface electrical stimulation (SES) is being investigated as a noninvasive method to evoke natural sensations distal to electrode location. This may improve treatment for phantom limb pain as well as provide an alternative method to deliver sensory feedback. The median and/or ulnar nerves of 35 subjects were stimulated at the elbow using surface electrodes. Strength-duration curves of hand sensation were found for each subject. All subjects experienced sensation in their hand, which was mostly described as a paresthesia-like sensation. The rheobase and chronaxie values were found to be lower for the median nerve than the ulnar nerve, with no significant difference between sexes. Repeated sessions with the same subject resulted in sufficient variability to suggest that recalculating the strength-duration curve for each electrode placement is necessary. Most of the recruitment curves in this study were generated with 28 to 36 data points. To quickly reproduce these curves with limited increase in error, we recommend 10 data points. Future studies will focus on obtaining different sensations using SES with the strength-duration curve defining the threshold of the effective parameter space.

Influence of Lumbar Disc Herniation on Chronaxie and Rheobase in Patients with Chronic Low Back Pain-A Quasi Experimental Pilot Study

Aim: To investigate the influence of lumbar disc herniation on rheobase and chronaxie in patients with low back pain. Methods: The Strength duration curve, rheobase and Chronaxie in 10 LBP patients with radiculopathy due to lumbar disc herniation and 10 healthy controls were obtained following stimulation of tibialis anterior and peroneus muscles. Results: Tibialis anterior and peroneus muscles in both Low back patients with radiculopathy due to lumbar disc herniation and healthy control showed normal pattern of strength duration curve typical of an innervated muscle. The rheobase and chronaxie values for tibialis anterior were 11.8 ± 1.93 and 0.54 ± 0.014 respectively in LBP patients and 7.4 ± 0.84 and 0.24 ± 0.14 respectively in control subjects. The rheobase and chronaxie values for peroneus muscles were 12.8 ± 2.93 and 0.41 ± 0.07 respectively in LBP patients and 7.6 ± 1.71 and 0.16 ±0.01 respectively control subjects. Rheobase and chronaxie were significantly higher in low back pain patients with radiculopathy than apparently healthy control subjects (P<0.00). Conclusion: Findings from this study demonstrated higher rheobase and chronaxie in patients with LBP and lumbar disc herniation than the control subjects. The outcome of this study will be used to do power size calculation for a large randomised controlled trial

Is the introduction of another variable to the strength-duration curve necessary in neurostimulation?

Neurostimulation is the process and technology derived from the application of electricity with different parameters to activate or inhibit nerve pathways. Pulse width (Pw) is the duration of each electrical impulse and, along with amplitude (I), determines the total energy charge of the stimulation. The aim of the study was to test Pw values to find the most adequate pulse widths in rechargeable systems to obtain the largest coverage of the painful area, the most comfortable paresthesia, and the greatest patient satisfaction. A study of the parameters was performed, varying Pw while maintaining a fixed frequency at 50 Hz. Data on perception threshold (Tp ), discomfort threshold (Td ), and therapeutic threshold (Tt ) were recorded, applying 14 increasing Pw values ranging from 50 µsec to 1000 µsec. Lastly, the behavior of the therapeutic range (TR), the coverage of the painful area, the subjective patient perception of paresthesia, and the degree of patient satisfaction were assessed. The findings after analyzing the different thresholds were as follows: When varying the Pw, the differences obtained at each threshold (Tp , Tt , and Td ) were statistically significant (p < 0.05). The differences among the resulting Tp values and among the resulting Tt values were statistically significant when varying Pw from 50 up to 600 µsec (p < 0.05). For Pw levels 600 µsec and up, no differences were observed in these thresholds. In the case of Td , significant differences existed as Pw increased from 50 to 700 µsec (p ≤ 0.05). The coverage increased in a statistically significant way (p < 0.05) from Pw values of 50 µsec to 300 µsec. Good or very good subjective perception was shown at about Pw 300 µsec. The patient paresthesia coverage was introduced as an extra variable in the chronaxie-rheobase curve, allowing the adjustment of Pw values for optimal programming. The coverage of the patient against the current chronaxie-rheobase formula will be represented on three axes; an extra axis (z) will appear, multiplying each combination of Pw value and amplitude by the percentage of coverage corresponding to those values. Using this new comparison of chronaxie-rheobase curve vs. coverage, maximum Pw values will be obtained different from those obtained by classic methods.

