A Novel Radiofrequency Device to Monitor Changes in Pulmonary Fluid in Dialysis Patients.

BACKGROUND AND OBJECTIVES: Fluid monitoring is an important management strategy in patients with chronic kidney disease (CKD) and heart failure (HF). The µCor™ Heart Failure and Arrhythmia Management System uses a radiofrequency-based thoracic fluid index (TFI) to track pulmonary edema. During hemodialysis, the acute removal of fluid through ultrafiltration offers a model for measuring a patient's fluid status. The objective of the study was to assess the relationship between the device measured TFI and ultrafiltration volume (UFV). DESIGN SETTING PARTICIPANTS AND MEASUREMENTS: Patients undergoing chronic dialysis with and without heart failure were enrolled in the study. The relationship between TFI and UFV in each individual subject was assessed by calculating the Pearson correlation coefficient (r). The average correlation across all subjects was calculated through the use of the Fisher's z transform. Responder analysis was performed to assess the magnitude of change in TFI before and after dialysis. RESULTS: Twenty subjects were enrolled in the trial. The mean volume of fluid removal was 3.63 L (SD 0.88 L). The mean correlation based on Fisher's transform was 0.95 CI (0.92-0.99). Responder analysis showed that the mean reduction of TFI after dialysis was 5.5% ± 3.8. CONCLUSION: The µCor system provides radiofrequency-based measurements of thoracic fluid which correlate well with total body fluid removal in a real-world setting. Fluid management based on the radar-derived TFI may provide benefits to dialysis patients and serves as a potential model for pulmonary edema common to the clinical course of heart failure.

Frequency and Implications of Ischemia Prior to Ventricular Tachyarrhythmia in Patients Treated With a Wearable Cardioverter Defibrillator Following Myocardial Infarction.

BACKGROUND: Autopsy studies imply that recurrent myocardial infarction (MI) accounts for the majority of sudden death early after acute MI, rather than primary arrhythmia. However, diagnosis of recurrent MI by autopsy is challenging and excludes electrocardiographic data to adjudicate arrhythmic causes. We examined the frequency of ischemia prior to treated ventricular tachycardia/fibrillation (VT/VF) and outcomes in patients using the wearable cardioverter defibrillator (WCD) following acute MI. HYPOTHESIS: Primary arrhythmia, rather than ischemia, is a frequent contributor to sudden death following MI. METHODS: All patients treated for VT/VF over a 6-year period while wearing a WCD following acute MI with advanced left ventricular dysfunction (ejection fraction ≤35%) were included. Patients with ST-segment changes ≥0.1 mV before VT/VF were classified ischemic. Demographics and clinical outcomes were compared between those with ischemia-mediated vs primary arrhythmia. RESULTS: Among 273 patients fulfilling study criteria, 15.4% had ischemia prior to VT/VF. Clinical and WCD use characteristics did not significantly differ between ischemic and primary VT/VF groups. Termination of VT/VF by WCD treatment approximated 96% in both groups. Survival 24 hours post-treatment was 88% and 84% (P = 0.54) for patients with and without ischemic VT/VF, respectively. Furthermore, 30-day cumulative survival for those with and without ischemic VT/VF was 77% and 70%, respectively (P = 0.57). CONCLUSIONS: Ischemia is an infrequent cause of VT/VF following MI, contradicting previous study conclusions that recurrent MI is responsible for most post-MI sudden death. Etiology of VT/VF, however, did not influence defibrillation success or survival, which was high for both groups.

Time course of functional and biomechanical improvements during a gait training intervention in persons with chronic stroke.

BACKGROUND AND PURPOSE: In rehabilitation, examining how variables change over time can help define the minimal number of training sessions required to produce a desired change. The purpose of this study was to identify the time course of changes in gait biomechanics and walking function in persons with chronic stroke. METHODS: Thirteen persons who were more than 6 months poststroke participated in 12 weeks of fast treadmill training combined with plantar- and dorsiflexor muscle functional electrical stimulation (FastFES). All participants completed testing before the start of intervention, after 4, 8, and 12 weeks of FastFES locomotor training. RESULTS: Peak limb paretic propulsion, paretic limb propulsive integral, peak paretic limb knee flexion (P < 0.05 for all), and peak paretic trailing limb angle (P < 0.01) improved from pretraining to 4 weeks but not between 4 and 12 weeks. Self-selected walking speed and 6-minute walk test distance improved from pretraining to 4 weeks and from 4 to 12 weeks (P < 0.01 and P < 0.05, respectively for both). Timed Up & Go test time did not improve between pretraining and 4 weeks, but improved by 12 weeks (P = 0.24 and P < 0.01, respectively). DISCUSSION AND CONCLUSIONS: The results demonstrate that walking function improves with a different time course compared with gait biomechanics in response to a locomotor training intervention in persons with chronic stroke. Thirty-six training sessions were necessary to achieve an increase in walking speed that exceeded the minimally clinically important difference. These findings should be considered when designing locomotor training interventions after stroke.Video Abstract available (see Video, Supplemental Digital Content 1, http://links.lww.com/JNPT/A63) for more insights from the authors.

Using submaximal contractions to predict the maximum force-generating ability of muscles.

INTRODUCTION: Muscle weakness can be caused by decreases in either the maximum force-generating ability of a muscle (MFGA) or neural drive from the nervous system (e.g., after a stroke). Presently, there is no agreed-upon practical method for calculating the MFGA in individuals with central nervous system pathology. The purpose of this study was to identify the best method for determining MFGA. METHODS: The predicted and estimated MFGA of the muscles of 23 non-neurologically impaired subjects (13 males, 21.9 ± 1.9 years) were compared using the burst superimposition, twitch interpolation, doublet interpolation, twitch-to-tetanus ratio, and the adjusted burst superimposition methods. RESULTS: The adjusted burst superimposition test was the most accurate predictor of MFGA. CONCLUSIONS: Further testing is needed to validate the use of the adjusted burst superimposition test in a neurologically impaired population.

Combined effects of fast treadmill walking and functional electrical stimulation on post-stroke gait.

Gait dysfunctions are highly prevalent in individuals post-stroke and affect multiple lower extremity joints. Recent evidence suggests that treadmill walking at faster than self-selected speeds can help improve post-stroke gait impairments. Also, the combination of functional electrical stimulation (FES) and treadmill training has emerged as a promising post-stroke gait rehabilitation intervention. However, the differential effects of combining FES with treadmill walking at the fast versus a slower, self-selected speed have not been compared previously. In this study, we compared the immediate effects on gait while post-stroke individuals walked on a treadmill at their self-selected speed without FES (SS), at the SS speed with FES (SS-FES), at the fastest speed they are capable of attaining (FAST), and at the FAST speed with FES (FAST-FES). During SS-FES and FAST-FES, FES was delivered to paretic ankle plantarflexors during terminal stance and to paretic dorsiflexors during swing phase. Our results showed improvements in peak anterior ground reaction force (AGRF) and trailing limb angle during walking at FAST versus SS. FAST-FES versus SS-FES resulted in greater peak AGRF, trailing limb angle, and swing phase knee flexion. FAST-FES resulted in further increase in peak AGRF compared to FAST. We posit that the enhancement of multiple aspects of post-stroke gait during FAST-FES suggest that FAST-FES may have potential as a post-stroke gait rehabilitation intervention.

A phenomenological model that predicts forces generated when electrical stimulation is superimposed on submaximal volitional contractions.