Importance of the Brow in Facial Expressiveness During Human Communication

The objective of this study was to evaluate laterality and upper/lower face dominance of expressiveness during prescribed speech using a unique validated image subtraction system capable of sensitive and reliable measurement of facial surface deformation. Observations and experiments of central control of facial expressions during speech and social utterances in humans and animals suggest that the right mouth moves more than the left during nonemotional speech. However, proficient lip readers seem to attend to the whole face to interpret meaning from expressed facial cues, also implicating a horizontal (upper face-lower face) axis. Prospective experimental design. Experimental maneuver: recited speech. image-subtraction strength-duration curve amplitude. Thirty normal human adults were evaluated during memorized nonemotional recitation of 2 short sentences. Facial movements were assessed using a video-image subtractions system capable of simultaneously measuring upper and lower specific areas of each hemiface. The results demonstrate both axes influence facial expressiveness in human communication; however, the horizontal axis (upper versus lower face) would appear dominant, especially during what would appear to be spontaneous breakthrough unplanned expressiveness. These data are congruent with the concept that the left cerebral hemisphere has control over nonemotionally stimulated speech; however, the multisynaptic brainstem extrapyramidal pathways may override hemiface laterality and preferentially take control of the upper face. Additionally, these data demonstrate the importance of the often-ignored brow in facial expressiveness. Experimental study. EBM levels not applicable.

Long Pacing Pulses Reduce Phrenic Nerve Stimulation in Left Ventricular Pacing

Phrenic nerve stimulation is a major obstacle in cardiac resynchronization therapy (CRT). Activation characteristics of the heart and phrenic nerve are different with higher chronaxie for the heart. Therefore, longer pulse durations could be beneficial in preventing phrenic nerve stimulation during CRT due to a decreased threshold for the heart compared with the phrenic nerve. We investigated if long pulse durations decreased left ventricular (LV) thresholds relatively to phrenic nerve thresholds in humans. Eleven patients, with indication for CRT and phrenic nerve stimulation at the intended pacing site, underwent determination of thresholds for the heart and phrenic nerve at different pulse durations (0.3-2.9 milliseconds). The resulting strength duration curves were analyzed by determining chronaxie and rheobase. Comparisons for those parameters were made between the heart and phrenic nerve, and between the models of Weiss and Lapicque as well. In 9 of 11 cases, the thresholds decreased faster for the LV than for the phrenic nerve with increasing pulse duration. In 3 cases, the thresholds changed from unfavorable for LV stimulation to more than a factor 2 in favor of the LV. The greatest change occurred for pulse durations up to 1.5 milliseconds. The chronaxie of the heart was significantly higher than the chronaxie of the phrenic nerve (0.47 milliseconds vs. 0.22 milliseconds [P = 0.029, Lapicque] and 0.79 milliseconds vs. 0.27 milliseconds [P = 0.033, Weiss]). Long pulse durations lead to a decreased threshold of the heart relatively to the phrenic nerve and may prevent stimulation of the phrenic nerve in a clinical setting.

Comparison of Different Pacing Strategies to Minimize Phrenic Nerve Stimulation in Cardiac Resynchronization Therapy

Phrenic nerve (PN) stimulation (PNS) frequently limits cardiac resynchronization therapy (CRT). Yet, pacing strategies to minimize PNS have not been systematically compared. We propose to: (1) compare different pacing strategies to minimize PNS in CRT and (2) evaluate differences between PN and left ventricular (LV) capture thresholds among LV pacing configurations. PN and LV thresholds were obtained using 6 LV configurations in 28 patients with any PNS during CRT implantation or replacement. Incidence of PNS was compared in all LV configurations by programming pacing output to (1) One Volt (V) above LV threshold, (2) triple pulse width (PW) at LV threshold, and (3) 1.5 times LV threshold for each patient. PN thresholds and PN strength-duration curves were statistically different between configurations (P < 0.05). Ring→RVcoil and Ring→Can had the largest difference between PN and LV thresholds. Pacing output programmed to 1.5 times LV threshold, 1 V above LV threshold, and triple PW at LV threshold had similar probability of PNS between LV configurations. However, 1 V above LV threshold and triple PW at LV threshold frequently resulted in poor (< 30%) LV capture safety margin (14-43% and 53-68%, respectively). Freedom from PNS (programmed output at twice LV threshold) was found in 88%, 84%, and 52% with 6, 3, or 2 available LV configurations, respectively. Multiple LV pacing configurations marginally increase the probability of avoiding PNS by electronic reprogramming. Pacing output programmed to 1.5 times LV threshold is an additional alternative to minimize PNS when electronic reprogramming options are limited.

Benzalkonium Chloride–Induced Denervation of Orbicularis Oculi Muscle in Rabbits

To examine the potential for benzalkonium chloride (BAK) to cause denervation of the orbicularis oculi muscle (OOM) in a rabbit model. Pigmented rabbits were separated into five groups consisting of five rabbits each. Group 1 was injected with 1 mL of BAK 0.25% in the OOM of the upper eyelid. Group 2 was injected with 1 mL of BAK 0.5%. Group 3 included untreated controls. Groups 4 and 5 underwent surgical severing of the facial nerve (to cause complete paralysis of the OOM). Strength-duration curves for electrical stimulation of muscle twitches were measured for each group and chronaxie values were calculated to determine innervation status. Groups 1 and 4 were stimulated at 1 week postintervention while groups 2 and 5 were stimulated at 4 weeks postintervention. The rabbits were then sacrificed and the eyelids sent for histological analysis. In group 1, all five rabbits demonstrated denervation of the OOM in the injected area. In group 2, one rabbit developed an abscess at the injection site and was sacrificed at 1 week. Of the remaining four rabbits, two showed complete denervation and two showed denervation with evidence of partial reinnervation. The histology demonstrated marked atrophy of the OOM for BAK-treated rabbits when compared with controls. In group 3, all five rabbits showed normal OOM function. In groups 4 and 5, all rabbits showed denervation of the OOM and histological evidence of muscle atrophy similar to groups 1 and 2. BAK causes denervation when injected into the OOM in rabbits. The clinical relevance of this finding may be the onset of lagophthalmos and eyelid retraction in patients with chronic BAK exposure.

Pulse width dependence of motor threshold and input–output curve characterized with controllable pulse parameter transcranial magnetic stimulation

ObjectiveTo demonstrate the use of a novel controllable pulse parameter TMS (cTMS) device to characterize human corticospinal tract physiology. MethodsMotor threshold and input–output (IO) curve of right first dorsal interosseus were determined in 26 and 12 healthy volunteers, respectively, at pulse widths of 30, 60, and 120μs using a custom-built cTMS device. Strength–duration curve rheobase and time constant were estimated from the motor thresholds. IO slope was estimated from sigmoid functions fitted to the IO data. ResultsAll procedures were well tolerated with no seizures or other serious adverse events. Increasing pulse width decreased the motor threshold and increased the pulse energy and IO slope. The average strength–duration curve time constant is estimated to be 196μs, 95% CI [181μs, 210μs]. IO slope is inversely correlated with motor threshold both across and within pulse width. A simple quantitative model explains these dependencies. ConclusionsOur strength–duration time constant estimate compares well to published values and may be more accurate given increased sample size and enhanced methodology. Multiplying the IO slope by the motor threshold may provide a sensitive measure of individual differences in corticospinal tract physiology. SignificancePulse parameter control offered by cTMS provides enhanced flexibility that can contribute novel insights in TMS studies.