Superimposition of electrical stimulation during voluntary contractions is used to produce functional movements in individuals with central nervous system impairment, to evaluate the ability to activate a muscle, to characterize the nature of fatigue, and to improve muscle strength during postsurgical rehabilitation. Currently, the manner in which voluntary contractions and electrically elicited forces summate is not well understood. The objective of the present study is to develop a model that predicts the forces obtained when electrical stimulation is superimposed on a volitional contraction. Quadriceps femoris muscles of 12 able-bodied subjects were tested. Our results showed that the total force produced when electrical stimulation was superimposed during a volitional contraction could be modeled by the equation T=V+S[(MaxForce-V)/MaxForce]N, where T is the total force produced, V is the force in response to volitional contraction alone, S is the force response to the electrical stimulation alone, MaxForce is the maximum force-generating ability of the muscle, and N is a parameter that we posit depends on the differences in the motor unit recruitment order and firing rates between volitional and electrically elicited contractions. In addition, our results showed that the model predicted accurately (intraclass correlation coefficient>or=0.97) the total force in response to a wide range of stimulation intensities and frequencies superimposed on a wide range of volitional contraction levels. Thus the model will be helpful to clinicians and scientists to predict the amount of stimulation needed to produce the targeted force levels in individuals with partial paralysis.

Novel patterns of functional electrical stimulation have an immediate effect on dorsiflexor muscle function during gait for people poststroke.

BACKGROUND: Foot drop is a common gait impairment after stroke. Functional electrical stimulation (FES) of the ankle dorsiflexor muscles during the swing phase of gait can help correct foot drop. Compared with constant-frequency trains (CFTs), which typically are used during FES, novel stimulation patterns called variable-frequency trains (VFTs) have been shown to enhance isometric and nonisometric muscle performance. However, VFTs have never been used for FES during gait. OBJECTIVE: The purpose of this study was to compare knee and ankle kinematics during the swing phase of gait when FES was delivered to the ankle dorsiflexor muscles using VFTs versus CFTs. DESIGN: A repeated-measures design was used in this study. PARTICIPANTS: Thirteen individuals with hemiparesis following stroke (9 men, 4 women; age=46-72 years) participated in the study. METHODS: Participants completed 20- to 40-second bouts of walking at their self-selected walking speeds. Three walking conditions were compared: walking without FES, walking with dorsiflexor muscle FES using CFTs, and walking with dorsiflexor FES using VFTs. RESULTS: Functional electrical stimulation using both CFTs and VFTs improved ankle dorsiflexion angles during the swing phase of gait compared with walking without FES (X+/-SE=-2.9 degrees +/- 1.2 degrees). Greater ankle dorsiflexion in the swing phase was generated during walking with FES using VFTs (X+/-SE=2.1 degrees +/- 1.5 degrees) versus CFTs (X+/-SE=0.3+/-1.3 degrees). Surprisingly, dorsiflexor FES resulted in reduced knee flexion during the swing phase and reduced ankle plantar flexion at toe-off. CONCLUSIONS: The findings suggest that novel FES systems capable of delivering VFTs during gait can produce enhanced correction of foot drop compared with traditional FES systems that deliver CFTs. The results also suggest that the timing of delivery of FES during gait is critical and merits further investigation.

Functional electrical stimulation of ankle plantarflexor and dorsiflexor muscles: effects on poststroke gait.

BACKGROUND AND PURPOSE: Functional electrical stimulation (FES) is a popular poststroke gait rehabilitation intervention. Although stroke causes multijoint gait deficits, FES is commonly used only for the correction of swing-phase foot drop. Ankle plantarflexor muscles play an important role during gait. The aim of the current study was to test the immediate effects of delivering FES to both ankle plantarflexors and dorsiflexors on poststroke gait. METHODS: Gait analysis was performed as subjects (N=13) with chronic poststroke hemiparesis walked at their self-selected walking speeds during walking with and without FES. RESULTS: Compared with delivering FES to only the ankle dorsiflexor muscles during the swing phase, delivering FES to both the paretic ankle plantarflexors during terminal stance and dorsiflexors during the swing phase provided the advantage of greater swing-phase knee flexion, greater ankle plantarflexion angle at toe-off, and greater forward propulsion. Although FES of both the dorsiflexor and plantarflexor muscles improved swing-phase ankle dorsiflexion compared with noFES, the improvement was less than that observed by stimulating the dorsiflexors alone, suggesting the need to further optimize stimulation parameters and timing for the dorsiflexor muscles during gait. CONCLUSIONS: In contrast to the typical FES approach of stimulating ankle dorsiflexor muscles only during the swing phase, delivering FES to both the plantarflexor and dorsiflexor muscles can help to correct poststroke gait deficits at multiple joints (ankle and knee) during both the swing and stance phases of gait. Our study shows the feasibility and advantages of stimulating the ankle plantarflexors during FES for poststroke gait.