Development of less invasive neuromuscular electrical stimulation model for motor therapy in rodents

BackgroundCombination therapy is essential for functional repairs of the spinal cord. Rehabilitative therapy can be considered as the key for reorganizing the nervous system after spinal cord regeneration therapy. Functional electrical stimulation has been used as a neuroprosthesis in quadriplegia and can be used for providing rehabilitative therapy to tap the capability for central nervous system reorganization after spinal cord regeneration therapy.ObjectiveTo develop a less invasive muscular electrical stimulation model capable of being combined with spinal cord regeneration therapy especially for motor therapy in the acute stage after spinal cord injury.MethodsThe tibialis anterior and gastrocnemius motor points were identified in intact anesthetized adult female Fischer rats, and stimulation needle electrodes were percutaneously inserted into these points. Threshold currents for visual twitches were obtained upon stimulation using pulses of 75 or 8 kHz for 200 ms. Biphasic pulse widths of 20, 40, 80, 100, 300, and 500 µs per phase were used to determine strength–duration curves. Using these parameters and previously obtained locomotor electromyogram data, stimulations were performed on bilateral joint muscle pairs to produce reciprocal flexion/extension movements of the ankle for 15 minutes while three-dimensional joint kinematics were assessed.ResultsRhythmic muscular electrical stimulation with needle electrodes was successfully done, but decreased range of motion (ROM) over time. High-frequency and high-amplitude stimulation was also shown to be effective in alleviating decreases in ROM due to muscle fatigue.ConclusionsThis model will be useful for investigating the ability of rhythmic muscular electrical stimulation therapy to promote motor recovery, in addition to the efficacy of combining treatments with spinal cord regeneration therapy after spinal cord injuries.

Strength–duration relationship for intra- versus extracellular stimulation with microelectrodes

Chronaxie, a historically introduced excitability time parameter for electrical stimulation, has been assumed to be closely related to the time constant of the cell membrane. Therefore, it is perplexing that significantly larger chronaxies have been found for intracellular than for extracellular stimulation. Using compartmental model analysis, this controversy is explained on the basis that extracellular stimulation also generates hyperpolarized regions of the cell membrane hindering a steady excitation as seen in the intracellular case. The largest inside/outside chronaxie ratio for microelectrode stimulation is found in close vicinity of the cell. In the case of monophasic cathodic stimulation, the length of the primarily excited zone which is situated between the hyperpolarized regions increases with electrode–cell distance. For distant electrodes this results in an excitation process comparable to the temporal behavior of intracellular stimulation. Chronaxie also varies along the neural axis, being small for electrode positions at the nodes of Ranvier and axon initial segment and larger at the soma and dendrites. As spike initiation site can change for short and long pulses, in some cases strength–duration curves have a bimodal shape, and thus, they deviate from a classical monotonic curve as described by the formulas of Lapicque or Weiss.

Application of a computational model of vagus nerve stimulation

The most widely used and studied neurostimulation procedure for medically refractory epilepsy is vagus nerve stimulation (VNS) Therapy. The goal of this study was to develop a computational model for improved understanding of the anatomy and neurophysiology of the vagus nerve as it pertains to the principles of electrical stimulation, aiming to provide clinicians with a systematic and rational understanding of VNS Therapy. Computational modeling allows the study of electrical stimulation of peripheral nerves. We used finite element electric field models of the vagus nerve with VNS Therapy electrodes to calculate the voltage field for several output currents and studied the effects of two programmable parameters (output current and pulse width) on optimal fiber activation. The mathematical models correlated well with strength-duration curves constructed from actual patient data. In addition, digital constructs of chronic versus acute implant models demonstrated that at a given pulse width and current combination, presence of a 110-μm fibrotic tissue can decrease fiber activation by 50%. Based on our findings, a range of output current settings between 0.75 and 1.75mA with pulse width settings of 250 or 500μs may result in optimal stimulation. The modeling illustrates how to achieve full or nearly full activation of the myelinated fibers of the vagus nerve through output current and pulse width settings. This knowledge will enable clinicians to apply these principles for optimal vagus nerve activation and proceed to adjust duty cycle and frequency to achieve effectiveness.

Disparity in Left Ventricular Stimulation Among Different Pacing Configurations in Cardiac Resynchronization Therapy

Cardiac resynchronization therapy (CRT) demands high energy utilization due to continuous biventricular pacing. Current technology allows 6 pacing configurations for a bipolar left ventricular (LV) lead. Understanding the energy requirements for each configuration will allow optimization of pacing output. Pacing impedance, LV voltage threshold at 1.5 ms (rheobase) and 0.4 ms and chronaxie were obtained in 6 LV configurations in 49 consecutive patients undergoing CRT implantation or replacement. Strength-duration curves were derived using the Lapicque formula. Pacing impedances and voltage thresholds at 1.5 and 0.4 ms, calculated minimum threshold energy at chronaxie, current drain, energy thresholds at 0.4 ms, and strength-duration curves were statistically different between LV configurations (P<0.05). The lowest threshold energy requirements were found in Tip→right ventricular (RV) coil and Tip→Can configuration. Energy strength-duration curves involving the ring as the cathode (Ring→RV, Ring→Can, and Ring→Tip) had the highest LV thresholds. The pacing configuration with the lowest energy threshold correlated 89% of the time with the lowest voltage threshold at 0.4 ms. The probability to reach LV thresholds <1.5 V at 0.4 ms was increased from 51% with 2 LV configurations to 67% with 6 LV configurations. Pacing impedance, LV thresholds, minimum threshold energy at chronaxie, current drain, voltage, and energy strength-duration curves were statistically different between LV pacing configurations. LV pacing configuration with the lowest voltage threshold does not always reflects the lowest energy threshold, particularly in the presence of a low impedance configuration. The availability of 6 LV configurations increases the probability of optimizing LV pacing output.

Serial electrophysiological changes in uraemic patients with tetrodotoxin intoxication

Tetrodotoxin (TTX) is a well-known biological neurotoxin, predominantly isolated from the viscera of puffer fish. TTX binds to the voltage-gated sodium channels and reduces the membrane excitability of vital tissues (Narahashi et al., 1967). As a result, paraesthesia, paralysis, respiratory failure and, eventually, coma with absence of electrical brain activities can appear depending on the amounts of ingested toxin. There are only a few reports about the electrophysiological studies of peripheral nerves in TTX intoxication because of the rarity of this intoxication and rapid alteration of clinical and electrophysiological status. We conducted serial nerve-conduction studies (NCSs) in two uraemic patients with acute TTX intoxication during the recovery period and obtained the strength–duration curves with nerve-excitability test at each visit. A brief case summary is as follows: Patient #1 was a 38-year-old man, who received peritoneal dialysis with end-stage renal disease for 1 year. He was referred to our hospital due to rapidly progressive quadriparesis. Three days before admission, he ate cooked puffer fish for dinner. On the next morning, he felt dizziness and peri-oral tingling sensation. On neurologic examination, he showed symmetric muscle weakness of Medical Research Council grade 2. Although he complained of mild dyspnoea, he did not need mechanical ventilation. On the 5th day of intoxication, an NCS on the median nerve revealed slow conduction velocity (30 m s ) but normal compound muscle action potential (CMAP) amplitude (8 mV). Serum blood urea nitrogen (BUN) and creatinine (Cr) were 99 and 14.9 mg dl , respectively. Serum potassium level was normal (3.9 mEq l ). His neurologic deficits slowly improved and disappeared at 14 days after ingestion. In the follow-up NCS performed on the 17th day, CMAP was increased in amplitude (11 mV) and conduction velocity demonstrated improvement (51 m s ) over the previous value. A nerve-excitability test was performed. Recording electrodes were attached in the abductor pollicis brevis muscle and the median nerve at the wrist was stimulated. We set the initial stimulation duration to 0.05 ms of square wave, and stimulation intensities were serially increased with 1-mA intervals until visible muscle twitching was observed. We conducted the same procedure with each increment of 0.1 ms up to 1 ms to measure each threshold.