Development of a mathematical model for predicting electrically elicited quadriceps femoris muscle forces during isovelocity knee joint motion.

BACKGROUND: Direct electrical activation of skeletal muscles of patients with upper motor neuron lesions can restore functional movements, such as standing or walking. Because responses to electrical stimulation are highly nonlinear and time varying, accurate control of muscles to produce functional movements is very difficult. Accurate and predictive mathematical models can facilitate the design of stimulation patterns and control strategies that will produce the desired force and motion. In the present study, we build upon our previous isometric model to capture the effects of constant angular velocity on the forces produced during electrically elicited concentric contractions of healthy human quadriceps femoris muscle. Modelling the isovelocity condition is important because it will enable us to understand how our model behaves under the relatively simple condition of constant velocity and will enable us to better understand the interactions of muscle length, limb velocity, and stimulation pattern on the force produced by the muscle. METHODS: An additional term was introduced into our previous isometric model to predict the force responses during constant velocity limb motion. Ten healthy subjects were recruited for the study. Using a KinCom dynamometer, isometric and isovelocity force data were collected from the human quadriceps femoris muscle in response to a wide range of stimulation frequencies and patterns. % error, linear regression trend lines, and paired t-tests were used to test how well the model predicted the experimental forces. In addition, sensitivity analysis was performed using Fourier Amplitude Sensitivity Test to obtain a measure of the sensitivity of our model's output to changes in model parameters. RESULTS: Percentage RMS errors between modelled and experimental forces determined for each subject at each stimulation pattern and velocity showed that the errors were in general less than 20%. The coefficients of determination between the measured and predicted forces show that the model accounted for approximately 86% and approximately 85% of the variances in the measured force-time integrals and peak forces, respectively. CONCLUSION: The range of predictive abilities of the isovelocity model in response to changes in muscle length, velocity, and stimulation frequency for each individual make it ideal for dynamic applications like FES cycling.

Predicting muscle forces of individuals with hemiparesis following stroke.

BACKGROUND: Functional electrical stimulation (FES) has been used to improve function in individuals with hemiparesis following stroke. An ideal functional electrical stimulation (FES) system needs an accurate mathematical model capable of designing subject and task-specific stimulation patterns. Such a model was previously developed in our laboratory and shown to predict the isometric forces produced by the quadriceps femoris muscles of able-bodied individuals and individuals with spinal cord injury in response to a wide range of clinically relevant stimulation frequencies and patterns. The aim of this study was to test our isometric muscle force model on the quadriceps femoris, ankle dorsiflexor, and ankle plantar-flexor muscles of individuals with post-stroke hemiparesis. METHODS: Subjects were seated on a force dynamometer and isometric forces were measured in response to a range of stimulation frequencies (10 to 80-Hz) and 3 different patterns. Subject-specific model parameter values were obtained by fitting the measured force responses from 2 stimulation trains. The model parameters thus obtained were then used to obtain predicted forces for a range of frequencies and patterns. Predicted and measured forces were compared using intra-class correlation coefficients, r2 values, and model error relative to the physiological error (variability of measured forces). RESULTS: Results showed excellent agreement between measured and predicted force-time responses (r2 >0.80), peak forces (ICCs>0.84), and force-time integrals (ICCs>0.82) for the quadriceps, dorsiflexor, and plantar-fexor muscles. The model error was within or below the +95% confidence interval of the physiological error for >88% comparisons between measured and predicted forces. CONCLUSION: Our results show that the model has potential to be incorporated as a feed-forward controller for predicting subject-specific stimulation patterns during FES.

Gait training after stroke: a pilot study combining a gravity-balanced orthosis, functional electrical stimulation, and visual feedback.

RATIONALE: This case report describes the application of a novel gait retraining approach to an individual with poststroke hemiparesis. The rehabilitation protocol combined a specially designed leg orthosis (the gravity-balanced orthosis), treadmill walking, and functional electrical stimulation to the ankle muscles with the application of motor learning principles. CASE: The participant was a 58-year-old man who had a stroke more than three years before the intervention. He underwent gait retraining over a period of five weeks for a total of 15 sessions during which the gravity compensation provided by the gravity-balanced orthosis and visual feedback about walking performance was gradually reduced. OUTCOMES: At the end of five weeks, he decreased the time required to complete the Timed Up and Go test; his gait speed increased during overground walking; gait was more symmetrical; stride length, hip and knee joint excursions on the affected side increased. Except for gait symmetry, all other improvements were maintained one month post-intervention. CONCLUSIONS: This case report describes possible advantages of judiciously combining different treatment techniques in improving the gait of chronic stroke survivors. Further studies are planned to evaluate the effectiveness of different components of this training in both the subacute and chronic stages of stroke recovery.

Effects of activation pattern on nonisometric human skeletal muscle performance.

During volitional muscle activation, motor units often fire with varying discharge patterns that include brief, high-frequency bursts of activity. These variations in the activation rate allow the central nervous system to precisely control the forces produced by the muscle. The present study explores how varying the instantaneous frequency of stimulation pulses within a train affects nonisometric muscle performance. The peak excursion produced in response to each stimulation train was considered as the primary measure of muscle performance. The results showed that at each frequency tested between 10 and 50 Hz, variable-frequency trains that took advantage of the catchlike property of skeletal muscle produced greater excursions than constant-frequency trains. In addition, variable-frequency trains that could achieve targeted trajectories with fewer pulses than constant-frequency trains were identified. These findings suggest that similar to voluntary muscle activation patterns, varying the instantaneous frequency within a train of pulses can be used to improve muscle performance during functional electrical stimulation.

Mathematical model that predicts lower leg motion in response to electrical stimulation.

Electrical stimulation of skeletal muscles of patients with upper motor neuron lesions can be used to restore functional movements such as standing or walking. Mathematical muscle models can assist in designing stimulation patterns that will enable patients to perform particular tasks more efficiently. In this study we extend our previous model to allow us to predict changes in knee joint angle in response to electrical stimulation of the human quadriceps femoris muscle. The model was tested both with and without inertial loads placed around the ankle joints of healthy subjects. Results showed that the model predicted the knee extensions with a RMS angle error that was generally

Modeling the length dependence of isometric force in human quadriceps muscles.

Functional electrical stimulation is used to restore movement and function of paralyzed muscles by activating skeletal muscle artificially. An accurate and predictive mathematical model can facilitate the design of stimulation patterns that produce the desired force. The present study is a first step in developing a mathematical model for non-isometric muscle contractions. The goals of this study were to: (1) identify how our isometric force model's parameters vary with changes in knee joint angle, (2) identify the best knee flexion angle to parameterize this model, and (3) validate the model by comparing experimental data to predictions in response to a wide range of stimulation frequencies and muscle lengths. Results showed that by parabolically varying one of the free parameters with knee joint angle and fixing the other parameters at the values identified at 40 degrees of knee flexion, the model could predict the force responses to a wide range of stimulation frequencies and patterns at different muscle lengths. This work showed that the current isometric force model is capable of predicting the changes in skeletal muscle force at different muscle lengths.