Left ventricular pacing with long pulse duration can avoid phrenic nerve stimulation

One of the major limitations of left ventricular pacing is phrenic nerve stimulation. We evaluated the characteristics of the strength-duration curves of the left phrenic nerve (PN) and epicardium of the left ventricle (LV). A total of 12 mongrel dogs were studied. Capture threshold was assessed during pacing with 180 beats/min and various pulse durations (0.25 to 2.0 ms) in an open chest model. A commercially available LV lead was inserted through a small hole created in the pericardium. The tip electrode position was maintained at the course of the left PN with gentle manual pressure under visual inspection. The LV threshold was evaluated at the epicardium just below the PN. The LV threshold was defined as the highest amplitude that showed loss of 1:1 capture. The PN threshold was defined as the highest amplitude that could not induce any diaphragmatic contraction. The strength-duration curve of the PN had a higher rheobase and shorter chronaxie than that of the LV. The LV threshold was higher than that of the PN at pulse duration <1.0 ms in bipolar pacing, but significantly lower than that of the PN at a pulse duration of 2.0 ms, especially during unipolar pacing (1.3 ± 0.21 V vs. 2.0 ± 0.56 V, LV vs. PN, P =.012). LV pacing without PN stimulation could be achieved in pacing with long pulse duration. The longer pulse duration option can be considered in the development of devices for LV pacing such as cardiac resynchronization therapy.

Left Ventricular Pacing with Long Pulse Duration Can Avoid Phrenic Nerve Stimulation

Background: One of the major limitations of left ventricular pacing is phrenic nerve stimulation. We evaluated the characteristics of the strength-duration curves of the left phrenic nerve (PN) and epicardium of the left ventricle (LV). Methods: A total of 12 mongrel dogs were studied. Capture threshold was assessed during pacing with 180 bpm and various pulse durations (0.25–2.0 ms) in an open chest model. A commercially available LV lead was inserted through a small hole created in the pericardium. The tip electrode position was maintained at the course of the left PN with gentle manual pressure under visual inspection. The LV threshold was evaluated at the epicardium just below the PN. Results: The strength-duration curve of the PN had a higher rheobase and shorter chronaxie than that of the LV. The LV threshold was higher than that of the PN at pulse duration <1.0 ms in bipolar pacing, but significantly lower than that of the PN at a pulse duration of 2.0 ms, especially during unipolar pacing (1.3±0.21 vs. 2.0±0.56 V, LV vs. PN, p=0.012). Conclusion: LV pacing without PN stimulation could be achieved in pacing with long pulse duration. The longer pulse duration option can be considered in the development of devices for LV pacing such as cardiac resynchronization therapy.

Muscle Contraction During Electro‐muscular Incapacitation: A Comparison Between Square‐wave Pulses and the TASER® X26 Electronic Control Device*

Electronic control devices (including the Advanced TASER(®) X26 model produced by TASER International) incapacitate individuals by causing muscle contractions. To provide information relevant to development of future potential devices, effects of monophasic square waves with different parameters were compared with those of the X26 electronic control device, using two animal models (frogs and swine). Pulse power, electrical pulse charge, pulse duration, and pulse repetition frequency affected muscle contraction. There was no difference in the charge required, between the square waveform and the X26 waveform, to cause approximately the same muscle-contraction response (in terms of the strength-duration curve). Thus, on the basis of these initial studies, the detailed shape of a waveform may not be important in terms of generating electro-muscular incapacitation. More detailed studies, however, may be required to thoroughly test all potential waveforms to be considered for future use in ECDs.

Strength-duration relationship for extracellular neural stimulation

Understanding the mechanisms and dynamics of extracellular neural stimulation is very important for the development of electro-neural interfaces in general, and for the design of stimulation waveforms and electrode configurations for neural prosthetic implants, in particular. The most common type of neural stimulation is extracellular, and yet its mechanisms and dynamics have scarcely been explored and described. Its strength-duration relationship is often assumed to be similar to the classical intracellular dependence, with a slope asymptotically approaching 1/τ at pulse durations τ shorter than chronaxy. The current study explores the basic mechanisms of extracellular stimulation and derives its strength-duration curve in a wide range of stimulus durations for various waveforms and cell shapes. We used two different models of active membrane properties: the Hodgkin-Huxley model of the squid giant axon [1], and a six-channel salamander retinal ganglion cell model [2]. Three cell geometries were analyzed: an idealized planar cell with two uniformly polarized flat surfaces, and more realistic spherical and cylindrical shapes corresponding to the soma and unmyelinated axon or axon hillock. The strength-duration relationship was found to differ significantly from classical intracellular models. For the Hodgkin-Huxley model at pulse durations between 4 μs and 5 ms stimulation is dominated by sodium channels, and has a slope of approximately –0.72 in log-log coordinates. At shorter durations it is dominated by the potassium channels, and has a much lower slope of about –0.13. With pulses shorter than cell polarization time (typically about 0.1-1 μs), it is dominated by polarization dynamics, and asymptotically approaches the classical –1 slope. For retinal ganglion cells we demonstrate that extracellular stimulation can have not only lower but also upper thresholds, and may be impossible below certain pulse durations. For both cell models we have found that in some stimulation regimes the stimulus can hyperpolarize cells, suppressing rather than stimulating spiking behavior. Thresholds for burst stimuli can be either higher or lower than that of a single pulse, depending on pulse duration. The modeled thresholds were found to be comparable to published experimental data obtained with rabbit retinal ganglion cells. These results provide a biophysical basis for understanding stimulation dynamics, and guidance for optimizing the efficacy and safety of extracellular neural stimulation.

In situ electric fields causing electro-stimulation from conductor contact of charged human

Contact currents flow from/into a human body when touching an object such as a metal structure with a different electric potential. These currents can stimulate muscle and peripheral nerves. In this context, computational analyses of in situ electric fields caused by the contact current have been performed, while their effectiveness for transient contact currents has not well been investigated. In the present study, using an anatomically based human model, a dispersive finite-difference time-domain model was utilised to computed transient contact current and in situ electric fields from a charged human. Computed in situ electric fields were highly localised in the hand. In order to obtain an insight into the relationship between in situ electric field and electro-stimulation, cell-maximum and 5-mm averaged in situ electric fields were computed and compared with strength-duration curves. The comparison suggests that both measures could be larger than thresholds derived from the strength-duration curves with parameters used in previous studies.

The Effects of Temperature on Cardiac Pacing Thresholds

Human core body temperature can fluctuate between 36 degrees C (sleep) and 42 degrees C (intense exercise). Also, efforts are underway to develop implantable pacing systems that minimize heating during magnetic resonance imaging (MRI) scans (i.e., MRI safe). Concerns exist that ventricular pacing capture thresholds (VPCT) are modified by changing cardiac temperatures. This project was designed to assess the effects of temperature on VPCT of the mammalian heart. Fresh ventricular specimens were obtained from healthy canine, healthy swine, and diseased human hearts. Isolated trabeculae were suspended in temperature-controlled tissue baths containing oxygenated Krebs buffer. Small active fixation pacing leads delivered pacing pulses to each specimen. Baseline strength-duration curves were determined at 37 degrees C, then at randomized temperatures ranging from 35 degrees C to 42 degrees C. Final thresholds were repeated at 37 degrees C to confirm baseline responses. All threshold data were normalized to a baseline average. Both canine and swine trabeculae elicited significant decreases in thresholds (10-14%) at pacing stimulus durations (pulsewidths) of 0.02 ms (P < 0.01) and 0.10 ms (P < 0.05) between the temperatures of 38 degrees C and 41 degrees C, compared to baseline. Thresholds at 42 degrees C trended back to baseline for both canine (NS) and swine trabeculae (P < 0.05 compared to 38 degrees C-41 degrees C). Human trabeculae thresholds increased >35% (P < 0.05) at 42 degrees C relative to baseline with no significant differences at other temperatures. Temperature is a significant factor on pacing thresholds for mammalian ventricular myocardium. Our data for the diseased human trabeculae indicate that cases where cardiac heating may occur (e.g., radiofrequency energy due to MRI scans, febrile events), patients without adequate VPCT safety margin may be at higher risk of loss of proper function of an implanted pacing or defibrillation system.

Are There Nociceptive-Specific Brain Potentials?

to the editor: In their study on factor analysis of brain responses to different stimulus modalities, [Mouraux and Iannetti (2009)][1] claimed that laser-evoked brain potentials do not reflect nociceptive-specific neural activity, although laser radiant heat pulses provide a purely nociceptive input